EPA-600/8-85-026 c.2 PB86-108636
WASTEWATER TREATMENT PLANT INSTRUMENTATION
HANDBOOK
EMA, Incorporated
St. Paul, MN
Sep 85
U.S. DEPARTMENT OF COMMERCE
National Technical Information Service
NTIS
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EE86-1CS636
EPA/600/3-85/026
September 1985
WASTEWATER TREATMENT PLANT INSTRUMENTATION HANDBOOK
by
Robert C. Manross
EMA, Inc.
St. Paul, Minnesota 55101
Contract No. 68-03-3130
Project Officer
Walter W. Schuk
Wastewater Research Division
Water Engineering Research Laboratory
Cincinnati, Ohio 45268
US. Environs r , ~, t .
• RcR.on \/_ , .- .. ' ' ••'--•ctj
), v •
"1!'',-,1' ""•"'' ''"'"': 5i
, iinf;o;s 60604
_ on /Agency.
230 ;>-, •
WATER ENGINEERING RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
REPRODUCED BY
NATIONAL TECHNICAL
INFORMATION SERVICE
US. DEPARTMENT OF COMMERCE
SPRINGFIELD. VA 22161
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA/600/8-85/026
2.
3. RECIPIENT'S ACCESSION NO.
P33 6 1 0 8 o 3 S /
4. TITLE AND SUBTITLE
Wastewater Treatment Plant
Instrumentation Handbook
5. REPORT DATE
September 1985
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
Robert C. Manross
9. PERFORMING ORGANIZATION NAME AND ADDRESS
EMA, Inc.
Control System Engineers
480 Cedar Street
St. Paul, MN 55391
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-03-3130
12. SPONSORING AGENCY NAME AND ADDRESS
Water Engineering Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268
13. TYPE OF REPORT AND PERIOD COVERED
- Final
14. SPONSORING AGENCY CODE
EPA/600/14
is. SUPPLEMENTARY NOTES Project Officer: Walter W. Schuk (513) 684-2621
Prepared as a follow-up to the Technology Transfer Wastewater Instrumentation and
Control Seminar, November 29 - December 2, 1983, in Chicago, Illinois.
16. ABSTRACT
•Instruments are required for proper operation of wastewater plants. To be of use the
instruments must be operable and maintainable. This requires care in the selection,-^
application and installation of instruments and control equipment. Contents of the
handbook address the "how-to" of designing and applying instrumentation and controls
for waste treatment operations. Special focus is given to problems, 'causes and
solutions.
The handbook covers instruments, valves and pumps commonly used in wastewater plants.
The material covers
. o Basic Theory of Operation
o Application
o Installation Requirements
o Maintenance and Calibration Requirements
o Selection and sizing Specifications
The material is intended for use by individuals with no previous background or
specialized knowledge of instrumentation or control equipment. Those responsible for
reviewing the work done by others, may find the designers checklist in each section a
helpful reference. If more technical information is required, a reference is included
17.
KEY WORDS AND DOCUMENT ANALYSIS at €he end of each section.
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATl Field/Group
Instrument
Pump
Valve
Control
Analytical
Flow
Level
Pressure
Measurement
18. DISTRIBUTION STATEMENT
Release Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
310
20. SECURITY CLASS (This page)
Unclassified
EPA Form 2220-1 (R«v. 4-77) PREVIOUS EDITION is OBSOLETE
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NOTICE
THIS DOCUMENT HAS BEEN REPRODUCED FROM THE
BEST COPY FURNISHED US BY THE SPONSORING
AGENCY. ALTHOUGH IT IS RECOGNIZED THAT CER-
TAIN PORTIONS ARE ILLEGIBLE, IT IS BEING RE-
LEASED IN THE INTEREST OF MAKING AVAILABLE
AS MUCH INFORMATION AS POSSIBLE.
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DISCLAIMER
The information in this document has been funded wholly or in part by the
United States Environmental Protec.xon Agency under Contract No. 68-03-3130 to
EMA, Inc. It has been subject to the Agency's peer and administrative review,
and it has been approved for publication as an EPA document. Mention of trade
names or commercial products does not constitute endorsement or recommendation
for use.
ii
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FOREWORD
The U.S. Environmental protection Agency is charged by Congress with-
protecting the Nation's land, air, and water systems. Under a mandate of
national environmental laws, the agency strives to formulate and implement
actions leading to a compatible balance between human activities and the
ability of natural systems to support and nurture life. The Clean Water
Act, the Safe Drinking Water Act, and the Toxic Substances Control Act are
three of the major congressional laws that provide the framework for
restoring and maintaining the integrity of our Nation's water, for
preserving and enhancing the water we drink, and for protecting the
environment from toxic substances. These laws direct the EPA to perform
research to define our environmental problems, measure the impacts, and
search for solutions.
The Water Engineering Research Laboratory is that component of EPA's
Research and Development program concerned with preventing, treating, and
managing municipal and industrial wastewater discharges; establishing
practices to control and remove contaminants from drinking water and to
prevent its deterioration during storage and distribution; and assessing the
nature and controllability of releases of toxic substances to the air,
water, and land from manufacturing processes and subsequent product uses.
This publication is one of the products of tnat research and provides a
vital communication link between the researcher and the user community.
This handbook was developed to provide users and designers with a guide
for selecting and applying instrumentation in wastewater treatment
plants. The material is intended for use by individuals with no previous
background or specialized knowledge of instrumentation.
Francis T. Mayo, Director
Water Engineering Research Laboratory
iii
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ABSTRACT
This handbook is to be used as a guide for selecting and maintaining
instruments and final control elements in wastewater treatment plants. Basic
applications covered include analytical measurement, flow measurement (liquid
and gas), level measurement, pressure measurement, pump control, and control
valves.
Priority has been given to basic proven instruments that meet specific
needs and provide tangible benefits. The material covers the theory of opera-
tion, application guidelines, installation requirements, maintenance and cali-
bration requirements, and selection*and sizing specifications.
The handbook is intended to be used by individuals with no previous
background or specialized knowledge of instrumentation and control equipment.
A designer's checklist is provided for each of the instruments described in the
handbook.
This report was submitted in partial fulfillment of Contract No. 68-03-
3130 by Dynamac, Inc., Rockville, Maryland, under the sponsorship of the U.S.
Environmental Protection Agency. The report was prepared under subcontract to
EMA, Inc., St. Paul, Minnesota. This report covers the period May 1982 to
December 1983, when work was completed.
iv
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CONTENTS
Foreword _x
Abstract *Y
Figures
Tables •-.'.:
Abbreviations and Symbols "
Acknowledgments
Introduction 1
1.0 Analytical Measurement 3
1.1 Total Chlorine Residual 3
1.2 Dissolved Oxygen Meters 11
1.3 pH 20
1.4 Suspended Solids 29
2.0 Flow Measurement, Closed Conduit Liquid Flow 40
2.1 Magnetic Flow Meters 40
2.2 Sonic Flow Meters 53
2.3 Turbine Flow Meters 64
2.4 Venturi Tubes and Flow Tubes 71
3.0 Flow Measurement, Closed Conduit Gas Flow 83
3.1 Orifice Plate 83
3,.2 Venturi Tubes and Flow Tubes 96
3.3 Averaging Pi trot Tubes 105
3.4 Turbine Flow Meters 113
4.0 Flow Measurement, Open Channel 119
4.1 Kennison Nozzle 119
4.2 Palmer-Bowlus Flume 125
4.3 Parshall Flume 132
4.4 Weir 140
5.0 Level Measurement 14g
5.1 Bubbler Level Measurement 14g
5.2 Capacitance Probe 155
5.3 Float Level Instruments 163
5.4 Sonic and Ultrasonic Level Sensors ....
6.0 Pressure Measurement
6.'1 Pressure Cells
6.2 Differential Pressure
7.0 Performance Testing JOQ
7.1 Flow 190
8.0 Pump Control Methods 2QQ
8.1 Variable Flow Service 2QO
9.0 Variable Speed Drive 213
9.1 Magnetic Coupling 213
9.2 Liquid Rheostat „,-
9.3 Variable Frequency __
9.4 Variable Pulley 224
9.5 Direct Current (SCR) 227
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10.0 Control Valves 230
10.1 Modulating Service 230
11.0 Control Valve Operators 251
11.1 Electric 251
11.2 Hydraulic 256
11.3 Pneumatic 260
Appendices
A. Title by Subject Reference List 263
B. Reference List Sources 272
Glossary 276
vi
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FIGURES
Number Page
1.1 Araperometric measurement ..... 4
1.2 Amperometric total chlorine residual analyzer .... 5
1.3 Sample point location 7
1.4 Sample transport S
1.5 Membrane-type DO probe cell 12
1.6 DO meter configuration 16
1.7 Typical pH sensor 22
1.8 Equivalent circuit 22
1.9 Effect of varying asymmetric potential at 23
constant (25° C) temperature
1.10 Effect of varying temperature at constant 23
asymmetric potential
1.11 Flow-through pH sensor installation 26
1.12 Submersion pH sensor installation 26
1.13 Light scattering 31
1.14 Transraissive-type optical suspended solids analyzer . 31
1.15 Nuclear solids analyzer 33
1.16 Jackson candle turbidimeter 34
1.17 Forward scatter turbidimeter 34
1.18 Side scatter turbidimeters 35
vii
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FIGURF-S
(Continued)
Number Page
2.1 Magnetic flow meter construction 43
2.2 Magnetic induced voltage 44
2.3 Mag meter electrode snape op» Ions 45
2.4 Bypass pipe installation 43
2.5 Mag meter grounding -. . . . 49
2.6 Transmissive sonic flow meter 55
2.7 Reflective sonic flow meter 56
2.8 Transmissive sonic meter axial configuration ^Q
2.9 Turbine flow meter 66
2.10 Turbine meter mounting 6g
2.11 Classic venturi tube 72
2.12 Proprietary flow tubes -^
2.13 Typical differential piping 75
3.1 Concrete orifice plate 34
3.2 pressure profile 35
3.3 Orifice straight run requirements 33
3.4 Steam flow installation 90
3.5 Gas flow installation 91
3.6 Classic venturi tube 96
3.7 Proprietary flow tubes 97
3.8 Venturi piping requirements 99
3.9 .Steam flow measurement installation diagram .....
viii
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FIGURES
(Continued)
Number Page
3.10 Gas flow measurement installation diagram ...... 102
3.11' Pitot tube probe ................... 106
3.12 Typical up/downstream requirements .......... 103
3.13 Installation for steam flow applications ....... 109
3.14 Orientation for gas flow applications ......... ng
3.15 Gas turbine meter .................. 114
3.16 Meter installation .................. 116
4.1 Kennison nozzle ................... 120
4.2 Typical rating curve for a 10-inch Kennison nozzle . . 120
4.3 Palmer-Bowlus flume ................. 126
4.4 Free flow/depth relation ............... 127
4.5 Parshall flume flow element ............. 133
4.6 Head/width parameters ................ 134
4.7 Free flow submergence ................
4.8 Sharp-crested weir ..................
4.9 Weir shapes .....................
4.10 Suppress rectangular weir ..............
5.1 Typical bubbler applications ............. 149
5.2 Open tank bubbler installation ............ 152
5.3 Illustration of probe/tank capacitive relationship . . 153
5.4 Capacitance level sensor ............... 159
-ix
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Number
5.5
5.6 '
5.7
5.8
5.9
5.10
5.11
6.1
6.2
6.3
6.4
6.5
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
FIGURES
(continued)
Counterweighted float level indicator .... ..
Float switch mounting installation
Manifolds
Wet leg
Slider— crank stroke adjustment .......
Variable speed versus throttling of centrifugal pumps ....
Two identical pumps in parallel with throttling valve ....
Page
161
1 fi?
1 £/,
1 £C
1 ^6
168
175
131
185
1S3
1S8
202
203
204
205
208
209
210
211
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FIGURES
(Continued)
Number Page
9.1 Magnetic coupling functional block diagram 214
9.2 Magnetic coupling control diagram 216
9.3 Liquid rheostat functional block diagram 213
9.4 Liquid rheostat control diagram 220
9.5 Variable frequency functional block diagram 222
9.6 Simplex VFD control diagram 223
9.7 Variable pulley control diagram 226
9.8 DC drive control diagram 229
2^1
10.1 Ball valve
10.2 Butterfly valve, swing through type with flangeless .... 233
pipe connection
10.3 Butterfly torque 234
10.4 Gate valve, multi-orifice 235
10.5 Globe valve 236
10.6 Plug valves 237
10.7 Traditional pressure drops for control valves 239
10.8 Pressure drop across a valve 24]
10.9 Intrinsic valve characteristics 242
10.10 Installed valve characteristics 243
10.11 Installed valve gain 244
10.12 Valve pressure drop in a pumping system 245
10.13 Gain of a single-ported globe valve 247
xi
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FIGURES
(continued)
Number Page
11.1 Electric operator control circuit-modulating service .... .253
11.2 Hydraulic piston actuator 257
11.3 Piston actuator control circuit-incremental modulating . . . 259
service
11.4 Pneumatic piston actuators 261
1] .3 Diaphragm actuator 262
xii
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TABLES
1.2 Recommended Application of pH Sensors 20
1.3 Suspended Solids Application Guidelines 29
2.1 Typical Applications for Mag Meters in Wastewater Treatment ... 41
2.2 Liner Selection Criteria for Specific Conditions 42
2.3 Transmissive Sonic Flow Meter Application Guidelines 53
2.4 Reflective Sonic Flow Meter Application Guidelines • 54
2.5 Turbine Flow Meter Application Guidelines 64
2.6 Venturi and Flow Tube Application Guidelines 71
3.1 Flow Measurement, Closed Conduit Gas Flow Applications Guidelines 83
3.2 Venturi Tubes and Flow Tubes Application Guidelines 96
3.3 Averaging Pitot Tubes Application Guidelines 105
3.4 Turbine Flow Meter Application Guidelines 113
4.1 Flow Measurement, Open Channel Application Guidelines 119
4.2 Limiting Slope for Approach Piping 122
4.3 Maximum Line Velocity for Kennison Nozzle Installations 123
4.4 Palmer-Bowlus Flume Application Guidelines 125
4.5 Parshall Flume Application Guidelines 132
4.6 Submergence Limits 135
4.7 Wastewater Treatment Facility Application Guidelines 140
5.1 Bubbler Level Measurement Application Guidelines 148
5.2 Uncompensated Capacitance Probes Wastewater Treatment
Facility Application Guidelines 155
xiii
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TABLES
(continued)
5.3 Compensated Capacitance Probes Wastewater Treatment
Facility Application Guidelines 155
5.4 Float Level Indicators Wastewater Treatment Facility
Application Guideline 163
5.5 Sonic and Ultrasonic Level Sensors Application Guidelines .... 168
6.1 Pressure Measurement Application Guideline 174
6.2 Differential Pressure Application Guideline 180
10.1 Comparision or Control Valves in Wastewater Treatment Processes . 230
10.2 Cv Conversion Factors 238
10.3 Valve Characteristic Selection Guide 248
11.1 Electric Valve Operators Application Guidelines 251
xiv
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ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
AC
cm
c
CPU
d
D
dc
DAS
DDC
DEAC
DO
P
ft/s
G
gal
gpm
H
Hz
h
hp
in.
in/s
I/O
kl/m
LCD
LED
L
Vm
Ibm
mAdc
m/s
mg/1
MHz
MUX
NEMA
ODC
Pa
psia
psig
alternating current
centimeter
centigrade
central processing unit (computer)
orifice or venturi throat diameter
pipe inside diameter
direct current
data acquisition system
direct digital control
digitally emulated analog control
dissolved oxygen
Fahrenheit
feet per second
specific gravity
gallon
gallons per minute
head
hertz
head in inches of water column •
horsepower
inch
inches per second
input/output
kilo-liters/minute
liquid crystal display
light emitting diode
length
liters/minute
pounds mass
milliamperes direct current
meters per second
milligrams per liter
mega-hertz
multiplexer
National Electrical Manufacturer's Association
optimizing digital control
Pascals
pounds per square inch absolute
pounds per square inch gauge
R a.n If i n A
XV
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SCADA — supervisory control and data acquisition system
scfh «• standard cubic feet per hour
SCR — silicon controlled rectifier
V — volts
vdc — volts direct current
VDT — video display terminal
W — watts
Xmtr — transmitter
SYMBOLS
% — percent
A — delta or differential
p — differential pressure
° — degrees
> — greater than
< — less than
p — specified weight
8 ~ d/D
xvi
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ACKNOWLEDGMENTS
The handbook was prepared by EMA, Inc., Control System Engineers, as
part of a technology transfer program entitled Wastewater Instrumentation
and Control Seminar. The author acknowledges the significant contribution
made by the following EMA personnel in producing the seminar and handbook:
Dag Knudsen
Rich Lackman
Claude Williams
Cheryl Schuebel
Roxamfe Sabean
Helpful project guidance and assistance was provided by U.S. Environ-
mental Protection Agency personnel, Walt Schuk, Water Engineering Research
Laboratory, and Jim Smith, Center for Environmental Research Information.
Special appreciation is given to Walt Schuk for his technical review of the
handbook. His contributions have made it a better reference manual.
Appreciation is also expressed to the wastewater treatment plant per-
sonnel who shared their time, experiences, and opinions so the seminar and
handbook could address real-world problems and solutions. In particular,
instrument and control support people need to be recognized for their
creativity in developing fixes, procedures, etc. for improved system per-
formance. How greatly we would benefit if we could compile the unreported
experiences that exist in all plants.
xvii
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INTRODUCTION
Instruments are required for proper operation of wastewater treatment
plants. Instrumentation for wastewater treatment was observed in various
states of working order during visits to wastewater treatment plants over
several years. Personnel at these sites often criticized the instruments
for their failure to meet expectations of reliability and usefulness.
However, the same instruments performed satisfactorily in other locations.
We therefore concluded that the need was great to educate the engineering,
user, and regulatory communities about the prerequisites for success with
instrumentation.
As part of the U.S. Environmental Protection Agency's continuing
technology transfer program, a 4-day seminar was held in November 1983 in
Chicago to teach engineers and operators how to make wastewater treatment
instruments operable and maintainable. The program presented examples of
real-life problems and how they could be prevented. The problems were
attributed to misapplication, incorrect installation, or improper maintenance.
This handbook grew out of the preparation for the seminar, which
included documenting the basis for the information that was presented.
The handbook addressed more instruments than time permitted at the
seminar, but it does not cover all instrumentation used in wastewater
treatment plants. Priority has been given to basic, proven instruments
that meet specific needs and provide tangible benefits. For the instruments
contained in the handbook, the material covers:
1. Basic theory of operation
2. Application
3. Installation requirements
4. Maintenance and calibration requirements
5. Selection and sizing specifications
The information contained here should not be considered all-inclusive;
rather, it is a beginning documentation of much-needed information on what
really works in the field. What makes an instrument reliable and maintainable?
Many of the answeres to this question lie outside the manufacturers' manuals.
The solutions are sometimes revised procedures, test methods, or physical
modifications. Too often this knowledge is not shared outside the treatment
plant because the persons responsible do not consider their solutions unique
or important.
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When a decision is made to use an instrument for measuring a specific
parameter there is an implied commitment to maintain that instrument in an
operational state; thus, information describing the maintenance and calibration
requirements of a particular instrument are a critical part of the equipment
selection process.
A list of technical references is included at the end of each section.
For the reader's convenience, Appendix A contains a complete list of all
references used in the handbook. For ease in ordering, Appendix B lists
the organization, address, an
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1.0 ANALYTICAL MEASUREMENT
1.1 TOTAL CHLORINE RESIDUAL
A. Applications
The most common method of disinfecting wastewater plant
effluents is chlorination. If free chlorine gas, liquid
hypochlorite, or chlorimines are added to the effluent, the
chlorine acts as an agent to destroy microscopic organisms
that are disease producing or otherwise objectionable. Over
the years criteria for rates of adding chlorine, detention
time, and chlorine residual at the end of the detention
period have been established. If this criteria is adhered
to, it is assumed the desired level of disinfection,
elimination of harmful organisms, will be achieved.
Chlorine residual analyzers monitor the residual chlorine in
an effluent stream. This is an indirect measure. Current
accepted technology of monitoring chlorine residual is based
on.the assumption that maintaining a minimum chlorine
residual (usually 1.0 mg/1) 30 minutes after adding chlorine
will result in an effective disinfection level.
If the chlorine residual drops below the desired minimum
level, the rate of chlorine addition is increased.
Conversely, an increase above the minimum results in a
decrease in the chlorine addition rate. A chlorine residual
analyzer can be used to determine on-line if the correct
chlorine dosage is being used. If corrective action, when
indicated by the meter, is taken, then the operation can run
closer to the minimum level, resulting in a minimum of
chlorine usage.
The alternative to using a chlorine residual analyzer is to
run tests in the laboratory. This is not as convenient or
timely as an on-line unit. The on-line analyzer provides a
continuous reading of the chlorine residual.
B. Principle of Operation
Several different measurement methods are used for chlorine
residual, including colorometric, amperometric, and
polarographic. For application in wastewater, amperometric
is the most common for Measuring total chlorine residual.
Therefore, discussion in this section is restricted to
amperometric measurement.
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The amperometric measurement method uses two dissimilar
metals held in a solution or electrolyte. A voltage is
applied to the two metals which act as electrodes. Electrons
flow from the negative electrode to the positive electrode
generating a current. Figure 1.1 illustrates the
amperometric cell. The amount of current flowing between the
electrodes is proportional to the amount of chlorine present
in the solution.
ELECTROLYTE
I PROPORTIONAL TO Cl CONCENTRATION
Figure 1.1. Amperoinetric measurement.
The basic amperometric chlorine residual analyzer is
illustrated in Figure 1.2. It consists of an inlet sample
tank and flow regulator, reagent solutions with metering
pumps, measurement cell, and electronic signal converter.
The metered sample stream acts as the electrolyte as it flows
through the measurement cell. Since chlorine in the sample
can exist in many different chemical forms, the sample is
conditioned with other chemicals in order for the cell to
measure all chlorine present in the stream.
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BUFFER
SOLUTION
pH4
SAMPLE
TANK 3
FLOW
REGULATOR
SAMPLE
OUTPUT
SIGNAL
WftSTE
TO
DRAIN
POTASSIUM
IODIDE
REAGENT
WflSTE
TO
DRAIN
Figure 1.2. Amperometric total chlorine residual analyzer.
The current generated in the measurement cell is very
sensitive to temperature variations. A reading can change as
much as 3% per degree C temperature change. Therefore,
automatic temperature compensation is necessary. A
temperature sensor is located in the measurement cell to
provide temperature feedback to the electronic converter.
This feedback is then used to correct the indicator and
output signals to null-out the temperature effects.
C. Accuracy and Repeatability
The accuracy of chlorine residual analyzers is ^3% of full
scale. Several ranges are normally available from 0-1 mg/1
to 0 - 20 mg/1. The measurement error could range from
0.03 - 0.6 mg/1 depending on the operating range used.
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Repeatability of measurement on samples of equal chlorine
concentration should be within .+1% of full scale.
Automatic temperature compensation should enable this
accuracy and repeatability to hold over a sample temperature
range of 0 - 50o c (32 - 122O F).
D. Manufacturer's Options
A few options are available from manufacturers of chlorine
residual analyzers. Some to consider are:
1. Local indicator in the analyzer case,
2. Supply of reagents,
3. Integral solids filter, and
4. Output signal for remote monitoring of the chlorine
residual.
E. Installation
Chlorine residual analyzers are normally housed in free
standing enclosures. A sample for measurement is piped from
the chlorine contact basin to the analyzer. This sample
system is a critical element for a successful analyzer
application. A complete installation consists of a sample
point, sample transport, and the analyzer.
1. Sample point location.
It is desirable to analyze the effluent after there is
sufficient contact time between the chlorine and
effluent stream for disinfection to occur. A commonly
accepted disinfection period is 30 minutes. Therefore,
it is desired to deliver a sample to the analyzer 30
minutes after adding chlorine. To do this, you must be
concerned about the time in the contact tank plus the
time to deliver a sample to the analyzer. This is the
total contact time as shown in Figure 1.3.
Physically it is desired to locate the sample point so
it does not contribute unnecessary deadtime in chlorine
residual analysis. In addition, take care to ensure the
sample point is clean, thoroughly mixed, and
representative of the monitored stream.
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TOTAL CONTACT TIME'
CONTACT TANK TIME (T,J + SAMPLE TRANSPORT TIME (T2)
Figure 1.3. Sample point location.
2. Sample transport.
A sample line and pump are required to deliver the
sample to the analyzer. Features of this sample
transport assembly are shown in Figure 1.4. You should:
a. Select a pump capable of delivering 20 - 40 1/m
(5 -.10 gpm).
b. Size the pipe for a sample velocity of 1.5 - 3.0
m/s (5 - 10 ft/s).
c. Determine the length of sample line so it will
provide the desired transport time.
d. Install a valve next to the analyzer so samples can
be taken for calibration checks on the analyzer.
e. Provide a source of clean water and required valves
so the sample line can be backflushed to prevent
plugging.
f. If solids are present, install a filter.
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50-100 PSI
CLEAN
WATER
SUPPUT
MOTOR
DRIVEN FILTER
(OPTIONAL;
X
-M-
SAMPLE
VALVE
DRAIN
Figure 1.4. Sample transport.
3. Chlorine analyzer.
Install the analyzer so it is easy to service and
maintain.
a. Provide ample space, minimum of 1 m (3 ft), around
all sides of the analyzer.
b. Locate it next to a floor drain.
c. Provide a table nearby with the necessary equipment
and chemicals to perform calibration checks.
d. Provide a separate circuit on a lighting panel to
power the analyzer.
F. Designer's Checklist
Ask the following questions when designing or reviewing
chlorine residual analyzer applications. All checklist
questions should be answerable with a "yes" for correct
installation.
1. Is remote monitoring required? If so, has an output
signal compatible with the receiving instrument been
called for in the specifications?
2. Is a local indicator provided?
8
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3. Is the sample point located so that during normal plant
flows the sum of the contact tank time and sample
transport time equals 30 minutes?
4. Is the sample pipe length tuned to provide the required
delivery time?
5. Will the sample point location be thoroughly mixed and
representative of the process stream?
6. Is the sample pump and pipe sized to provide the
recommended flow rates and velocities?
7. Has a sample valve been provided adjacent to the
analyzer?
8. Can the sample line be backflushed?
9. Is there adequate space around the analyzeL for
servicing the instrument?
G. Maintenance and Calibration
Task
1. Check reagent
supply.
2. Check analyzer
calibration.
3. Check sample flow
through analyzer.
4. Check reagent flow
to sample line.
5. Calibrate analyzer.
6. Replace tubing on
reagent pumps.
7. Backflush sample
line.
8. Clean analyzer
drain lines.
9. Clean cell
electrodes.
Frequency
Daily.
Daily.
Daily.
Daily.
When need is indicated by
calibration check.
Monthly.
Weekly.
Weekly.
Monthly.
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H. Deficiencies
The following problems have been encountered in existing
chlorine residual analyzer installations:
1. The sample point does not provide a representative mixed
sample. This may be due to poor mixing, sample point
location, or contact tank design.
2. Contact time from the point of chlorine addition to the
analyzer is too long.
3. Sample lines plug and cannot be backflushed.
4. No provision for taking a sample at the analyzer for
calibration checks.
5. Cramped space around the analyzer making maintenance
difficult.
6. No reagents in the analyzers.
7. Using reagents with the wrong concentration.
I. References
1. Liptak, B. G. and K. Venczel. Instrument Engineers
Handbook of Process Measurement. Chilton Book Company,
Radnor, Pennsylvania, 1969, Revised, 1982.
2. American Petroleum Institute. Manual'on Installation of
Refinery Instruments and Control Systems. Part II -
Process Stream Analyzers. API RP550, Washington, DC,
1977, 3rd Edition.
3. Kulin, G. Recommended Practice for Measuring Residual
Chlorine in Wastewater Treatment Plants with On-Line
Analyzers. U.S. Environmental Protection Agency,
Cincinnati, Ohio.
10
-------
1.2 DISSOLVED OXYGEN METERS
A. Applications
Generally, dissolved oxygen (DO) meters in wastewater
processes provide an approximate measurement of the oxygen
available to support biological activity; in receiving
waters, the DO meters monitor one parameter of water
quality.
TABLE 1.1. DISSOLVED OXYGEN METER APPLICATION GUIDELINES
Recommended Not Recommended
Aeration tanfc Chlorine contact tank
Oxygenation basins H2S bearing streams
Mixed liquor streams
Secondary effluent
Plant effluent
Sample systems
B. Principles of Operation
Commercial DO meters consist of an electrochemical cell
(called the probe), and a signal conditioner or
transmitter.
The two principal types of electrochemical cell used in DO
probes are:
1. Galvanic cell.
2. Polarographic cell.
Galvanic and polarographic cells have very similar
operating principles. Both cells consist of an
electrolyte and two electrodes as shown in Figure 1.5.
11
-------
THERMISTOR
TO SIGNAL
CONDITIONER/
TRANSMITTER
ANODE
0-RING
MEMBRANE
ELECTROLYTE
Figure 1.5. Membrane type DO probe cell.
One difference is that the polarographic cell requires a
polarizing voltage in the range of 0.5 to 1.0 vdc.
Another difference lies in selection of electrode
materials. For example, in one proprietary probe design,
the electrodes are emersed directly in the process stream
where process fluid acts as the electrolyte. However, in
most commercially available probes, the electrolyte is
contained by a gas permeable membrane, and oxygen is
brought into contact with the electrodes by the action of
diffusion. In either case, fluid flow past the probe
greater than 30 cm/s (1 ft/s), is generally required to
maintain a representative sample. Whether the probe is of
the membrane or non-membrane type, oxygen is reduced at
the cathode, where the half cell reduction reaction is
generally:
02 + 2H2O + 4 electrons-*4 OH"
and at the anode, the anode metal is oxidized. The result
of this reduction/oxidation process is a flow of electrons
from the cathode to the anode proportional to the oxygen
dissolved in the process stream. The rate of this
reduction/oxidation process is strongly affected by
temperature. Therefore, accurate temperature measurement
and compensation is essential to an accurate DO
measurement. Temperature is usually monitored by a
thermistor located in the probe, and compensation is made
in the signal conditioner/transmitter electronics.
1.2
12
-------
Suspended and dissolved substances in the process stream
can also affect electron flow. When solids accumulate on
the membrane, they reduce the rate of oxygen transfer to
the electrodes.
A mechanical grindstone continuously polishes the surface
of non-membrane probes to keep the electrodes clean. To
maintain gas permeability, fouled membrane probes must be
manually cleaned. Certain dissolved gases interfere with
DO measurement by either non-membrane or membrane probes.
Common gases to be avoided are chlorine, hydrogen sulfide,
carbon dioxide, and sulfur dioxide. Chlorine will be read
by the probe as oxygen; carbon dioxide can neutralize some
electrolytes; and hydrogen sulfide and sulfur dioxide can
poison some metals used for an anode.
C. Accuracy and Repeatability
Claims made by individual meter manufacturers for the
combined accuracy of the probe and signal
conditioner/transmitter vary from _+l-3% of full scale at
the calibration temperature.
Additional error of +1% can be expected for each 5.0° C
(9.0° F) of change from the calibration temperature.
Even with temperature compensation, an additional error of
3 to 4% can be expected over an operating range of 0° to
50°C (32°-122°F).
A combined DO measurement accuracy of +2-4% of full scale
can be achieved in most meters under the conditions
generally encountered at municipal wastewater treatment
plants. For a meter with a range of 0.0-10 mg/1 DO, this
DO measurement accuracy represents a measurement
uncertainty of 0.4-0.8 mg/1; however, the uncertainty
would double (0.8-1.6 mg/1) for a meter with a range of
0.0-20.0 mg/1.
The calibration of most probes will change after initial
installation, or reinstallation after probe repair.
Stabilization time, depending on the manufacturer, ranges
from a couple of hours to several days. Output readings
must be stabilized before the accuracies above apply.
D. Manufacturer's Options
1. Remote calibration unit can be installed near the
probe to permit calibration where the transmitter is
not within 50 ft or is not located within sight of
the probe.
13
-------
2. Ranges can be switch selectable; some of the more
common range selections are:
a. 0-3 mg/1, 0-15 rag/1.
b. 0-5 mg/1, 0-10 mg/1, 0-20 mg/1.
3. Transmitter Output Signals.
a. 4-20 mAdc into 650 ohms isolated.
b. 10-50 mAdc into 250 ohms isolated.
c. 0-5 vdc isolated.
4. Input power.
a. 115 vAC, 60 Hz.
b. 220 VAC, 60 Hz.
c. 24 VDC
5. Transmitter enclosure.
a. Panel mounted, NEMA IB - general purpose.
b. Surface mounted, NEMA 4 - watertight.
6. Probe mounting.
a. Handrail brackets.
b. Tank side wall brackets.
c. Probe holder available in lengths of 3-6 m
(10-20 ft).
7. Probe cable in lengths of 7.5-15 ra (25-50 ft).
8. Agitator or ultrasonic cleaner for use where fluid
velocity across the probe is less than 30 cm/s (1
ft/s).
9. High/low alarm outputs.
10. Junction box for terminating probe cable if the
transmitter or remote calibration station is more
than 10 feet from the probe connection head.
1.4
-------
E. Installation
1. In open tanks and channels.
In most cases, open tanks have a convenient guard
rail on which to mount the probe and transmitter (or
the junction box, where the transmitter is remote
from the probe). If the guard rail is not
conveniently located, bolt the probe holder brackets
to the free board area of the tank walls. The probe
holder must be rigidly supported, but it must also be
readily removable for probe maintenance (see Figure
1.6 for a typical configuration). Tilt the probe at
an angle away from the general component of process
flow to prevent air bubbles and debris from
accumulating on the membrane. Submerge probes 60-90
cm (2-3 ft) in an area having sufficient agitation,
and is representative of the process. Determine the
final probe location through testing during startup.
2. In closed oxygenation tanks.
Probe holders can be inserted in the process stream
through a flanged opening in the tank cover. The
holder must be rigidly fixed to the tank cover by a
flange or quick connect sample port cover that is
removable for probe maintenance. Also, a stilling.
well can be installed to provide a gas seal and a
lateral support for an extended probe length, probe
placement in the process fluid is the same as in open
tanks.
Mount the transmitter or junction box on a stand near
the probe.
3. On pipelines.
Do not subject probes mounted on pipelines to
pressures greater than the manufacturer's
recommendation, usually about 350 Pa (50 psi). The
selected probe must be installed to ensure equal
pressure on both sides of the membrane. Usually,
probes are mounted in a tee in the pipeline with
either a corporation seal on the probe or a bypass
line to allow removal of the probe for maintenance.
If the probe is part of a sample system, care must be
taken to have a short transport time from the process
to the probe. Where possible, direct measurement at
the point of interest is preferable to transporting a
sample for DO measurement.
15
-------
'SIGNAL
CONDITIONER
CD
HI/LO
ALARMS
ISOLATED
CURRENT •
OUTPUT
5-8M
•(15-23 FT)-
FLEXIBLE CONDUIT
NOTE
~^>r~n
-A1" L
i
, POWER
115 VAC 60H2
RAILING OR TANKWALL /
MOUNTING BRACKET/CLAMP -^
PROBE HOLDER-
NOTE
IF IT IS NOT POSSIBLE TO LOCATE THE SIGNAL
CONDITIONER NEAR THE PROBE, INSTALL A REMOTE
CALIBRATION STATION CLOSE TO THE PROBE.
INSTALL RIGID CONDUIT 8. EXTEND THE SIGNAL
BETWEEN THE CALIBRATION STATION 8 THE SIGNAL
CONDITIONER. DO NOT EXCEED 120 M (4OO FT).
DETAIL A
Figure 1.6. DO meter configuration.
16
-------
P. Designer Checklist.
Ask the following questions when designing or reviewing
dissolved oxygen meter applications. All checklist
questions should be answered "yes."
1. If a non-membrane probe is being considered, is
the process stream's conductivity greater than 100
micro - mhos? Is the conductivity stable?
2. Are interfering dissolved gases absent (chlorine,
hydrogen sulfide, carbon dioxide, and sulfur dioxide)?
3. Does the process fluid wash the probe at a rate
greater than 30 cra/s (1 ft/s)?
4. Does mounting eliminate the possibility of air bubbles
being trapped at the measuring surface?
5. Does the probe see a sample representative of the
process?
6. Is the probe mounted securely without causing likely
collection sites for debris?
7. Can the probe be removed easily for inspection or
maintenance?
8. Is installation of the meter designed so one person
can calibrate it?
9. Is the transmitter protected from the weather where
necessary?
10. Does the meter have automatic temperature compensation?
11. Is the probe installed so it is always immersed in the
process liquid?
G. Maintenance and Calibration
1. Membrane probe meters.
Task Frequency
a. Clean membrane. Depends on process stream
characteristics. Some
membranes must be cleaned
daily; but more typically,
every two days or once a
week. Membrane breakage is a
17
-------
common problem. To reduce
cleaning time, check
calibration with a portable
DO probe and clean membrane
only when the two meter
readings differ by more than
0.5 mg/1.
b. Replace membrane. Whenever membrane breaks or
when electrolyte is replaced.
c. Electrolyte Every three to six months.
replacement.
d. Calibration to Every other day.
portable probe.
e. Air calibration. After membrane cleaning.
f. Calibration to On initial installation or
standard. after major repair.
2. Non-membrane probe meters.
Task Frequency
a. Inspect Every two months.
and clean
grindstone.
b. Replace Every six months.
grindstone.
c. Calibrate to Weekly or bi-weekly are
standard. typical calibration intervals.
H. Deficiencies
The following problems are commonly reported for dissolved
oxygen meters.
1. Agitator or cleaner becomes fouled with hair or
fibers. Where 'possible, avoid use of agitators,
mechanical cleaners, probe guards, or shields.
2. The probe becomes fouled within a few hours due to
process stream characteristics such as grease or
slime growth.
L8
-------
3. The probe cannot be withdrawn from the process stream
because of mounting.
4. One person cannot calibrate the probe because of the
mounting bracket, difficult alignment, or awkward
mounting.
5. The probe cannot be calibrated by one person because
the transmitter is too far from the probe and no
remote calibration unit is installed.
6. The probe is placed in a "dead" area of the tank.
Poor mixing with the rest of the tank results in a
false signal that does not show the true state of the
process.
I. References
1. Liptak, B.G. and K. Venczel. Instrument Engineers
Handbook of Process Measurement. Chilton Book
Company, Radnor, Pennsylvania, 1969, Revised 1982.
2. APWA Research Foundation. Comparison of Field
Testing of DO Analyzers. Chicago, Illinois,
September, 1982.
3. Kulin, G. and W.W. Schuk. Evaluation of a Dissolved
Oxygen Field Test protocol. EPA 78-D-X0024-1, U.S.
Environmental protection Agency, Cincinnati, Ohio,
1978.
4. American Public Health Association, American Water
Works Association, and Water pollution Control
Federation. Standard Methods For the Examination of
Water and Wastewater; 15th Edition. American Public
Health Association, Washington, DC, 1980.
19
-------
1.3 pH
A. Applications
In wastewater treatment, pH sensors are used to monitor plant
conditions, to monitor biological treatment process
conditions, and to control acid/base additions for pH
adjustment.
Regulatory agencies require measurement of plant influent and
effluent pH to describe overall plant conditions. It may
also be necessary to monitor the pH of specific industrial
discharges to give advance warning of possible toxic
conditions.
While the activated sludge and most other biological
processes can tolerate a variance of pH 5 "to pH 9, some—such
as anerobic digestion—are very pH sensitive. Normally
monitoring of plant influent and primary effluent or MLSS (if
applicable) is sufficient to warn of impending toxic
conditions. The anerobic digestion process requires pH in
the range 6.6 to 7.6 and will fail at pH's below 6.2.
Because of this sensitivity/ it is important to monitor the
pH of anerobic digester liquor. However, because of sensor
fouling, continuous monitoring of digester pH is 'not
recommended.
PH adjustment is required for other processes where an
on-line pH monitor can be used. This provides control loop
feedback. For example, pH adjustment may be required to
neutralize very low pH industrial wastes, to enhance
phosphorous removal by alum addition or to adjust pH to
optimum ranges for nitrification/denitrification.
Table 1 summarizes recommended applications for continuous
monitoring with pH sensors.
TABLE 1.2. RECOMMENDED APPLICATION OF pH SENSORS.
Recommended Not Recommended
Plant influent Digesting sludge
Primary effluent 1)
MLSS (if applicable) 1)
Plant effluent
1) In activated sludge plants primary effluent pH is optional if
MLSS pH is available.
B. Principle of Operation
Commercial pH sensors all employ a glass membrane electrode
that develops an electrical potential varying with the pH of
the process fluid. A reference electrode is used to measure
the potential generated across the glass electrode.
20
-------
Figure 1.7 shows a typical pH sensor arrangement. The heart
of the sensor is the glass membrane. An electrical potential
varying with pH is generated across the membrane. This
potential is measured and amplified by an electronic signal
conditioner. The complete electric circuit includes the
glass electrode wire, the glass membrane, the process fluid,
the reference electrode fill solution and finally, the
reference electrode wire.
Figure 1.8 shows an equivalent electric circuit of the pH
sensor in Figure 1.7. Voltage at the input of the amplifier
is:
EI + Er - Eg - 0
and:
Ei - Eg - EC
Where:
EI = amplifier input, mv
Eg » glass electrode potential, mv
Er - reference electrode potential, mv
The glass electrode has the approximate characteristic:
Eg » Kl + K2(pH)
So voltage at the amplifier input is:
Ei - Kl + K2(pH) - Er
Kl * asymmetric potential, mv
K2 » electrode gain, mv/pH
The reference electrode is designed so its potential Ec is
constant with pH and other chemical characteristics of the
process fluid. The asymmetric potential Kl varies from
sensor to sensor. It also changes as the sensor ages. For
this reason pH sensors must be periodically standardized
against buffer solutions of known pH. Figure 1.9 illustrates
the effects of varying asymmetric potential.
The electrode gain K2 is a function of temperature. ' For this
reason most commercial pH sensors include automatic
temperature compensation. A temperature sensor in the
process fluid adjusts amplifier gain to compensate for
changes in electrode gain which are caused by temperature.
Figure 1.10 illustrates the effects of varying electrode
gain.
21
-------
PREAMPLIFIER
FILL SO-
LUTION
BUFFERED
TO PH OF 7
PROCESS
FLUID
GLASS
ELECTRODE
— SILVER WIRE-
W/SILVER
CHLORIDE
COA'ING
"GLASS
MEMBRANE
METER
REFERENCE
ELECTRODE
SATURATED POTTASSIUM
CHLORIDE SOLUTION
PROCESS
FLUID
POROUS
Figure 1.7. Typical pH sensor.
PREAMPLIFIER
E > K| 4- K2 (PH)
ER » CONSTANT
RQ » RESISTANCE OF GLASS ELECTRODE
RR• RESISTANCE OF REFERENCE ELECTRODE
RS « RESISTANCE OF PROCESS FLUID SOLUTION
Figure 1.8. Equivalent circuit,
22
-------
E,.MV OFFSET DUE TO
ASYMMETRIC a REFER-
ENCE POTENTIALS
DESIRED
RESPONSE
Figure 1.9. Effect of varying asymmetric potential at
constant (25oc) temperature.
RESPONSE AT lOO^C
I 1 1 1 1 1 1
RESPONSE AT
2S«C
RESPONSE AT
0«C
Figure 1.10. Effect of varying temperature at constant
asymmetric potential.
23
-------
C. Accuracy and Repeatability
Manufacturer claims for pH meter accuracy range from +_0.02 pH
units to +_0.2 pH units. This represents the combined
accuracy of the electrodes and the signal
conditioner/transmitter.
Most pH meters include automatic temperature compensation.
Temperature effects are negligible with these meters.
Without temperature compensation, an additional error of .002
pH per degree centigrade difference from the calibration
temperature can be expected.
The repeatability of pH meter measurements varies by
manufacturer from 0.02 pH units to 0.04 pH units.
Stability (drift) is an important performance parameter that
indicates how often meters must be recalibrated.
Manufacturer claims for stability vary from .002 pH units
drift per week to 0.2 pH units drift per week. Wit-li flow
through probe mounts, the velocity of the sample oan cause a
shift (0.2 to 0.3 pH) in measured values.
Methods of reporting performance specifications vary among
manufacturers. Adjustment of the method of reporting
performance specifications to equal units of measure will
show there is a large variance.in both the accuracy and the
stability claimed by different manufacturers. Generally, pH
meters can achieve the following performance standards in
wastewater treatment plants:
Accuracy: +_0.1 pH
Repeatability: ^0.03 pH
Stability: +0.02 pH/week
D. Manufacturer Options
Most manufacturers sell several pH probes and mounting
assembly configurations, each compatible with 3 or 4
transmitter/indicator/controller units commonly called pH
analyzers. Probe and analyzer options are listed below.
1. Probe options:
a. Mounting configurations for in-tank submersion,
insertion in-process pipes, or side stream
flow-through installation,
b. Ultrasonic cleaning,
c. Flow-powered cleaning,
d. Mechanical wiper cleaning, and
e. Double-junction reference electrodes.
24
-------
2. Analyzer options:
a. Analog or digital (LCD) indicator,
b. Signal outputs available:
0-16 MA isolated
0-20 MA isolated
4-20 MA isolated
0-1 VDC isolated
0-5 VDC,
c. Field selectable output spans from 2-14 pH in 2 pH
increments,
d. Alarm contacts; both dry contact and triac,
e. One manufacturer includes self-diagnosis for both
electrodes and signal conditioner. A failure alarm
contact output is enclosed,
f. Integrated process controller.
B. Installation
1. Where pH is one parameter of a sample system.
The best installation of a pH meter, where pH control is
not the objective of the measurements, is as part of a
sample system along with other on-line analytical
' instruments. This locates the pH meter with other high
maintenance instruments for ease of service. Buffer
solutions needed for standardization can be conveniently
stored with other analytical instrument reagents.
Use flow-through pH sensors in sample system
installations. Provide bypass and shutoff valves for
instrument removal and service. Select sensors with
electrodes that are easily removed from the flow-through
housing for cleaning and replacement. The flow-through
sensor should be designed so that electrode tips are
flush with the tube wall and do not obstruct flow.
Locate the pH analyzer near the probe mounting assembly
for easy standardization. Provide work surface for
setting containers of buffer solution during
standardization. Install a sample valve next to the
sensor to collect a sample for conformance checks.
Figure 1.11 shows sample system installation.
25
-------
3/4 CONDUIT
MOUNT SENSOR 8
TRANSMITTER ON WALL
OR SAMPLE PANEL
BALL
VALVE
SAMPLE
FLOW
Figure 1.11. Flow-through pH sensor installation.
^FLEXIBLE CABLE
TRANSMITTER
GUARDRAIL'
•JUNCTION BOX
CONDUIT SUPPORT
BRACKET
'PIPE CLAMP
^^1" PVC CONDUIT
-SENSOR W/
INTEGRAL
PREAMPLIFIER
Figure 1.12. Submersion pH sensor installation.
26
-------
2. Submersion in open tanks and channels.
For in-tank and open-channel installations, use a
submersion type electrode assembly with an integral
preamplifier. Figure 1.12 shows a typical
installation. The electrode assembly is attached to a
pvc pipe with a bracket normally mounted on a
guardrail. Design the bracket so that the pipe and
electrode assembly can be removed for maintenance
without the use of tools. Secure all fastening devices
to prevent dropping them in the tank or channel. Mount
the signal conditioner/transmitter next to the electrode
assembly mounting bracket. Provide enough spare cable
to allow the sensor/pipe assembly to be lifted clear of
the tank.
Install a submersion probe in a well mixed zone at a
point that will provide a representative sample of the
process. If the probe is installed in an open channel,
locate it in a free flowing zone. Design the electrode
assembly and support pipe installation to discourage
collection of debris.
Designer's Checklist
1. What are the process fluid temperature and pressure?
Can the selected probe and probe assembly handle the
expected range?
2. All are wetted parts of corrosion resistant material?
3. Can the electrode assembly and the electrodes be removed
easily for maintenance?
4. Is the measuring system installation designed to allow
maintenance and calibration by one person?
5. Are the electrodes exposed to a representative sample of
the process fluid?
6. Is the electrode assembly securely mounted?
7. Is there potential for debris to hang up on the
electrode assembly?
8. Is the process fluid likely to coat the electrodes?
Maintenance and Calibration
Task Frequency
1. Clean electrodes. Depends upon process fluid.
Once per month for plant
effluent. Once per week for
plant influent and sludges.
-------
Task Frequency
2. Add reference electrode Check weekly. Add as necessary.
fill fluid (Free - flowing type electrode)
3. Replace reference As dictated by operating
electrode experience.
(Non flowing gel type electrode)
4. Standardization. Check once per 'week after
initial installation. Reduce to
once per month if justified by
experience.
5. Transmitter calibration. Check once per 6 months.
H. Deficiencies
The following problems are commonly reported for pH meters:
1. Electrodes become coated with grease or sludge.
Mechanical wipers have demonstrated some success in
sewage treatment applications. Ultrasonic cleaners do
not work on soft coatings like grease, oil, and
sludges. The best solution to coating is periodic
cleaning by trained personnel.
2. Plugging of reference electrode. Switch to a double
junction reference electrode when plugging is a problem.
3. No provision for easy removal of probes for cleaning or
replacement. Probes should be removable without
shutting down process piping.
4. The pH meter can't be easily calibrated or standardized
because the transmitter/indicator is too far away from
the probe.
I. References
1. Liptak, B. G. and K. Venczel. Instrument Engineers
Handbook of Process Measurement. Chilton Book Company,
Radnor, Pennsylvania, 1969, Rev. 1982.
2. Consdidine, D. M. ed. Process Instruments and Controls
Handbook. McGraw Hill, New York, 1974.
/
3. Krigman, A. Guide to Selecting pH and ORP
Instrumentation. InTech, August, 1982, p. 31.
28
-------
1.4 SUSPENDED SOLIDS
A. Application
Suspended solids analyzers are used in wastewater treatment
plants to continuously measure the concentration of solids in
various process streams. Concentrations of interest range
from effluent quality, 10-30 mg/1, to thickened sludge of
several percent solids. A variety of instruments are
commercially available to accommodate this wide spectrum of
concentrations. This section of the handbook will review
light emitting and nuclear type solids analyzers.
TABLE 1.3. SUSPENDED SOLIDS APPLICATION GUIDELINES
Recommended
Optical Analyzers
Not Recommended
Solids concentrations from
20 mg/1 - 8%
Return activated sludge
Waste activated sludge
Mixed liquor
Plant effluent
Gravity thickened sludge
Centrifuge supernatant
Primary solids
Flotation thickened sludge
Solids concentration
greater than 8%
Nuclear Analyzers
Recommended
Thickened sludge with
concentrations greater than
8%
Centrifuged sludge
Not Recommended
Streams with solids
concentrations less than 15%
Streams with entrained air
bubbles
Line sizes larger than 35 cm
(14 in.)
Line size smaller than 15 cm
(5 in.)
29
-------
Ultra-sonic Analyzers
Recommended
(Note 1) Solids concentration from
1-8% solids
Primary sludge
Waste activated sludge
Return activated sludge
Gravity thickened sludge
Not Recommended
Mixed liquor
Secondary effluent
Plant effluent
Pipe sizes greater than 30 cm
(12 in.)
Pipe sizes less than 10 cm
(4 in.)
Notes:
1. Ultrasonic analyzers are not described in this section but they are
commercially available for measuring suspended solids^
B. Principle of Operation
Commonly used suspended solids instruments are based on the
attentuation or scattering of a beam of radiation. The type
of beam used can be light, ultrasound or nuclear. This
section will address those instruments which use light or
nuclear radiation.
1. Optical technique.
Optical techniques for measuring suspended solids are
based on scattering of a beam of light by the suspended
particles (see Figure 1.13). The portion of the light
scattered is a function of the number and size of
particles. Light transmitted through the stream is
reduced in proportion to the light that is scattered;
therefore, an instrument which can measure the scattered
light, transmitted light or both, provides a measure of
the suspended solids present.
The optical type of suspended solids analyzer consists
of a lamp which acts as a source of light and a
photocell which measures the transmitted or scattered
light (see Figure 1.14). Arrangement of the lamps and
photocells depends on the manufacturer. An electronic'.
package analyzes the received light and correlates this
to the suspended solids in the sampled stream.
30
-------
LENS
LIGHT
SOURCE
SCATTERED
4 LIGHT RAY
TRANSMITTED
LIGHT RAY
Figure 1.13. Light scattering.
LIGHT
SOURCE
SIGNAL
TO
TRANSMITTER
Figure 1.14. Transmissive type optical suspended solids analyzer.
31
-------
Since solids buildup and coating is a problem in
wastewater plants, manufacturers have devised several
different methods to minimize or eliminate the effects
of solids buildup. One technique uses a small sample
chamber in which a piston draws and dispels a sample.
The piston is designed with a flexible edge that
mechanically cleans the glass which separates the lamp
and photocell from the sample. Another design uses
multiple lamps and photocells. By measuring the
transmitted light at different angles to the lamp,
comparisons can be made to null the effect of solids
buildup. Still another design measures the light
reflected at an angle to a falling stream, because the
sample does not contact the lens, solids buildup does
not occur.
2. Nuclear radiation.
A nuclear density gauge is a non-contact measurement of
solids density. It does not measure percent solids
directly. But rather it measures the specific gravity
of the material. If the specific gravity of the liquid
and solids is constant, then a correlation can be made
between measured specific gravity and percent solids
concentration.
In operation, a radioactive source emits gamma rays
which are absorbed by material in the measured stream.
High density materials absorb more radiation than low
density materials. Thus a nuclear gauge averages the
density of all material in the stream. The detector
senses the total radiation passing through the stream to
determine the material density. Figure 1.15 shows a
solids analyzer.
3. Turbidity.
Turbidity can be classified as forward scatter or side
scatter measurement types. In forward scatter
turbidimeters, the measurement is in Jackson Turbidity
Units (JTO). The JTU unit was derived from the Jackson
candle turbidimeter shown in Figure 1.16. In this
instrument, the sample is poured into the glass tube
until the candle flame is seen to disappear, leaving a
uniform field of light. At this transition point, the
height of the column is read and converted into JTU's
from a standard table.
32
-------
LEAD SHIELDING
RADIATION SOURCE
MEASURED PRODUCT
PIPE CLAMP
RADIATION
DETECTOR
TO
TRANSMITTER
Figure 1.15. Nuclear solids analyzer.
33
-------
JJ M:
Figure 1.16. Jackson candle turbidimeter.
Figure 1.17 shows a forward scattering type
turbidimeter. This instrument measures the amount of
light scattered by particles in the forward direction
from the light beam. By establishing and maintaining a
ratio of scattered light to the transmitted light, the
effects of color changes can be eliminated and a direct
measurement made of the particulates.
LIGHT
SOURCE
Figure 1.17. Forward scatter turbidimeter.
34
-------
In side scatter turbidimeters, the turbidity is
determined by measuring the amount of light scattered at
some angle (usually 9QO) from the light path by
particles suspended in the sample. Figure 1.18
illustrates two styles of turbidimeters which use the
side scatter method of measurement.
LIGHT
SOURCE
LENS
PHOTOCELL
LIGHT SCATTER
LENS
PHOTOCELL
LIGHT
S°URCE
\
'
TURBID
SAMPLE
.
^ I / ',"
'
TURBID
METER
BODY
SAMPLE
OVERFLOW
TO DRAIN
SAMPLE
DRAIN
SAMPLE
INLET
SURFACE SCATTER
Figure 1.18. Side scatter turbidimeters.
3.S
-------
The units for side scatter turbidimeters are
Nephelometric Turbidity Units (NTU). The word
"nephelometric" describes the optical technique of
measuring scattered light at an angle to the light path.
Formazin polymer has gained acceptance as the turbidity
reference suspension standard. It is easy to prepare
and is reproducible in its light scattering properties.
Although a sample of formazin suspension measured by
forward scatter (JTU's) and side scatter (NTU's)
turbidimeters will read approximately the same, they are
not identical. A poor correlation exists when measuring
a wastewater sample because of the variation in the
absorption and optical scattering properties of the
suspended particles. Because of this, turbidity units
are not interchangeable between different types of
turbidity meters. JTU or NTU can be correlated to
suspended solids for a specific application.
C. Accuracy and Repeatability
1. Optical solids analyzers.
The accuracy of a suspended solids analyzer is typically
+5% of full scale. Several ranges of operation are
available. On a range of 0-3000 mg/1, the instrument
error is +_150 mg/1 of the actual reading. For a 0-10%
range the error would be _+0.5%.
The repeatability is not readily available from the
manufacturer's literature. However ^1% of full scale is
a reasonable estimate of the repeatability.
2. Nuclear solids analyzers.
Nuclear gauges offer accuracy of +_0.05% of full scale.
However, since the instrument measures specific gravity
and is empirically calibrated to read out in percent
solids the accuracy of solids measurement can be
affected by changes in the specific gravity of the
particulate or the fluid.
3. Turbidity analyzers.
Turbidity is a relative measurement. For this reason it
is inappropriate to apply conventional standards of
accuracy to this measurement. For purposes of this
discussion, consider turbidimeter accuracy, _+5% of full
scale and repeatability j^2% of full scale.
D. Manufacturer's Options
Some options are common to all types of solids analyzers.
These common options are listed first followed by options
which apply to a specific analyzer type.
36 .. _
-------
1. Common options.
a. Low and high alarm contact outputs.
b. Voltage or current output signals for remote
monitoring. This is a standard offering on some
analyzers.
c. Wall or panel mounting for the transmitter
enclosure.
d. Length of interconnecting cable between the sensor
and transmitter.
2. Optical analyzers.
a. Light shields to prevent stray light from
introducing measurement errors.
b. In-line pipe mounting adapters.
c. Mounting brackets for installing on hand rails.
d. Length or style (depends on manufacturer) of the
sensor probe.
e. Test standards for troubleshooting and calibration
of transmitter electronics.
3. Nuclear analyzers.
a. Mass flow computer (requires flowmeter input).
b. Pipe spool pieces with cleanout ports.
c. Radiation source decay compensation.
d. Automatic temperature compensation.
4. Turbidity analyzers.
a. Sample system accessories such as pumps and bubble
traps.
b. Installation Kits.
c. Extended high ranges.
d. Test standards for calibration.
37
-------
E. Installation
Installation details for solids analyzers are unique to each
manufacturer. The variations of installation are too
numerous to list here. Manufacturers installation manuals
should be obtained and used when designing for a solids
analyzer installation. Some general considerations for
installing solids analyzers follow:
1. Solids analyzers require frequent attention and
calibration checks. Provide space for servicing and
locate the sensor so it can be easily reached.
2. If sample lines are required, make sure they are large
enough and that flow velocity is high enough to minimize
line plugging.
3. Provide flushing water for the instrument and sample
valve.
4. Provide a sample valve next to the sensor so samples can
be taken to check analyzer calibration.
5. Mount the transmitter within sight of the sensor.
6. Locate sensors or sample line taps where air bubbles are
least likely to be present. Preferably a vertical line
with an up-»flow.
P. Designer's Checklist
The designer is referred to the manufacturer's installation
instructions or operation and maintenance manuals for details
of analyzer installation. Also review the general
installation suggestions contained in Part E of this
section. Caution: the performance of solids analyzers is
directly related to ease of maintenance and calibration.
G. Maintenance and Calibration
Refer to the manufacturer's operation and maintenance manuals
for specific reconunendations on frequency of tasks. The
following are general maintenance considerations for all
suspended solids and turbidity analyzers.
Task Frequency
1. Check analyzer Weekly.
calibration.
2. If sample line is used, Daily.
check sample and drain
flows.
33
-------
Task Frequency
3. .If sample line is used, Weekly.
backflush sample line.
4. Calibrate analyzer with When need is indicated by
a solution of known a conformance check.
solids concentration.
H. Deficiencies
Some problems encountered in existing solids analyzer
installations are described below.
1. The solids analyzer or the sample line tap is not
located where there is a well mixed representative
process sample.
a. Air bubbles in the sample.
2. No provision for taking a sample at the analyzer for
calibration checks.
3. Locating the sensor and/or transmitter so servicing and
maintenance are difficult due to inaccessibility.
4. Analyzer operating range does not match the range of
solids in the process.
5. For optical and turbidity analyzers, stray light causes
erroneous readings.
I. References
1. Standard Methods for the Examination of Water and
Wastewater. American Public Health Association,
American Water Works Association, Water Pollution
Control Federation, Washington, DC, 1975, 14th edition.
2. Condrashoff, G. Wastewater In-Line Turbidity and
Suspended Solids Measurements. Monitor Technology,
Inc., Redwood City, CA.
3. Simms, R. J. A Return to Accurate Turbidity
Measurement. Monitor Technology, Inc., Redwood City, CA.
4. Liptak, a. G. and K. Venczel. Instrument Engineers
Handbook of Process Measurement, Chilton Book Company,
Radnor, PA, 1969, Revised, 1982.
39
-------
2.0 FLOW MEASUREMENT, CLOSED CONDUIT LIQUID FLOW
2.1 MAGNETIC FLOW METERS
A. Applications
1. Operating conditions where magnetic flow meters (mag
meters) are suitable include:
a. Streams in which head losses must be minimized.
b. Liquids with a conductivity greater than 5 micro-mhos
per centimeter.
c. Corrosive and/or abrasive process streams.
d. Liquid streams with a solids concentration less than
10% by weight.
2. Mag meters are not recommended for the following
applications:
a. Non-conducting liquid process streams.
b. Gas streams.
c. Streams with powdered or granular dry chemicals.
d. Liquid streams with a solids concentration greater
than 10% by weight.
40
-------
TABLE 2.1. TYPICAL APPLICATIONS FOR MAG METERS IN WASTEWATER TREATMENT
Service
Liner Material
Raw Sewage
Settled Sewage
Primary Sludge
Mixed Liquor
Return Activated
Sludge
Waste Activated
Sludge
Thickened Sludge
Digester Sludge
Digester Supernatant
Polymer Solutions
Polyurethane
Clean (Process)
Water
Strongly Corrosive
Polyurethane
Polyurethane
Polyurethane
or Teflon
Polyurethane
Polyurethane
Polyurethane
Polyurethane or Teflon
Polyurethane or Teflon
Polyurethane or Teflon
Teflon, Rubber,
Rubber
Polyurethane, Rubber
Teflon or Kynar
Gasket Material
Asbestos, Rubber,
Neoprene
Asbestos, Rubber,
Neoprene
Teflon/Asbestos
Asbestos, Rubber,
Neoprene
Asbestos, Rubber,
Neoprene
Asbestos, Rubber,
Neoprene
Teflon/Asbestos
Teflon/Asbestos
Teflon/Asbestos
Teflon/Asbestos,
Rubber, Neoprene
Teflon/Asbestos
41
-------
TABLE 2.2. LINER SELECTION CRITERIA FOR SPECIFIC CONDITIONS
Resistance to Abrasion
Liner Material
Teflon
Kynar
Polyurethane
Butyl rubber
Neoprene
(Mild)
(Severe)
Good Not Recom.
Good Not Recom.
Excellent Excellent
Excellent Good
Excellent Good
Resistance to
Corrosion
Excellent
Excellent
Not Recommended
Not Recommended
Not Recommende^
Maximum
Temperature
150°C (300°F)
100°C (212°F)
88°C (190°F)
71°C (160°F)
93°C (200°F)
4. Cost considerations regarding liner materials.
If a base price is assumed for a polyurethane liner, the
following costs may be used for comparison:
a. Rubber or neoprene costs will be approximately
$5.90/cm ($15.00/in) of meter diameter, greater
than an equivalent meter with a polyurethane liner.
b., Teflon costs will be approximately $87.00/cm
($220.00/in) of meter diameter, greater than an
equivalent meter with a polyurethane liner.
B. Principle of Operation
Magnetic flow meters (mag meters) operate by using Faraday's
principle of electro-magnetic induction in which the induced
voltage generated by an electrical' conductor moving through a
magnetic field is proportional to the conductor's velocity.
Figures 2.1 and 2.2 illustrate the application of this
principle to volumetric flow rate measurements of wastewater
treatment process streams.
Commercial power is applied to the meter, and the coil driver
energizes the magnetic coils which encase the spool pipe,
creating a magnetic field. If the process liquid has enough
conductivity, it will act as an electrical conductor and will
induce an electrical voltage. This voltage io a summation of
all the incremental voltages developed within each liquid
particle occupying the magnetic field and is proportional to
the field strength, pipe diameter, and "conductor velocity."
The more rapid the rate of liquid flow, the greater the
instantaneous value of electrode voltage.
42
-------
The induced voltage is received by the two electrodes mounted
180° apart in the meter. This signal is sent to the
converter/transmitter where it is summed, referenced, and
converted from a magnetically induced voltage to the
appropriate scaled output. The output signal then goes to any
of the appropriate operating interfaces, e.g., a control panel
indicating meter, a control system computer, etc.
Two basic types of mag meters are available, the AC mag meter
and the dc mag meter. With the AC mag, line voltage is applied
to the coils and a continuous flux is created producing a
continuous low level AC electrode voltage. With the dc mag
meter, the magnetic coils are periodically energized, thereby
producing two induced electrode voltages — one when energized,
the other when de-energized. The energized electrode voltage
is a combination of both true signal and noise, while the
de-energized electrode voltage represents only noise. The
difference between the two voltages is measured yielding a
'clean" signal. Because of this operating scheme, the pulsed
dc mag meters are zeroed every cycle whereas AC meters require
stopping the flow for periodic re-zeroing.
POWER
SOURCE
I20VAC
PROCESS
LIQUID
FLOW
METER OUTPUT SIGNAL
CONVERTER/TRANSMITTER
LINER MATERIAL
FLOW TUBE
ELECTRODE
MAGNETIC COILS
Figure 2.1, Magnetic flow meter construction.
-------
TURBULENT
VELOCITY
FLOW
PROFILE
MAGNET
COIL
OUTPUT VOLTAGE = E0 = KBDV x 10 "4 VOLTS
WHERE K = COEFFICIENT TO ACCOUNT FOR NON-IDEALITY
B = MAGNETIC FLUX DENSITY, TELSA
D = PIPE DIA. (DISTANCE BETWEEN ELECTRODES), METERS
V = AREA - AVERAGE VELOCITY OF FLOW, M/S
Figure 2.2. Magmeter induced voltage.
C. Accuracy and Repeatability
The accuracy of a magnetic flow meter should be within £1.0% of
full scale and should not exceed £3.0% of indicated flow when
operating in the lower one-third of the meter range.
The repeatability should be within £0.5% of full scale.
The above accuracies reflect the expected performance under
typical field conditions. These meters are capable of
improved performance under ideal conditions.
These requirements can be met by the inherent characteristics
of present-day mag meter design; however, several circumstances
will degrade these levels of operation, including:
1. Plow conditions.
Flow-disturbing piping obstructions located too near the
meter inlet and outlet may add an additional 1 - 10% of
uncertainty to the measured flow. Avoid locating the
following obstructions nearer than five pipe diameters to
the meter inlet or outlet:
a. Valves,
b. Gates,
-------
c. Tees,
d. Elbows,
e. Pumps, and
f. Severe reducers and expanders ( 30° included angle).
Refer to Part E, Installation, for detail.
2. Meter orientation.
Meter orientation leading to a non-full meter pipe (i.e.,
trapped gases) or resulting in material buildup on the
electrodes severely degrades accuracy. Refer to Part E,
Installation, for detail.
D. Manufacturer's Options
1. Electrodes. .
a. Shape (see Figure 2.3),
WASHER
NUTS'
•INSULATING BUSHING
FLUSH ELECTRODE
(STANDARD)
vV-LINER
ELECTRODE
INSULATOR
—PIPE TUBE WALL-*
INSULATING WASHER
LOADING SPRING
TERMINAL
INSULATING BUSHING
BULLET-NOSED ELECTRODE
(PROTRUDING)
Figure 2.3. Mag meter electrode shape options.
45
-------
b. Materials:
1) 316 Stainless steel,
2) Platinum/iridium,
3) Tantalum,
4) Hastelloy, and
5) Nickel.
c. Self-cleaning .
1) High frequency ultrasonic (continuous or
portable).
2) Heat.
d. Field replaceable.
1) Available with self-sealing liners only, e.g.-,
neoprene or rubber (not available for
Teflon-lined meters).
2. Liner material.
a. The corrosive and/or abrasive characteristics of the
process liquid dictate proper selection of the liner
material and electrode construction (see Tables 2.1
and 2.2).
i
3, Mag/flow converter.
a. Auto zero calibration.
b. Output signal: 4-20 mAdc, digital pulse, scaled
digital pulse.
c. Face-mounted indicating meter.
4. Grounding rings, straps, probes.
5. Environment.
a. Corrosive resistant epoxy paint.
b. Protected from accidental or continuous submergence,
NEMA 6 - submersible, watertight.
46
-------
E. Installation
1. Locate the meter on the discharge side of pumps and on the
upstream side of throttling valves.
2. Locate the meter in a straight run of pipe free of valves
or fittings with a minimum of five diameters upstream and
downstream length.
3. The process conduit must flow full of liquid.
4. Meter sizing is critical. Size the meter to provide a
fluid velocity within the following ranges:
a. Non solids-bearing liquids: 1-9 m/s (3-30 ft/s).
b. Solids-bearing liquids: 1.0 - 7.5 m/s (3.0 - 25.0 ft/s
c. Abrasive solids-bearing liquids: 1.0 - 2.0 m/s
3.0 to 6.0 ft/s
Appropriate reducers/expanders may be required to achieve
recommended operating velocities.
Use the flow that will exist at startup for meter sizing.
DO NOT USE 20 YEAR FLOW ESTIMATES FOR METER SIZING.
5, The meter must have self-cleaning electrodes, ultrasonic
or heated, for all applications except where process water
is equivalent to or better than secondary effluent quality.
6. Install the mag meter so it can be taken out of service
for calibration and/or maintenance without disrupting the
associated process. Recommended isolation and bypass
piping configurations are shown in Figure 2.4.
47
-------
5 OIA. WIN.
A
MAG METER
5 DIA. MIN.
NORMAL
WITH CLEAN OUT TEE
Figure 2.4. Bypass pipe installation.
When the meter is to measure process liquids containing
solids, e.g., primary sludge, RAS, WAS, thickened sludge,
or when continuous electrode cleaning is not used, install
a cleanout tee as ""-own in Figure 2.4.
The decision to install bypass piping is a value judgment
involving consideration of many factors including:
a. Pipe size,
b. Available space, and
c. The ability to shut down the line while maintaining
process operation or shifting to a parallel process
unit.
48
-------
7. Properly ground all mag meters using stainless steel
grounding rings and grounding straps supplied by the meter
manufacturer. The grounding rings should have an inside
diameter one cm (1/4 in) less than that of the meter (for
meters 10 cm 4 in in diameter or larger). Place them on
both flanges with grounding straps as shown in Figure 2.5.
Pn f
i "*' ~i r-
GROUNDING RING
GROUNDING STRAPS
GROUNDING STRAP
TO EARTH GROUND
Figure 2.5. Mag meter grounding.
Always ensure that the plant electrical system ground near
the meter location provides adequate grounding. If a
plant-wide grounding grid is available, ground the meter
to it.
8. Additional installation information.
a. Avoid locating mag meters near heavy induction
equipment because it causes meter operational
problems (100 HP motors and larger, no closer than 20
feet).
b. Provide sufficient space to facilitate calibration,
in-line maintenance, or meter removal.
c. Orient the meter so the electrodes lie in a plane
parallel to the floor.
d. Wall-mount the transmitter/converter within sight of
the meter in a NEMA 4 enclosure (NEMA 6 if possible
submergence), or flush panel mounted so the cable
length from the meter does not exceed 60m (200 ft).
-------
e. Use driven-shield signal leads and route them between
the transmitter and meter through dedicated 2 cm
(3/4 in) conduit. Route power wiring in separate
conduit.
f. Torque flange connections to the manufacturer's
installation specifications.
g. Wire power to the transmitter/converter and the coil
driver through the same dedicated circuit. If
separate circuits are used for transmitter/converter
power and coil driver power, both circuits must
originate from the same phase of the primary power
feed.
h. Mount the meter in a vertical pipe run with the flow
direction upward. Install air bleed valves for
meters mounted horizontally.
i. Metered lines should not self-drain when shut down.
j. Provide for flushing and filling with clean water in
sludge applications where intermittent operation is
expected.
P. Designer Checklist
Use the following checklist when designing or reviewing mag
meter applications. Verify affirmative all checklist items for
proper application and installation.
1. Does the process liquid to be measured have a measured
conductivity greater than 5 micro-mhos per centimeter?
2. Will the pipe flow full under ALL conditions, excluding
shutdown?
3. If intermittent flow is expected, will the meter remain
full at no flow? Does the transmitter have low flow zero
cut-out circuitry?
4. Does the meter size ensure a flow velocity between 1.5-7.5
m/s (5-25 ft/s) for solids-bearing liquids or 1-9 m/s
(3-30 ft/s), for non solids-bearing liquids?
5. Has the proper liner material been selected for the
particular application?
6, DO the electrodes require and have continuous cleaning
'capabilities?
50
-------
7. Are all piping elements/obstructions located at a minimum
distance of five pipe diameters upstream or downstream of
the meter?
8. Have grounding rings and straps been provided, and is the
meter grounded to a true ground?
9. Have bypass piping and valving been provided?
10. Is signal wiring between the transmitter and meter as
specified by the meter manufacturer and routed in separate
conduit?
11. Has a dedicated power source been provided for the mag
meter?
12. Has the proper electrode material been"selected to avoid
excessive wear?
13. Is the selected liner material compatible with the
expected operating temperature?
14. Does the design provide environmental temperatures within
the range specified by the manufacturer for both the meter
tube and the transmitter/converter?
G. Acceptance and Performance Monitoring
Include provisions for acceptance testing and performance
monitoring as described in Section 7,2, relating to:
a. Hydraulic flow testing,
b. Electrical ground testing, and
c. Verification of manufacturer accuracy and factory
calibration documentation.
H. Maintenance and Calibration
Task Frequency
1. Calibrate transmitter. Once each month.
2. Plow calibrate Every three months.
the meter.
I. Deficiencies
The following problems are commonly encountered with existing
mag meter installations.
51
-------
1. Velocity skewing created by piping obstructions located
too near to the meter cause accuracy problems and liner
wear. In severe cases the obstructions cause the liner to
be ripped away.
2. Meter sizing does not maintain adequate flow velocities.
Often this results from over design for "future" flow
rates.
3. Improper installation results in non-full pipe during low
flows.
4. Solids coating the electrodes due to lack of automatic
electrode cleaning cause low flow velocity and/or
intermittent flow.
5. Meter and/or transmitter located so that calibration and
maintenance accessibility are difficult.
6. Isolation and bypass piping is not installed, requiring
shutting down the process for meter zeroing and meter
removal (when required).
7. Infrequent calibration.
8, No provisions are made for meter calibration.
9. Improper grounding.
J. References
1. Liptak, B.G., and K. Venczel. Instrument Engineers
Handbook of Process Measurement. Chilton Book Company,
Radnor, Pennsylvania, 1969, Revised, 1982.
2. Kulin, G. Recommended Practice For The Use Of
Electromagnetic Plow Meters In Wastewater Treatment
Plants. EPA 600/2-84-187 U.S. Environmental Protection
Agency, Cincinnati, Ohio, November, 1984.
3. Fisher & Porter Company. Instruction Bulletin No.
10D1435A Warminster, Pennsylvania, 1969, Revision 1.
4. Sybron/Taylor Corp. Magnetic Flow Meter - Basic Theory.
Product Data, PDS-15E001 Issue 3. Rochester, New York.
5. Sybron/Taylor Corp. Magnetic Flow Meter - Application.
Product Data, PDS-15E002 Issue 3, Rochester, New York.
6, Sybron/Taylor Corp. Magnetic Flow Meter - Installation.
Product Data, PDS-15E003 Issue 2. Rochester, New York.
52
-------
2.2 SONIC FLOW METERS
A. Applications
Sonic flow meters are available in two basic types; the
transmissive (through beam) type and the reflective
(frequency-shift) or Doppler type.
1, Transmissive type.
Application of transmissive type (through-*beam) sonic
flow meters are indicated where the following
conditions exist:
a. Head losses must be minimized.
b. Process pipe flows full.
c. The amount of suspended solids and entrained air
bubbles in the process liquid together are
"equivalent" to no greater than 3% suspended
solids by weight.
d. Process liquid temperatures range between
00-800 c (320-180° F).
e. Line size is small enough so the sonic signal
attentuation does not cause a problem. Consult
meter manufacturer about applications in lines
larger than 100 cm (42 in.).
TABLE 2.3. TRANSMISSIVE SONIC FLOW METER APPLICATION GUIDELINES
Recommended Not Recommended
Primary effluent Raw sewage
Mixed liquor Primary sludge
Secondary clarifier effluent Thickened sludge
Plant final effluent Nitrification RAS
Process (wash) water Nitrification WAS
Return activated sludge (RAS)
Waste activated sludge (WAS)
53
-------
2. Reflective type.
Conditions for suitable applications of reflective
type (Doppler) sonic flow meters are as follows:
a. Head loss must be minimized.
b. Process pipe flows full.
c. The amount of solids ind the entrained air
bubbles in the process liquid must be equivalent
to a suspended solids concentration greater than
2% but less than 4% by weight.
d. Flow velocities at the transducer must be
maintained between 1-9 m/s (3-30 ft/s).
e. Pipe wall thickness mast be less than 5 cm (2
in) thick.
f. The pipe is not constructed of, or lined with,
an aggregate material.
g. The thickness of the pipe wall is exactly known.
TABLE 2.4. REFLECTIVE SONIC FLOW METER APPLICATION GUIDELINES
Recommended Not Recommended
Raw sewage Secondary clarifier effluent
Primary sludge Plant final effluent
Thickened sludge Process (wash) water
3. Varying stream conditions.
A note of caution for the "recommended/not
recommended" process applications: Determine the
range of conditions under wh-' ;h a sonic flow meter
will have to operate; fluctuating flow conditions
may cause intermittent operation of the meter.
54
-------
B. Principle of Operation
1. Transmissive sonic meters.
The transmissive sonic flow meter (also called
through beam or time-of-travel meter) measures fluid
velocity by measuring the difference in the time
required for a sonic pulse to travel a specific
distance through the fluid in the same general
direction as fluid flow, and the time required for a
sonic pulse to travel the same distance in the
opposite direction. This meter is available in two
types: (1) a pipe section with integral well-mounted
transducers and (2) a direct-mounted version with the
transducers mounted externally to an existing pipe.
Both types use the same operating principle; Figure
2.6 shows the pipe section type.
NOTES'
METER BODY RECOMMENDED
TO BE MOUNTED IN
VERTICAL PIPE.
TRANSMITTER RECOMMENDED \ 1
TO BE MOUNTED ON THE ^
METER BODY.
WIRING:
-FLEXIBLE CONDUIT TOR
TRANSDUCER/TRANSMITTER
WIRING.
-RIGID CONDUIT (2) FOR
POWER 8 SIGNAL.
PROCESS PIPING
METER FLANGE
AVERAGE FLUID VELOCITY = 2COS9
AF= DOWNSTREAM FREQUENCY
UPSTREAM FREQUENCY
D = DISTANCE BETWEEN
TRANSDUCERS
TRANSDUCER HOUSING
(TRANSMITTER/RECEIVER)
FLOW
Figure 2.6. Transmissive sonic flow meter.
Sonic transducers are energized alternately by
electrical pulses and emit sonic pulses across the
flow. The pulse whose directional component is
downstream traverses the pipe in a shorter time than
the pulse traveling against the flow (upstream).
This time difference is proportional to the flow
velocity, and an output signal linearly proportional
to the flow rate is computed in the meter
transmitter.
55
-------
2. Reflective (Dopplet) sonic meters.
Figure 2.7 illustrates that operation of the
reflective, or Doppler, sonic flow meter and is based
on a principle different from the transmissive type.
The single transducer used is mounted on the external
wall of the pipe. A signal of known frequency is
sent into the fluid where it is reflected back to the
transducer by suspended particulates and/or gas
bubbles. Because the reflective matter is moving
with the process stream, the frequency of the sonic
energy waves is shifted as it is reflected. The
magnitude of the frequency shift is proportional to
the particle (flow) velocity and is converted
electronically to the meter output signal linear to
flow.
FREQUENCY CHANGE
= FLUID VELOCITY
Figure 2.7, Reflective sonic flow meter.
C. Accuracy and Repeatability
The accuracy and repeatability of sonic flow meters vary
between the two types; the transmissive type provides a
more accurate flow rate signal than does the reflective
type. The following limits should be required when
considering either type for any of the applications
previously listed:
56
-------
Transmissive: accuracy, .+2% of actual flow
(through beam) repeatability, _+!% of actual
flow
Reflective: accuracy, _+5% of actual flow
(Doppler type) repeatability, jKL% of actual
flow
Purchase of sonic flow meters which meet these accuracy
limits does not assure that the limits will be met in
operation. Several factors can degrade accuracy and must
be adequately addressed during the design phase. These
factors are:
1. Flow conditions.
Flow-disturbing piping obstructions located too near
the meter inlet may add up to 10% of error to the
measured flow rate. The following obstructions
should not be located nearer than 7 to 10 pipe
diameters from the meter inlet, or 5 pipe diameters
from the outlet (on flow-tube, transmissive meters)
or within these distances from the external
transducer of the Doppler type meter.
a. Valves (modulating and isolating).
b. Gates.
c. Elbows and tee's.
d. Pumps.
e. Severe reducers and expanders (> 30 degrees
included angle).
Also, skewing of the velocity profile will result if
the recommended straight lengths of pipe are not
provided upstream and downstream of the meter.
Skewing will cause errors in the flow measurement.
Refer to Part E, Installation, for additional details.
2. Meter orientation.
Meter orientation leading to a non-full pipe or
resulting in material buildup or deposition will
severely degrade meter accuracy. Refer to Part E,
Installation, for detail.
57
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D. Manufactured Options
1. Transmissive (through beam) - only.
a. Meter tube construction:
1) Stainless steel.
2) Carbon steel.
b. Meter tube end connections:
1) 150 Ib ANSI RF flange.
2) 300 Ib ANSI RF flange.
3) Victaulic.
•
4) Plain.
c. Transducer mounting:
1) Wetted, with flush water port.
2) Wetted, with epoxy window (Teflon-optional)
3) Wetted, removable without process
disruption,
2. Reflective (Doppler) - only:
a. Transducer mounting:
1) External, clamp-on.
2) Wetted, with flow tube.
3, Common options:
a. Input power:
1) 115 VAC, 50-60 Hz.
2) 220 VAC, 50-60 Hz.
3) 24 VDC.
b. Transmitter:
'58
-------
1) Outputs:
a) 4-20 mAdc.
b) 0-10 VDC.
c) Pulse rate.
2} Integral flow rate indicator/totalizer.
3) Adjustable relay contact alarm outputs.
c. Environment:
1) Temperature: -2QO to 60° c (-4° to
1400 F).
E. Installation
1. Install transmissive sonic flow meters having wetted
transducers so the meters can be taken out of service
for calibration and/or maintenance without disrupting
the associated process. Recommended bypass
configurations are the same as those recommended for
magnetic flow meters and are shown in Figure 2.4.
The decision to -install bypass piping involves such
considerations as pipe.size, available space, and the
i ability to shut down the line while maintaining
process operation or shifting to a parallel process
unit.
2. Locate meters on the discharge side of pumps and on
the upstream side of throttling valves if these
devices are near the required meter location.
3. Flow velocities through the meter should be
maintained between 1-9 m/s (3-30 ft/s).
Appropriate reducers/expanders may be required to
achieve recommended operating velocities.
Use the flow that will exist at start-up for meter
Sizing. DO NOT USE 20 YEAR FLOW ESTIMATES FOR METER
SIZING.
4. Provide straight runs of pipe upstream and downstream
of the meter as described in Part C, Accuracy and
Repeatability.
59
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5. Orient spool-piece type meters in, or locate clamp-on
type meters on/ vertical process piping where
possible, with flow direction upward only. Install
air bleed valves when horizontally mounted.
6, Metered lines should not self-drain when shut down.
7. Locate the meter in an accessible location with
sufficient space for calibration, in-line
maintenance, or meter removal. Install the
ttansmitter as close as possible (3.7 m, 12 ft) to
the clamp-on transducer. When a meter tube is used,
mount the transmitter directly on the tube.
8, Install clamp-on transducers according to the
manufacturer's suggested procedures. Be sure that no
air babbles are present in the epoxy sealant compound.
9. Use separate conduit to wire line power and signal
wiring.
10. Follow precisely the manufacturer's guidelines for
aligning sonic transducers to the pipe.
11. Install the transmissive meter as a spool piece for
pipes ranging in size from 91 - 7.6 cm (36 - 3 in).
For pipes smaller than this, mount the transducers in
an axial configuration as shown in Figure 2.8.
METER
FLANGE
PROCESS
PIPE
/
FLOW
jUDO CnDC
Y1
\
CONDUIT
CONNECTION
I—I 1
a
5
\
TRANSDUCER
HOUSING
NOTE:
L METER TUBE SHALL ALWAYS BE
MOUNTED IN HORIZONTAL PROCESS PIPE.
Figure 2.8. Transmissive sonic meter, axial configuration.
60
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F. Designer Checklist
Use the following checklist when designing or reviewing
sonic flow meter applications. All checklist items should
be answered yes for proper application and installation.
1. Common items.
a. Will the pipe flow full under all conditions?
b. If intermittent flow is expected, will the pipe
remain full at no-flow conditions?
c. Are all piping elements/obstructions located a
minimum distance of 7-10 pipe diameters upstream
and 5 diameters downstream?
d. Are the meter and transmitter easily accessible?
e. Will adequate flow velocities be realized, 1-9
m/s (3 - 30 ft/s)7
f. Is the meter located correctly in relation to
pumps and throttling valves?
g. Does the design provide environmental
temperatures within the manufacturer's specified
' range?
h. Has a sample of the process fluid been tested
for sonic transmittance?
2. Transmissive (through beam) items.
. a. Have the proper spool piece material and end
connections been provided for?
b. Is the transmitter mounted on the spool piece?
c. Is the process liquid recommended in Table 2.3?
Is the amount of air bubbles and solids less
than 3% by volume?
d. Is the process liquid temperature between
00-80° C (30-180° F)?
61
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3. Reflective (Doppler) items.
a. Is the process liquid recommended in Table 2.4?
Does it have a solids and/or air bubble content
greater than 2% but less than 4%?
b. Is the clamp-on transducer located where no
excessive pipe and/or liquid-transmitted
vibration will occur?
c. Is the pipe inside diameter and wall thickness
known accurately?
H. Acceptance and Performance Monitoring
Provide for acceptance testing and performance monitoring
as described in Section 7.1, relating to:
1. Hydraulic flow testing.
2. Verification of manufacturer accuracy and factory
calibration documentation (reflective type meters are
rarely factory calibrated).
I. Maintenance and Calibration
The recommended calibration interval for sonic flow meters
is every two months.
J. Deficiencies
The following problems are commonly encountered with
existing sonic flow meter installations:
1. Piping obstructions located too near to meter causing
accuracy problems.
2. Meter sizing is such that adequate flow velocities
are not maintained. Many times this results from
over design for "future" flow rates,
3. Installation resulting in non-full pipe during low
flows.
4. Solids coating due to low flow velocity and/or
intermittent flow.
5. Meter and/or transmitter located so that calibration
and maintenance accessibility is difficult.
62
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6. Infrequent calibration.
7. No provisions for flow rate testing/calibration.
8. Solids concentration and/or entrained air greater
than acceptable for transmissive type, resulting in
poor accuracy or an unacceptable signal.
9. Solids concentration and/or entrained air less than
required for reflective type, resulting in poor
accuracy or an unacceptable signal.
10. Grease and scum buildup on pipe walls and wetted
transducers,
K. References
1. Liptak, B, G., and K. Venczel. Instrument Engineers
Handbook of Process Measurement, Chilton Book
Company, Radnor, Pennsylvania, 1969, Revised, 1982.
2. Brown, A. E. Application of Flowmeters to Water
Management Systems, Presented to Instrument Society
of America, ISA/81 Conference, Anaheim, CA, (October,
1981).
3. Powell, D. J. Ultrasonic Flowmeters, Basic Design,
Operation and Criteria Application. Plant
Engineering, May, 1979.
4. Hall, J. Choosing a Flow Monitoring Device.
Instruments and Control Systems, June, 1981.
6-3
-------
2.3 TURBINE FLOW METERS
A. Application
The following general conditions provide suitable
applications for turbine flow meters:
1, The typical head loss through a turbine meter of
21-35 kPa (3-5 psi) can be tolerated.
2. The process piping is full under flowing conditions.
3. The process liquid is relatively 'clear," i.e., a
solids concentration less than 0.1% by weight (1000
mg/1) and is free of fiberous materials and/or debris.
4. A maximum meter rangeability of 10:1 is acceptable.
5, An intermittent flow may be expected.
TABLE 2.5. TURBINE PLOW METER APPLICATION GUIDELINES
Recommended Not Recommended
Plant final effluent Raw sewage
Secondary clarifier effluent Primary sludge
Process (wash) water Secondary sludge (RAS & WAS)
Steam condensate Mixed liquor
Primary effluent
Chemical slurries
B. Meter Sizing
Due to the nature of their linear-to-flow relationship,
turbine meters must be properly sized by volumetric flow
rate. A meter sized to a specified range of linear flow rate
measurement should not be used for flow rates outside that
range.
)
Follow these guidelines when sizing a turbine meter:
1. The flow meter should be sized for 120-130% of the
maximum expected process flow rate.
64
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2. If the meter is sized by volumetric flow rate (guideline
No. 1), it will have a diameter smaller than the process
pipe. See Figure 2.10 for reducer/straight pipe
installation.
3, If the meter size is the same diameter as the process
pipe, its range will be severely reduced (to 2:1 or
3:1); however, the head loss through the meter will be
less than if volumetrically sized.
4. Liquid cavitation may occur in the meter if upstream
line pressure is not sufficient. To ensure sufficient
pressure, the downstream line pressure must be a minimum
of 2 times the meter head loss plus 1..25 times the
liquid vapor pressure. If this condition cannot be met,
a larger size meter with a correspondingly reduced meter
range is required.
5, Available turbine meter sizes range from 0.5-60.0 cm
(3/16-24 in.) in diameter.
C. Principle of Operation
Turbine flow meters consist of a pipe section with a
multi-bladed impeller suspended in the fluid stream on a free
running bearing (see Figure 2.9). The direction of rotation
of the impeller is perpendicular to the flow direction, and
the impeller blades sweep out nearly the full bore of the
pipe. The impeller is driven by the process liquid impinging
on the blades. Within the linear flow range of the meter,
the impeller's angular velocity is directly proportional to
the liquid velocity which is, in turn, proportional to the
volumetric flow rate. The speed of rotation is monitored by
an electromagnetic pickup coil which operates either on a
reluctance or inductance principle to produce a pulse. The
output signal is a continuous voltage pulse train with each
pulse representing a discrete volume of liquid. Associated
electronics units then convert and .display volumetric flow
(flow rate) and/or total accumulated flow.
D. Accuracy and Repeatability
The accuracy and repeatability characteristics of turbine
flow meters, when properly applied and installed, should be:
Accuracy: +0.25% of actual flow, within
the linear range of the meter.
Repeatability: ^0.05% of actual flow, within
the linear range of the meter.
-------
PICKUP COIL
ROTOR
UNIT
UPSTREAM ASSEMBLY
HANGER UNIT
HOUSING
DOWNSTREAM
HANGER
UNIT
SPINDLE
Figure 2.9. Turbine flow meter.
Each turbine flow meter has a unique "K" factor (the number
of pulses per unit volume) which is determined during factory
calibration. This factor is adversely affected by two
conditions:
1. The liquid viscosity is significantly greater than that
of clean water. This condition should not occur in
wastewater treatment facility applications recommended
in Table 2.5.
2. The moving components become impaired by buildup of
solids and/or fiberous materials.
E. Manufactured Options
Turbine flow meter options available to the buyer are limited
due to meter design standardization. Typically, options are
limited to wetted parts materials for additional protection
against corrosion and some additional equipment listed below:
1. Wetted parts materials:
a. Stainless Jteel (standard).
b. Hastelloy "C".
c. P.T.F.E. (bearings).
66
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2. Plow straightening vanes/elements.
3. An additional electromagnetic pickup and associated
electronics for increased accuracy.
4. Turbine meters may require additional equipment for
secondary readout and/or transmitter devices. These
should be purchased from the meter manufacturer. If
another supplier is used, take care to ensure that both
units are compatible with regard to pulse shape,
amplitude, width, and signal frequency.
5. Typical secondary elements may include:
a. Electromechanical rate indicator and totalizer,
b. Pulse-to-current transducer.
c. Signal pulse "preamplifier (for long distance pulse
signal transmission).
F. Installation
1. Flow-disturbing piping obstructions severely affect
turbine meter accuracy. Figure 2,10 shows the
recommended installation piping and details, including a
flow-straightening element.
2. When a flow-straightening element is used, the flow-
disturbing effects of the following obstructions will be
adequately damped in a minimum upstream distance of 10
pipe diameters (including the straightener). NOTE: If
no flow-straightening element is used, extend this
minimum distance to 25 to 30 pipe diameters.
a. Valves.
b. Gates.
c. Tees.
d. Elbows.
e. Severe reducers and expanders ( 30 degrees
included angle).
3. Locate piping obstructions no nearer than 5 pipe
diameters downstream from the meter.
4. Install the meter in a horizontal pipe run.
67
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5. Install the meter on the discharge side of pumps and on
the upstream side of throttling valves.
6. Shield the cable between the turbine meter and
electronics; minimize its length, and do not route it
through areas of high electrical noise.
CONCENTRIC
CONE
NOMINAL SIZE
0 INCHES
CONCENTRIC
CONE
ALTERNATIVE FLOW
STRAIGHTENING VANES
METER 8 STRAIGHTENER
CONNECTIONS
Figure 2,10. Turbine meter mounting.
G. Designer Checklist
Use the following checklist when designing or reviewing
turbine flow meter applications. All checklist items should
be verified affirmative for proper application and
installation.
1. Is the intended process liquid as recommended in Table
2.5?
2. Can the expected head loss be tolerated from a hydraulic
standpoint?
3. Is the expected upstream line pressure great enough to
prevent cavitation in the meter?
4. If the meter diameter is equal to the process pipe, is
an instrument rangeability of 3:1 acceptable?
68
-------
5. Are all pipe obstructions located at least a distance of
5 pipe diameters downstream and a minimum upstream
distance of:
a. 10 pipe diameters (when a flow straightener is
utilized)?
b. 25-30 pipe diameters (when a flow straightener is
not utilized)?
6. Will the turbine meter be full under flowing conditions?
7. Is the process liquid essentially free of solids,
fiberous materials, and/or debris?
8. Has the proper secondary flow indication device(s) been
provided; has line power been provided, if required, for
the device(s)?
9. Is the meter easily accessible for maintenance?
10. Have provisions been made for calibrating the meter?
H. Acceptance and Performance Monitoring
Provide for acceptance testing and performance monitoring as
described in Section 7.1, relating to:
1. Hydraulic flow testing. •
2, Verification of manufacturer accuracy and factory
calibration documentation.
The meter constant "K", in clean water, is determined by the
manufacturer prior to meter shipment. If the intended
application process liquid has physical characteristics that
significantly differ from clean water, consult the
manufacturer for additional testing data.
I. Maintenance and Calibration
Turbine meters normally do not require periodic calibration.
When meter accuracy becomes questionable (as observed through
performance monitoring), examine it to determine maintenance
requirements, or if none are required, determine a new "K"
factor by hydraulic testing.
69
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J, Deficiencies
The following problems are commonly encountered with existing
turbine meter applications:
1. Inadequate upstream and downstream straight run piping,
resulting in poor meter accuracy.
2. No flow straightening vanes, resulting in poor accuracy.
3, Meter sized too large, resulting in non-full pipe and/or
poor accuracy at low flows.
4. Meter sized correctly, but reducers located too near
inlet and outlet.
5. Meter applied to a process liquid with an excessive
solids concentration.
K.' References
1. Liptak, B. G., and K. Venczel. Instrument Engineers
Handbook of Process Measurement, Chilton Book Company,
Radnor, Pennsylvania, 1969, Revised, 1982.
2. Foxboro Company. Technical Bulletin No. TI, 16-6a.
Foxboro, Massachusetts, January, 1971.
70
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2.4 VENTURI TUBES AND PLOW TUBES
A. Applications
Recommended applications for venturi and proprietary flow
tubes as primary elements include nearly all wastewater
treatment process streams. The most critical aspect of
proper application is the type of pressure sensing system
used to measure the differential pressure produced by the
primary tube.
Therefore, the recommended applications for these
instruments are listed by type of pressure sensing
system. The major types of sensing systems (described in
Part C) are:
1. Opefi connection, without flushing (including
piezometric rings).
2. Open connection, with flushing (excluding piezometric
rings),
3. Diaphragm sealed connections.
TABLE 2.6. VENTURI AND FLOW TUBE APPLICATION GUIDELINES
Recommended
Recommended With Flushing or
Without Flushing Diaphragm Seals Not Recommended
Secondary effluent Raw sewage Primary sludge
Final effluent Primary effluent Thickened sludge
Process (wash) water Return activated Chemical (corrosive)
sludge slurries
Waste activated
sludge
Mixed liquor
4, Additional requirements for venturi and flow tube
applications are:
a. The metering tube must flow full.
b. A maximum to minimum measuring range of 4:1 is
acceptable.
c. The Reynolds number of the process flow at the
meter should be greater than 150,000.
71
-------
Do not use venturi or flow tube meters in line with a
positive displacement pump. The resultant flow
pulsations will produce excessive signal noise and
measurement inaccuracy.
B. Principle of Operation
1. Venturi tube.
A venturi tube operates on the principle that a fluid
flowing through a pipe section that contains a
constriction of known geometry will cause a pressure
drop at the constriction area. The difference in
pressure between the inlet and the constriction area
(throat) is proportional to the square of the flow
rate. Figure 2.11 shows a cut-away of a typical
venturi tube.
-HIGH PRESSURE TAP
-LOW PRESSURE TAP
INLET
INLET CONE
OUTLET CONE
Figure 2.11. Classic venturi tube.
2. General equation.
W » 353Yd2 VhP/(l-B2)
Where:
d » throat diameter of pipe.
D » pipe inside diameter.
h m differential produced (in inches of water)
Y m net expansion factor.
S » d/D (Beta ratio).
P » specific weight.
W » flow rate (pounds/hr).
72
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Flow tube.
Several manufacturers provide differential-causing
flow tubes which are modified versions of the
classical venturi tube. These devices operate on the
same principle as the classical venturi; however,
they provide features which make them more attractive
for some applications, e.g., less space is required
for installation, less overall head loss, and lower
installed cost. Figure 2.12 shows three commonly
used flow tubes.
Both the venturi tube and the proprietary flow tubes
are primary sensing elements and require a secondary
element to measure pressure differential.
LO-LOSS FLOW TUBE
GENTILE OR BETHLEHEM
FLOW TUBE
CALL TUBE
Figure 2.12. Proprietary flow tubes.
73
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C. Pressure Sensing Systems
The differential pressure created by a venturi or flow
tube is generally measured by connecting a differential
pressure (Ap) transmitter to the sensing taps with pipe or
tubing (tap lines). The discussion here focuses on three
common Ap transmitter connection methods. Further
information regarding the Ap transmitter and tap line runs
is presented in Section 6.2 of this handbook.
1. Open connection.
For this method, the venturi tube pressure taps are
connected directly to the Ap transmitter and the tap
lines are allowed to fill with process liquid. To
avoid tap line clogging, do not use this method for
liquids with greater than 30 mg/1 solids.
Providing a flushing water system for this sensing
method allows the measurement of process liquids
containing solids which would normally clog the
sensing lines. There are two methods of operating
flushing water systems. In one, flushing water is
applied intermittently to purge solids from the tap
line (measurement' is interrupted during the purge
cycle). In the second, a continuous equal flow of
purge water is applied to both taps to act as a
barrier to solids. As the purge water back pressure
is measured in the latter method, it is critical that
purge flows are equal. Direct and flushing water
connections are illustrated in Figure 2.13.
2. Piezometric rings.
Piezometric rings may be used to sense inlet and
throat pressures. These are normally used in very
large diameter tubes where an average pressure is
required to compensate for velocity profile
variations. The rings consist of several holes for
each tap (in a plane perpendicular to flow) connected
to an annular ring. They should be used or.j.y on
clean liquids. Flushing water systems cannot be used
because the purge water short circuits within the
annular ring to the nearest tap hole.
3. Diaphragm sealed sensors.
The diaphragm sealed sensor method allows
solids-bearing liquids to be measured without a tap
flushing system. The process liquid is separated
from the tap lines and transmitter by a diaphragm.
74
-------
TO TRANSMITTER
HGH PRESSURE
FLUSHING
PURGE
TO TRANSMITTER
WITH CONTINUOUS FLUSHING
Figure 2.13. Typical differential piping.
75
-------
D. Accuracy and Repeatability
The accuracy and repeatability of venturi and proprietary
flow meters vary. The characteristics of the secondary
element (transmitter) must also be included in the total
accuracy figure. The following limits are generally
attainable for the previously listed applications:
1. Classical Accuracy, _+!% of actual flow.
venturi and Repeatability, _+!%.
Ap transmitter
2. Proprietary Accuracy, _+!% - 3% of actual flow.
flow tubes Repeatability, _+!%.
and Ap
transmitter
These levels of accuracy reflect optimum values achievable
for proper application and installation. Factors which'
degrade these levels include improper tube sizing (Beta
ratio to high or low for the expected flow range), and
piping elements which disrupt the velocity profile.
E. Manufactured Options
1. Primary meter tube:
a. Single sensing ports.
b. Piezometer ring sensing.
c. Inspection openings.
d. Manual rodders for cleanout of the sensing ports.
2. Sensing system:
a. Conventional, or
b. Diaphragm sealed.
3. Secondary element (transmitter):
a. Differential pressure transmitter (see Section
6.2).
b. Manometer transmitter (not recommended).
76
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F. Installation
1. Primary system.
a. Venturi and flow tubes may De installed in any
position to suit the requirements of the
application and piping as long as the meter
flows full.
b. For best accuracy, flow disturbing obstructions
(fittings) should not be located too near the
meter inlet. The following guidelines indicate
the minimum upstream distance of straight pipe
recommended between the fitting "and the meter
inlet:
1) Reducers 8 diameters of reduced pipe
size.
2) Expanders 4 diameters.
3) Fully open 5 diameters.
valve
4) Check valve 12 diameters.
5) Throttled 20 diameters.
gate or ball
valve
6) 900 bend(s) 4 diameters.
For more information on lengths of straight pipe
runs, see References 8, 9 and 10.
c. If a flow control valve is required in the line,
it should be placed a minimum of 5 pipe
diameters downstream of the meter tube. As seen
in 5) above, to place the valve upstream
requires a much greater straight run of pipe.
d. Locate all downstream pipe fittings a minimum
distance of 4 pipe diameters downstream of the
throat tap(s).
e. Place the meter tube a minimum distance of 10
diameters downstream from the pump discharge.
The meter tube can be located on the suction
side of a centrifugal pump only if
subatraospheric pressure can be avoided.
77
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f. Install the primary and secondary flow elements
in an accessible location with suitable space
for maintenance and calibration.
g. Orient single tap meters (at inlet and throat)
in the process piping so that the taps lie in
the upper half of the meridian plane.
2. Secondary system.
a. Place the differential pressure transmitter
below the hydraulic grade line to facilitate
positive gas bleeding.
b. Place an indicator gauge (A p) near the primary
element for convenience in calibration and
performance checking.
The following refer to all installations except those
having diaphragm-sealed sensors (see Figure 2.13).
c. If there is a possibility of the tap lines
freezing, use insulation and heat tape to wrap
them.
d. Install tap line (connecting) tubing so that it
has a minimum downward slope from the meter of 1
in 12.
e. Install a bleed valve or gas collector at the
highest point in the tap line run,
f. Provide valves to isolate the transmitter for
calibration,
g. Provide a flushing water system if the process
liquid contains greater than 30 mg/1 of solids.
h. Use connecting tubing no smaller than 1 cm (3/8
in) in diameter.
The following refer to installations where continuous
flushing is required (see Figure 2.13).
i. The head loss in the tubing between the flushing
water connection and the sensor tap should be
the same in both lines so the pressure
differential is unaffected.
-------
j. The flushing water supply pressure should be at
least 70 Kpa (10 psi) higher than process
pressure.
k. Equip the flushing water supply line for each
tap with a rotameter for visual inspection and
adjustment of purge flow.
G. Designer's Checklist
Use the following checklist when designing or reviewing
venturi and proprietary flow tube applications. All
checklist items should be answered yes for proper
application and installation.
1. Is the process liquid recommended in Table 2.6
compatible with the type of meter under consideration?
2. Will the meter tube flow full?
3. Is a maximum to minimum measurement range of 4:1
acceptable?
4. Is the Reynolds number at the meter expected to be
150,000 or greater?
5. Has the meter tube been sized to accommodate the
present flow range .(bear in mind that meters sized
for a 20 year projected flow are typically oversized)?
6, If the meter is to measure a solids-bearing liquid:
a. Are single sensor taps being used as opposed to
a piezometric ring?
b. Has either a flushing system or diaphragm-sealed
sensor system been provided?
7. Is adequate straight run piping provided up and
downstream from the meter tube (see Part F,
Installation)?
8, Have provisions been made for bleeding and flushing
tap lines?
9. Are the tap lines sloped properly?
10. Are both the meter and secondary elements readily
accessible?
-------
11. Will the meter be placed in a process line having
smooth dynamics, e.g., not pulsating as in positivfe
displacement pump applications?
H. Acceptance and Performance Monitoring
Provide for acceptance testing and performance monitoring
as described in Section 7.1, relating to:
1. Hydraulic flow te>r _ing.
2. Verification of manufacturer accuracy and factory
calibration documentation.
I. Maintenance and Calibration
1. Primary system.
a. If the tube has manual rodders (bayonets), use
these weekly or when the flushing water (if
used) flow rate decreases.
b. If annular rings are included with the tube,
bleed off gas periodically.
c. If performance monitoring indicates an accuracy
change, test the primary with a portable
manometer.
2. Secondary systems.
a. Bleed tap lines of entrapped air regularly.
b. Re-calibrate the transmitter monthly using a
portable manometer.
J. Deficiencies
The following problems are commonly encountered with
existing venturi and flow tube applications:
1. Meter oversized, low flow measurements are lost due
to square root function cut-off.
2. Met-.ers installed in process lines having pulsating
flow (reciprocating pumps), causing erroneously high
flow rates.
3. Tap lines inadequately sloped and/or not provided
with bleed valves, causing gas buildup.
80
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4. Improper differential range selection for the ^p
transmitter.
5. Inadequately designed flushing systems which skew the
pressure differential.
6. Insufficient straight run piping upstream and
downstream of the meter.
K. References
1. Liptak, B. G., and K. Venzcel. Instrument Engineers
Handbook of process Measurement. Chilton Book
Company, Radnor, Pennsylvania, 1969, Revised, 1982.
2. Spink, K. L. Principles and Practice of Flow Meter
Engineering, 9th edition. The Foxboro Co., 1967.
•
3. Water pollution Control Federation. Instrumentation
in Wastewater Treatment plants. WPCF Manual of
Practice No. 21, 1978.
4. International Standards Organization. Measurement of
Fluid Flow by Means of Orifice Plates, Nozzles and
Venturi Tubes Inserted in Circular Cross-Section
Conduits Running Full. ISO/DIS 5167, 1976, draft
revision of R781.
5. International Standards Organization. Fluid Flow in
Closed Conduits—Connections for pressure Signal
Transmissions Between Primary and Secondary
Elements. ISO 2186 - 1973.
6. American society for Testing and Materials. Standard
Method of Flow Measurement of Water by the Venturi
Meter Tube. ASTM D2458-69.
7. Benson, J. E. Process Instrumentation Manifolds.
Instrument Society of America, Research Triangle
Park, NC, 1981.
8. Fluid Meters, Their Theory and Application. Report
ASME Research Committee on Fluid Meters, American
Society of Mechanical Engineers, New York, New York,
1971, 6th Edition.
81
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9. _ Sprenkle, R. E. Piping Arrangements for Acceptable
Flowmeter Accuracy. ASME Transactions 67:345, New
York, New York, 1945.
10. Starret, P. S., P. F. Halfpenny and H. B. Noltage.
Survey of Information Concerning the Effects of
Nonstandard Approach Conditions Upon Orifice and
Venturi Meters. Paper presented at Winter Annual
Meeting, American Society of Mechanical Engineers,
New York, New York, 1965.
82
-------
3.0 FLOW MEASUREMENT, CLOSED CONDUIT GAS FLOW
3.1 ORIFICE PLATE
•
A. Application
The following general conditions provide suitable
applications for orifice plate gas flow meters:
1. Clean gas, or steam.
2. A relatively large head loss is acceptable.
3. The Reynolds number at minimum flow is greater than
10,000.
4. A meter maximum to minimum ratio of 3:1 is acceptable.
TABLE 3.1. FLOW MEASUREMENT, CLOSED CONDUIT GAS FLOW
APPLICATION GUIDELINES
Recommended Not Recommended
Boiler steam Wet steam
Compressed digester gas Low pressure (uncompressed)
Natural gas digester gas
Activated sludge treatment Strongly corrosive gases
- blower air
- oxygen
Incinerator draft/blower air
Aerated grit chamber, air flow
The discussions, in this section pertain to concentric orifice
plates. Other configurations, segmental and eccentric, are
available to accommodate particular application problems.
B. Sizing Guidelines
Proper sizing of the orifice plate is required for accurate
flow measurement. Use the following guidelines:
1.
Plate Thickness
a. Pipe I.D. from 5-20 cm (2-12 in): 0.3 cm (1/8 in)
thick.
i
Pipe I.D. 35 cm (14 in) and larger: 0.6 cm (1/4
in) thick. ~
83
-------
b. Bevel plates thicker than 0.3 cm (1/8 in) on the
downstream orifice edge as shown in Figure 3.1.
BEVEL WHERE
THICKNESS IS
GREATER THAN
3mm (1/8 IN)
OR THE ORIFICE
DIAMETER IS LESS
THAN 25mm (UN)
PIPE
INTERNAL
DIAMETER
3mm (I/8IN)
MAXIMUM
- 3-l3mm
(1/8-1/2 IN)
Figure 3.1. Concentric orifice plate.
2. Orifice diameter.
The Beta ratio is defined as the ratio of orifice
diameter (d) to the pipe I.D. (D) and is critical for
accurate flow measurement. For air and steam flow
measurement, it is recommended that the Beta ratio be
greater than 0.2 but less than 0.7(5). The flow
calculations used for determining the Beta ratios are
standardized but rather complex. These calculation
methods are thoroughly covered in References 2 and 7.
C. Principle of Operation
Orifice plates are differential-producing head-type flow
measuring devices made of a thin, flat plate having an
opening (orifice). When installed in cross section with the
process pipe, orifice plates cause .an increase in the flow
velocity as the process gas moves through the orifice,
causing a corresponding decrease in downstream pressure. A
differential pressure measuring device is connected across
the orifice plate to sense the differential pressure. Figure
3.2 illustrates this principle.
-------
104-
DIFFERENTIAL
| PRESSURE I
L_MEASUREMENTj
Figure 3.2. Pressure profile.
The orifice plate flow meter is a primary sensing element
which creates a pressure differential proportional to the
square of the flow rate. This pressure is measured by a
differential pressure sensor (Ap) which converts the
differential pressure to either a voltage or a current
signal. A secondary element to convert the non-linear
differential pressure into a linear flow rate is required.
This can be done using a square root scale, square coot
extractor or computer.
Location of the pressure taps determines the exact
relationship between differential pressure and flow rate.
These relationships are stated in standard reference material
(2, 7). The basic equations are:
For gas flow: Q =
(eq.l)
For steam flow: W = K
Q
K
(eq.2)
volume flow rate (scfh)
basic orifice expansion, flow and conversion factors
that usually are constant for a given application
h * differential pressure across the orifice in inches of
water
P = absolute flowing pressure, psia
T * absolute flowing temperature (OR a OF + 460)
G 3 specific gravity of gas (air » 1.0)
W = mass flow in pounds per hour (Ib /hr)
V » specific volume (ft3 AD ) determined from Standard
Steam Tables
85
-------
D. Accuracy and Repeatability
The total accuracy and repeatability of an orifice plate
flow-measuring system must include tha accuracy and
repeatability of the orifice,Ap sensor, and the square root
extractor. The following limits are achievable by orifice
plate meters for the applications listed in Table 3.1:
1. Accuracy: +1/2 to ±2% of full scale.
2. Repeatability: +1.0% of full scale.
These accuracy levels reflect c^cimum values achievable by
the measuring system when properly applied and installed.
The main factors which will degrade these levels include:
.improper orifice sizing with respect to the Beta ratio; flows
either less or greater than anticipated, and" piping
configurations which disrupt the velocity profile.
E. Manufactured Options
1. Type of construction materials.
2. Pressure connection location.
a. Flange taps (standard).
b. Vena contracta taps.
c. Radius taps.
d. Corner taps.
3. Orifice shape and location:
a. Concentric (standard), with or without drain and
vent holes.
b. Eccentric, with or without drain and vent holes.
c. Segmental, with or without drain and vent holes.
4. Removable orifice plate (without process disruption).
5. Secondary element:
a. Differential pressure transmitter (standard).
b. Manometer transmitter (not recommended).
6. Additional sensors (for measuring line temperature and
pressure) and a module for calculating gas flow in
standard units.
86
-------
Installation
1. Primary system.
a. Mount the orifice plata in either horizontal or
vertical process piping. Pressure tap locations
and AP transmitter locations will differ according
to the orientation selected.
b. Install beveled or cut away plates (Figure 3.1)
with the flat surface upstream.
c. Use 1.6 millimeter (1/16 in) thick gaskets,
graphited on the side next to the plate. The
gaskets must not extend into the pipe or obstruct
vent and drain holes (if used).
d. Provide straight run smooth piping upstream and
downstream of the orifice plate. The length of
straight run required depends on the Beta ratio.
Recommended lengths are shown in Figure 3.3. Use
straightening vanes when it is not practical to
install the meter with the recommended straight
pipe length.
e. Pressure taps should be free of any burrs or
protrusions into the pipe.
87
-------
-jof
o 020 040 ota tuo
DIAMETER RATIO,/9
(A) FOR ORIFICES AND FLOW NOZZLES
ALL FITTINGS IN SAME PLANE
0 Om 040 06O O«0
DIAMETER RATIO,/)
(B) FOR ORIFICES AND FLOW NOZZLES
ALL FITTINGS IN SAME PLANE
ORIFICE OR FLOW NOZZLE.
10
OIAM
-
-
/
)
ELLS,TUBE TURNS OH
LONG RADIUS BENDS
(U) ORIFICE OH FLOW NOZZLE
^tm. c J^k tf
S~\--\— A »1 | IS
2DIAM ' -I-^A
"" STRAIGHTENING •
VANE 2DIAM.LONG
0 020 040 OCO 0-80
DIAMETER RATIO,/)
(C) FOR ORIFICES AND FLOVV NOZZLES
FITTINGS IN DIFFERENT PLANES
020 04O O6C 080
DIAMETER RAflO./)
(D) FOR ORIFICES AND FLOW NOZZLES
FITTINGS IN DIFFERENT PLANES
00
00
OAIFICE OR FLOW NOZZLE.
^
0 020 040 060 060
(KAUETER RATIO, 0
(E) FOR ORIFICES AND FLOW NOZZLES
WITH REDUCERS AND EXPANDERS
.Baffin
0 020 040 060 OM
DIAMETER RATIO, />
(F) FOR ORIFICES AND FLOW NOZZLES
IN ATMOSPHERIC INTAKE
020 040 aw 0*0
DIAMETER RATIO, £
(G) VALVES AND REGULATORS
FIG. ll-II-l RECOMMENDED MINIMUM LENGTHS t P I'IPE PIIUCEDING
AND fOLLOHlNG ORIFICES. FLOW NOZZLES AND
VENTUIII TUBES (ALL CONTIIOL VALVES. INCLUDING
REGULATORS, SHOULD HE LOCATED ON OUTLET SIDE
OF PRIMARY ELEMENT.)
*jto oto o«o 040 oeo
DIAMETER RATIO,/)
(H) FOR VENTURI TUBES
-------
2. Secondary system.
Installation of the secondary system (pressure
connections, tap line run, and Ap transmitter location)
differs between applications for steam flow
(condensible) and gas flow measurement.
a. Steam flow measurement.
1) A typical installation diagram is shown in
Figure 3.4.
2) Always use condensing chambers on tap line
runs. Mount each chamber at the same level.
3) Install horizontal portions of tap line runs
so they slope downward from the orifice at a 1
in 12 grade.
4) Install the AP transmitter below the orifice
plate location for both vertical and
horizontal piping runs.
5) If the transmitter must be mounted above the
pressure connections either vena contracta or
pipe taps are recommended. Flange taps are
not recommended.
89
-------
CONDENSING
CHAMBERS -
PLUGGED
TEES
DIFFERENTIAL
PRESSURE
TRANSMITTER
PLUGGED i> SLOPE ALL HORIZONTAL
TEES RUNS AT LEAST l" PER I'-O"
Figure 3.4. Steam flow installation.
b. Gas flow measurement.
1) A typical installation diagram is shown in
Figure 3.5.
2) Mount the AP transmitter above the orifice
plate for both vertical and horizontal piping
runs.
3) If the gas is corrosive, use a liquid seal
with diaphragm pressure connections to isolate
the transmitter.
4) Install horizontal portions of tap line runs
so they slope upward from the orifice at a 1
to 12 grade.
90
-------
3 -VALVE
MANIFOLD
DIFFERENTIAL
PRESSURE
TRANSMITTER
SLOPE ALL HORIZONTAL
RUNS AT LEAST l" PER l'-0"
,, ^—TO TRANSMITTER
-TO ORIFICE
PLUGGED A '
TEES-" x
N
N
x i
^
* >
"N *
U^
r^ b
V
^
*
^
-DRIP POTS
INSTALL WHEN TRANSMITTER
HAS TO 8E MOUNTED BELOW
THE PRESSURE TAPS
Figure 3.5. Gas flow installation.
91
-------
3. Additional recommendations.
a. Locate the transmitter to facilitate easy access
for calibration and maintenance.
b. Tap line runs should not exceed 15 m (50 ft) and,
if freezing is possible, should be insulated or
heated.
c. Take special care to mount theAp transmitter
pressure connection plumbing so the differential
neasurement is not affected.
d. An isolation valve manifold and quick disconnects
should be installed in the tap lines to facilitate
sensor calibration.
G. Designer Checklist
Use the following checklist when designing or reviewing
orifice plate gas flow meter applications. All checklist
items should be verified affirmative for proper application
and installation.
1. General items.
a. Is the process gas or steam recommended in Table
3.1?
b. Is a large head loss acceptable?
c. Is the Reynolds number at minimum flow greater than
10,000?
d. Has the proper orifice size based on Beta ratio,
(d/D ratio) been determined for the expected flow
range and allowable pressure loss?
e. Is the Beta ratio greater than 0.2 and less than
0.7?
f. Will the meter construction materials withstand the
corrosive properties of the fluid to be measured?
g. Has the proper differential range been selected for
theAp transmitter?
h. Have the flange gaskets been properly sized to
insure no protrusion into the inside diameter of
the process i pipe?
92
-------
i. Does the straight run piping conform to the minimum
requirements in Figure 3.4?
3. Are the tap line runs less than 15 m (50 ft) long?
K. If freezing is possible, are the tap lines
adequately insulated or heated?
1. Has an isolation valve manifold and quick
disconnects been installed in the tap lines to
faciliate &p sensor calibration?
m. Will the Ap sensor be mounted in a vibration free
location?
2. Steam flow measurement.
a. Have condensing chambers been provided in the tap
lines and are they of adequate size?
b. Is the transmitter mounted below the process
connections? If it is not,.have either vena
contracta or pipe taps been used rather than flange
taps? Insulate from thermal shocks with tube loop
(pig tail) or other means.
3. Gas flow measurement.
a. Is the transmitter mounted above the process
connections?
b. Have condensata traps been installed at the lowest
point of the tap line runs?
c. If the gas is corrosive, have diaphragm sealed
pressure connections been provided?
H. Acceptance and Performance Monitoring
Recommended acceptance testing and performance monitoring
procedures are described in Section 7.2 related to:
1. Verification of manufacturer accuracy and factory
calibration documentation.
2. On-site testing.
I. Maintenance and Calibration
1. Primary system (orifice assembly).
a. Test the primary with a portable manometer monthly.
93
-------
b. If accuracy problems persist, remove orifice plate
and inspect orifice for solids buildup and/or wear.
2. Secondary system (Ap sensor).
a. For gas flows, empty condensate traps once per week.
b. Recalibrate transmitter monthly using a portable
manometer or other suitable calibration test set.
J. Deficiencies
The following problems are commonly encountered with existing
orifice plate gas flow meter applications.
1. Orifice oversized (Beta ratio too high) thus generating
a differential too low to be accurately "monitored by
the Ap transmitter provided.
2. Orifice properly sized, but the wrong _\p sensor range
was selected.
•
3. Condensate traps not provided, causing water
accumulation and occasional freezing in the lines.
4. Unequal tap line lengths or elevations, causing
differential measurement errors.
5. Insufficient straight run piping provided.
6. Insufficient calibration of primary and secondary
systems.
K. References
1. Liptak, B. G. and K. Venczel. Instrument Engineers
Handbook of Process Measurement. Chilton Book Company,
Radnor, Pennsylvania, 1969, Revised, 1982.
2. Fluid Meters, Their Theory and Application. Report of
ASMS Research Committee on Fluid Meters, American
Society of Mechanical Engineers, 345 East 47th Street,
New York, New York, 1971, 6th Edition.
3. The Foxboro Company. Technical Bulletins 6-110, 7-110,
7-251. Foxboco, Massachusetts.
94
-------
4. Instrument Society of America. Flange Mounted Sharp
Edged Orifice Plates for Flow Measurement.
ISA-RP3.2-1978. Research Triangle Park, NC.
5. American Petroleum Institute. Manual on Installation of
Refinery Instruments and Control Systems. Part 1 -
Process Instrumentation and Control. Section 1 - Plow.
API RPS50, Washington, DC, 1977, 3rd Edition.
6. Henson, J. S. Process Instrumentation Manifolds.
Instrument Society of America, Research Triangle Park,
NC., 1981.
7. Spink, L.K. Principles and Practice of Flow Meter
Engineering. The Foxboro Company, Foxboro, MA, 1967,
9th Edition.
8. Cusick, C. F. Flow Meter Engineering Handbook.
Honeywell, Inc., Fort Washington, PA, 1977, 3rd Edition.
9. Sprenkle, R. E. Piping Arrangements For Acceptable Flow
Meter Accuracy. ASME Transactions 55:345, New York, NY,
1945.
10. Starret, P. S., Halfpenny, P. F. and Noltage, H. 3.
Survey of Information Concerning the Effects of
Nonstandard Approach Conditions Upon Orifice and Venturi
Meters. Paper presented at Annual Winter Meeting,
American Society of Mechanical Engineers, New York, NY,
1965.
95
-------
3.2 VENTURI TUBES AND FLOW TUBES
A. Applications
General conditions which are suitable for the application of
venturi and proprietary flow tube gas flow meters:
1. The Reynolds number of the process stream at the meter
is greater than 150,000.
2. A meter rangeability of 4:1 is acceptable.
TABLE 3.2. VENTURI TUBES AND FLOW TUBES
APPLICATION GUIDELINES
Recommended
Not Recommended
Any low-pressure (uncompressed)
gas flows
Boiler steam
Compressed digester gas
Incinerator draft/blower air
Air flow
Oxygen flow
Corrosive gasses
3. Principle of Operation
1. Venturi tube.
A venturi tube operates on the principle that a gas
flowing through a meter section containing a convergence
and constriction of known shape and area will cause a
pressure drop at the constriction area. The difference
in pressure between the inlet and the constriction area
(throat) is proportional to the square of the flow
rate. Figure 3.6 shows a cut-away of a typical venturi
tube.
-HIGH PRESSURE TAP
•LOW PRESSURE TAP
INLET
INLET CONE -/ ^OUTLET CONE
Figure 3.6. Classic venturi tube.
96
-------
2. Flow tube.
Several manufacturers provide differential-causing flow
tubes that are modified versions of the classical
venturi tube. These devices operate on the same
principle as the classical venturi; however, they
provide features which make them more attractive for
some applications, i.e., less space is required for
installation and overall head loss is reduced. Figure
3.7 shows three commonly used flow tubes.
GENTILE OR BETHLEHEM
FLOW TUBE
LO-LOSS FLOW TUBE
CALL TU3E
Figure 3.7. Proprietary flow tubes.
97
-------
3. Pressure sensing.
Both the venturi tube and the proprietary flow tubes are
primary elements and require a secondary element to
sense the pressure differential and convert it to a
usable signal. The secondary element in most
applications is a differential pressure (A?) transmitter.
The A p produced by a flow tube is representative of the
volumetric flow rate at the actual operating temperature
and pressure. Appropriate temperature and pressure
sensors are required to correct the transmitter output
to standard reference conditions.
The AP in the tube is measured at the inlet and throat
(Figure 3.6).
One AP measurement method uses single connections in the
inlet and throat of the tube. The pressure tap lines
are coupled directly to the tap holes and run to the AP
transmitter.
As an alternative method, piezometric rings may be used
to sense inlet and throat pressures. These consist of
several holes around the circumference of the tube at
the inlet and throat tap locations. Each set of holes
is connected to an annulur ring to give an average of
the pressure at each tap hole connected to the ring.
Piezometric rings are usually used in large diameter
tubes to minimize velocity profile skewing.
C. Accuracy and Repeatability
The accuracy and repeatability of these meters vary with the
type used. The characteristics of the secondary element
(transmitter) must also be included in the total accuracy
figure. The following limits can be expected when
considering these types of flow meters for applications
previously listed:
Accuracy: +1% of actual flow.
Repeatability: +1% of actual flow.
The values shown are 'jr a complete system (primary and
secondary) when properly applied and installed. Factors
which will degrade these levels during operation include:
improper tube sizing, improper AP range, insufficient
straight pipe before and after the meter (see Part F,
Designers Checklist) and piping elements which disrupt the
velocity profile (see Part E, Installation).
Venturi tubes, flow tubes, and flow nozzles are all capable
of exceeding the accuracy values shown. Consult the
manufacturer if greater accuracy is required. Caution: both
the primary and secondary element must be considered when
designing for optimum accuracy.
98
-------
E.
Manufactured Options
1. Primary metar tube.
a. Single sensing ports.
b. Piezometer ring sensing.
c. Inspection openings.
d. Manual rodders for cleaning sensing ports.
2. Secondary element (transmitter).
a. Differential pressure transmitter (see Section 6.2)
b. Manometer transmitter (not recommended).
c. Temperature and pressure correction system
Installation
1. Primary system.
a. Venturi and flow tubes may be installed in any
position to suit the requirements application;
however, the primary and secondary system must be
accessible for maintenance and calibration.
b. Plow disturbing obstructions, pipe fittings and
valves will produce meter inaccuracies and should
not be located too near the metar inlet. Use
Figure 3.3 is to determine the minimum upstream
distance of straight pipe recommended between
fittings and the meter inlet.
3IHO
- — m — =
^fc
-flfcxfTT" — fl
0"~* fl
1
— — n
— — — i
T
-ilrHU w
J— 12-
DIA
^
VENTURI
A
rfTK
w
VENTURlTT
<±±
TWO ELBOWS NO
IN SAME _ PLANE,
PARTIALLY
' THROTTLED
-GATE VALVE
. GLOBE UALV
CHECK VALV1
£~
^
E,
E
1
^
*^
•=
^x
/
i
/
^ ^
~—
r
**,
1
/^
/
i^
»—
1
1
V
y
,
A
*B
s
*•
s
-------
c. If a flow control valve is required in the line, it
should be placed downstream from the meter tube as
shown in Figure 3.8. .
d. Locate all downstream pipe fittings a minimum
distance of two pipe diameters downstream of tire
throat tap(s).
e. Install the primary and secondary flow elements in
an accessible location with suitable space provided
for maintenance and calibration.
2. Secondary system.
Installation of the secondary system (pressure
connections, tap line run, andAp transmitter location)
differs between applications for steam.flow
(condensible) and gas flow measurement.
a. Steam flow measurement.
1) The tap lines are normally flooded with
condensate.
2) Use condensing chambers on tap line runs.
Mount each chamber at the same level.
3) Slope horizontal tap line runs downward from
the pressure taps at a 1 in 12 grade.
4) Install the ^p transmitter below the process
pipe for horizontal runs, or below the
pressure connections for vertical runs.
5) A. typical installation diagram is shown in
Figure 3.9.
100
-------
CONDENSING
CHAMBERS -
PLUGGED
TEES
DIFFERENTIAL
PRESSURE
TRANSMITTER
PIPE
STAND
PLUGGED
TEES
SLOPE ALL HORIZONTAL ,
RUNS AT LEAST l' PER I-0'
Figure 3.9. Steam flow measurement installation diagram.
b. Gas flow measurement.
1) The tap lines are normally dry.
2) Mount the AP transmitter above the process
pipe for horizontal runs, or above the
pressure connections for vertical runs.
3) If the gas is corrosive, use a liquid seal
with diaphragm pressure connections to isolate
the transmitter.
4} Slope horizontal tap line runs upward from the
pressure taps at a 1 and 12 grade.
5) A typical installation diagram is shown in
Figure 3.10.
101
-------
3-VALVE
MANIFOLD
DIFFERENTIAL
PRESSURE
TRANSMITTER
SLOPE ALL HORIZONTAL
RUNS AT LEAST l" PER l'-0"
PLUGGED V
TEES -- - -
\
TO ORIFICE
\
\
\
TO TRANSMITTER
*-
hsJ INSTALL WHEN TRANSMITTER
HAS TO BE MOUNTED BELOW
THE PRESSURE TAPS
Figure 3.10. Gas flow measurement installation diagram.
102
-------
3. Additional recommendations.
a. The transmitter should be located to facilitate
easy access for calibration and maintenance.
b. Tap line runs should not exceed 15m (50 ft.) in
length.
c. If a freezing potential exists, heat trace and
insulate the tap lines.
d. Mount the transmitter so that the high and low
pressure connections are at exactly equal
elevation. Failure to do so will create a bias in
the indicated AP which will in turn introduce an
error in flow measurement.
e. An indicator guage (AP) should be placed near the
primary element for convenience in calibration and
performance monitoring (usually mounted on the
secondary element).
f. The Beta ratio and overall diameter of the flow
tube should be carefully determined for the
expected flow range. Accordingly, the range of
the AP transmitter must match that of the flow tube
over the expected flow range.
F. Designer Checklist
Use the following checklist when designing or reviewing
orifice plate gas flow meter applications. Answer all
checklist items with a yes for proper application and
installation.
1. Is the process gas or steam recommended in Table 3.2?
2. Is the Reynolds number greater than 150,000?
3. Has the primary element been properly sized to generate
• a suitable differential pressure over the range of the
expected flow? Bear in mind that meter sizing for 20
year projected flow typically results in oversizing.
4. Has the proper differential range been selected for the
secondary device?
5. Has adequate straight run piping been provided both up
and downstream?
6. Are the tap lines sloped properly?
7. If measuring steam flow; is the transmitter installed
below the process connections, and have condensing
chambers been provided in the tap lines?
103
-------
8. Are the tap line runs less than 15m (50 ft)?
9. Is freezing a possibility? If so, are the tap lines
heated and insulated?
10. Are both the primary and secondary systems readily
accessible for maintenance?
11. When metering gas flow; is tne transmitter installed
above the process connections?
12. When metering gas flow; have condensate traps or drip
legs been installed at the low point of the tap line
tuns?
G. Maintenance and Calibration
1. Primary system.
a. If performance monitoring indicates a large
accuracy loss, test the primary with a portable
manometer.
2. Secondary system.
a. Bleed off condensate in the tap lines regularly.
b. Zero and recalibrate transmitter monthly using a
portable manometer.
H. Deficiencies
The following problems are commonly encountered with existing
venturi and flow tube applications:
1. Meter oversized. Low flow differentials are lost due to
square root function cut-off.
2. Gas buildup caused by tap lines that are not properly
sloped and/or not equipped with bleed valves.
3. Improper differential range selection for the £.p
transmitter.
4. Tap lines are plugged or restricted.
I. References
1. Liptak, B. G. and K. Venczel. Instrument Engineers
Handbook of Process Measurement. Chilton Book Company,
Radnor, Pennsylvania, 1969, Revised, 1982.
104
-------
2. Spink, K. L. Principle and Practice of Flow Meter
Engineering. The Foxboro Co., Foxboco, Massachusetts,
1967, 9th Edition.
3. Water Pollution Control Federation. Instrumentation in
Wastewater Treatment Plants, WPCF Manual of Practice No.
21, 1978.
3.3 AVERAGING PITOT TUBES
A. Applications
The following general conditions are suitable for the
application of averaging pitot tubes:
1. Clean gas or steam (free of solids),
2. Minimize head loss, and
3. An acceptable meter rangeability of 3:1.
TABLE 3.3. AVERAGING PITOT TUBES APPLICATION GUIDELINES
Recommended
Not Recommended
Gas or steam with particulace
solids
Low pressure (uncompressed
digester gas)
Corrosive gases
Boiler steam
Compressed digester gas
Natural gas
Activated sludge treatment
- blower air
- oxygen
Incinerator draft/blower air
Aerated grit chamber, air flow
B. Principle of Operation
Averaging pitot tubes are differential producing flow
measuring devices that consist of an insertion probe with
multiple upstream sensing ports and a single downstream
static port. The probe is geometrically constructed such
that an average upstream pressure is measured. Figure 3.1
shows the probe.
105
-------
/w» ^x
FOR EQUAL /VV"*
ANNULAR
AREAS
/w—
/
/w*. ^^/
*///////
\
1
s
VELOCITY
AVERAGE
SENSING
(TYPICAL
CF FOUR)
/ / / S / / J
\
c
5
>
-v
J
X.
/
,/
V
X1
Jt,
S
{
STATIC
PORT
////// /S///S/S
Figure 3.11. Pitot tube probe.
106
-------
The averaging pitot tube, like the venturi meter is a primary
sensing element that creates a pressure differential
proportional to the square of the flow rate. Like the
venturi, a £P cell is used to measure the differential
pressure and convert it to a voltage or current signal. A
secondary element to convert the nonlinear differential into
a linear flow rate is required. In most cases, this device
is a square root extractor.
C. Accuracy and Repeatability
The accuracy and repeatability of the averaging pitot tube is
good; however, the characteristics of the&p transmitter and
square root extractor must be included in the total accuracy
figure. The following performance limits are attainable by
averaging pitot tubes for the previously recommended
applications:
1. Accuracy: +1.0 to 3% of full scale.
2. Repeatability: +1.0% of full scale.
These accuracy levels reflect optimum values achievable for
proper application and installation. Factors that will
degrade these levels include operation at actual flows
outside the expected flow range, and piping elements that
disrupt the velocity profile.
D. Manufactured Options
1. Mounting:
a. Mounting coupling,
b. Flange, and
c. Hot tap.
2. Secondary element:
a. Differential pressure transmitter (standard), and
b. Manometer transmitter (not recommended).
3. .Calculation modules for standard pressure, temperature
correction and linearization to flow units.
B. Installation
1. Primary system.
a. Mount the averaging pitot tube in either horizontal
or vertical process piping. However, the tap line
configuration and Ap transmitter locations will
differ.
107
-------
b. Install the probe with the multiple ports facing
upstream.
c. Provide adequate straight-run smooth piping
upstream and downstream of the pitot tube. When it
is not practical to install the pitot tube with the
recommended straight pipe length, use straightening
vanes. Recommended lengths with and without vanes
are shown in Figure 3.12.
MINIMUM
DIAMETERS
OF
STAIGHT PIPE1
°~
% , 'alfl
o
fJrci"c-iDid
o
«i— A— LB$
-O-
«&fc£J2=L«jl
P ?
' ( A U-lB
P -9- 1
tS-TSteJ* '
rr + ,-n
^U_A_LaJ^
ft* * --n '
Ss.Ttso.r
Y
CLJ I i:n
1— A— Ul
f
'-U—.H I.,,.!-1-1
tS.TfcSd.BJ
Hi ? ,
t»J i 1
1 * — 1-8-1 SEE-
tl) ^ NCTTE
n=s*tij 2
UPSTREAM DIMENSION
WIT>
var
= IN
s
A
7
9
19
8
8
24
HOUT
JES
OUT
s
9
14
24
8
8
24
WITH
VANES
A LP
6
a
9
8
8
9
W
4
4
4
4
4
C
3
4
5
4
4
5
DOWNSTREAM
DIMENSION
B'
3
3
4
3
3
4
Figure 3.12. Typical upstream/downstream requirements.
2. Secondary systems.
Installation of the secondary system (pressure
connections, tap line run, and AP transmitter location)
differs oetween applications for steam flow
(condensible) and gas flow measurement.
a. Steam flow measurement.
1) A typical installation diagram is shown in
Figure 3.13.
108
-------
2) Always use condensing chambers on tap line
runs. Mount each chamber ac the same level.
Size condensation chambers large enough for
the application. This is to prevent flooding
between routine maintenance checks.
3) Install horizontal portions of tap line runs
so they slope upward from the primary element
at a 1 in 12 grade.
4) Install the &p transmitter below the process
pipe for horizontal piping runs, below the
pitot tube location for vertical piping runs.
HORIZONTAL LINE (HL)
VERTICAL LINE (VL)
5 VALVE
MANIFOLD
LOW DISPLACEMENT
DP TRANSMITTER
(NON-MOTION BALANCE)
= TO REMOTE OR
LOCAL READOUT
RECORDER OR
CONTROLLER
ANNUBAR INSTRUMENT CON-
NECTIONS V/ILL BE POSITIONED
90° FROM STANDARD.
Figure 3.13. Installation for steam flow applications.
b. Gas flow measurement.
1) A typical orientation diagram is shown in
Figure 3.14.
2) Mount the AP transmitter above the process
pipe for horizontal piping runs, above the
pitot tube for vertical piping runs.
3) Install horizontal portions of tap line runs
so they slope upward at a minimum of a 1 in
12 grade.
109
-------
Figure 3.14. Orientation for gas flow applications.
3. Additional recommendations.
a. Locate the transmitter to facilitate easy access
for calibration and maintenance.
b. Tap line runs should not exceed 15 m (50 ft) and,
if freezing is possible, should be insulated.
c. Mount the transmitter so that the high and low
pressure connections are at exactly equal elevation.
d. Provide a 3-valve manifold and quick connects for
connecting a manometer during calibration.
Designer Checklist
Use the following checklist when designing or reviewing pitot
tube gas flow meter applications. All checklist items should
be verified affirmative for proper application and
installation.
1. General items.
a. Is the process gas or steam recommended in Table
3.3 and free of particulates?
b. Has the proper differential pressure range been
selected for the secondary device, i.e., AP
transmitter?
c. Has adequate straight-run piping been provided to
conform to the minimum requirements in Figure 3.12?
110
-------
d. Do the horizontal tap line runs slope upward at a
minimum of a 1 to 12 grade?
e. Are the tap line runs less than 5M (15 ft) long?
f. If freezing is possible, are the tap lines
insulated and heat traced?
g. Have a 3-valve manifold and quick disconnects for a
manometer been provided for calibration?
h. Will the meter be mounted in a vibration free
location?
2. Steam flow measurement.
a. Have condensing chambers been provided in the tap
lines and are they of adequate size?
b. Is the transmitter mounted below the process
connections?
3. Gas flow measurement.
a. Is the transmitter mounted above the process
connections?
b. Have condensate traps been installed at the lowest
point of the tap line runs?
G. Acceptance and Performance Monitoring
Provide for acceptance testing and performance monitoring as
described in Section 7.2, related to:
1. Hydraulic testing.
2. Verification of manufacturer accuracy and factory
calibration and documentation.
H. Maintenance and Calibration
1. Primary system.
a. When performance monitoring indicates a large
accuracy loss, test the primary with a portable
manometer.
b. Extended periods of poor accuracy: remove pitot
tube and inspect orifice for solids buildup and/or
wear.
Ill
-------
2. Secondary system.
a. For gas flows, empty condensate traps once a week.
b. Check the transmitter calibration monthly using a
portable manometer or other suitable calibration
test set.
I. Deficiencies
The following problems are commonly encountered with existing
pitot tube gas flow meter applications.
1. The wrong range on the AP transmitter was selected.
2. Condensate traps were not provided, causing water
accumulation and occasional freezing in the lines.
3. Unequal tap line lengths or elevations used, causing
differential errors.
4. Insufficient straight-run piping provided.
5. Infrequent maintenance.
J. References
1. Liptak, B. G. and X. Venczel. Instrument Engineers
Handbook of Process Measurement. Chilton Book Company,
Radnor, PA, 1969, Revised, 1982.
2. Considine, D. M. Process Instruments and Controls
Handbook. McGraw Hill Book Company, New York, NY, 1974.
112
-------
3.4 TURBINE FLOW METERS
A. Application
General conditions which are suitable for turbine meters to
measure gas flow include:
1. An intermittent flow may be expected.
2. A maximum meter rangeability of 15:1 is acceptable.
TABLE 3.4. TURBINE FLOW METER APPLICATION GUIDELINES
Recommended Not Recommended
Steam Low pressure '(uncompressed)
Compressed digester gas digester gas
Natural gas
B. Principle of Operation
Turbine flow meters consist of a pipe section with a
multi-bladed rotor suspended in the fluid stream on a free
running bearing, see Figure 3.15. The,plane of rotation of
the rotor is perpendicular to the flow direction and the
rotor blades sweep out nearly to the full bore of the pipe.
The rotor is driven by the process gas impinging on the
blades. Within the linear flow range of the meter, the
angular velocity of the rotor is directly proportional to the
liquid velocity which is in turn, proportional to the
volumetric flow rate. The speed of rotation is sensed by an
electromagnetic pickup coil which produces a pulse. | The
output signal is a continuous voltage pulse train with each
pulse representing a discrete volume of gas. The turbine
output frequency is proportional to the volumetric flow rate
at the actual operating temperature and pressure. An
appropriate temperature and pressure correction system is
required to convert the meter output into a volumetric rate
at standard reference conditions.
113
-------
PICXUP COIL
UPSTREAM
ROTCR
UNIT
ASSEMBLY
HANGER UNIT
HOUSING
SPINDLE
DOWNSTREAM
HANGER
UNIT
C.
0.
Figure 3.15. Gas turbine meter.
Accuracy and Repeatability
When properly applied and installed, the accuracy and
repeatability characteristics of turbine flow meters, should
be:
1. Accuracy:
2. Repeatability:
+0.25% of actual flow, within the
linear range of the meter.
+0.05% within the linear range of the
meter.
Each turbine flow meter has a unique "K" factor (the number
of pulses generated per unit volume) which is determined
during factory flow calibration. This factor and thus the
accuracy of the meter, is affected by mechanical wear.
Manufactured Options
1. Wetted parts materials.
a. Stainless steel (standard).
b. Hastelloy "C".
c. Teflon bearings.
2. Flow straightening vanes.
3. Additional electromagnetic pickup and associated
electronics for increased accuracy.
114
-------
4. Pressure and temperature correction system for
calculating volumetric flow under standard conditions.
5. Turbine meters require secondary elements for indicating
flow at the meter or retransmitting for remote
monitoring. It is suggested these elements be purchased
from the same manufacturer. If another supplier is
used, take care to ensure that both units are compatible
with regard to signal pulse shape/ amplitude, width, and
frequency.
Typical secondary elements include:
a. Electromechanical rate indicator and totalizer.
b. Pulse-to-current signal converter-.
c. Signal pulse preamplifier (for long distance pulse
signal transmission).
E. Installation
1. Piping obstructions which severely disturb the flow
profile severely affect turbine meter accuracy. Figure
3.16 shows recommended piping installation including
flow straightening vanes.
2. Turbine meters have a linear flow relationship. They
are sized by volumetric flow rate. Use the following
guidelines when sizing a turbine meter:
a. Each meter size has a specified minimum and maximum
range of flow linearity and should not be used for
flow rates outside that range.
b. The maximum flow rate for the application should be
70% to 90% of the maximum flow rate specified for
the meter.
c. Size the meter on actual volume flow and not on
reference or standard units.
d. The meter size should be less than the diameter of
the process piping.
e. Available turbine meter sizes range from 0.5 - 60.0
cm (3/16 to 24 inches) in diameter.
115
-------
3. The recommended minimum upstream straight-run for
optimum accuracy is 25 to 30 pipe diameters. If
necessary, this distance may be reduced to 10 pipe
diameters by installing straightening vanes. The
following pipe fittings produce flow disturbances that
will degrade meter accuracy if placed closer than the
specified distances.
a.
b.
c.
d.
e.
Valves,
Gates ,
Tees,
Elbows ,
Severe i
and
redu<
Severe reducers and expanders (>30 degrees
included angle).
CONCENTRIC
CONE
NOMINAL SIZE
CONCENTRIC
CONE
ALTERNATIVE FLOW
STRAIGHTENING VANES
METER a STRAIGKTENER
CONNECTIONS
4.
Figure 3.16. Meter installation.
Locate downstream piping obstructions at least 5 pipe
diameters'from the meter.
5. Use shielded cable between the turbine meter and
secondary electronics.
6. Route power wiring and signal cable in separate conduit.
116
-------
F. Designer Checklist
The following checklist should be used when designing or
reviewing turbine flow meter applications. All checklist
items should be answered affirmative for proper application
and installation.
1. Is the intended process gas recommended in Table 3.4?
2. Can the expected head loss be tolerated?-
3. Are all pipe obstructions located a minimum upstream
distance of:
a. 10 pipe diameters when flow straightening vanes are
used?
b. 25-30 pipe diameters without flow straightening
vanes?
4. Is there a minimum downstream distance of 5 pipe
diameters to any flow disturbing fittings?
5. Has the proper secondary flow indicator/signal
conditioner been provided?
6. Is power available for the secondary element?
7. Is the meter accessible for maintenance?
8. Is the meter diameter smaller than the process piping?
9. If temperature and pressure correction are desired, have
provisions been made?
G. Maintenance and Calibration
The meter constant "K" is determined at the manufacturer's
facility prior to meter shipment and is performed under
standard conditions. If the intended application process gas
has significantly differing physical characteristics, the
manufacturer should be consulted for additional testing data.
Turbine meters do not require calibration, but periodic
calibration may be required on the secondary element. When
meter accuracy becomes questionable (as observed through
performance monitoring) check the "K" factor by physical
testing.
117
-------
H. Deficiencies
The following problems are commonly encountered with existng
turbine raeter applications:
1. Inadequate upstream and downstream straight run piping,
resulting in poor meter accuracy.
2. Meter sized too large, resulting in poor accuracy at low
flows.
3. Mett=i and secondary element are not compatible because
of differing electrical specifications.
4. Although rare, the factory determined "K" factor is
incorrect.
I. References
1. LiptaK/ B. G. and Venczel, K. Instrument Engineers
Handbook of Process Measurement. Chilton Book Company,
Radnor, Pennsylvania, 1969, Revised, 1982.
2. Foxboro Company. Technical Bulletin No. TI, 16-6a.
Foxboro Company, Foxboro, Massachusetts, January, 1971.
118
-------
4.0 FLOW MEASUREMENT, OPEN CHANNEL
4.1 KENNISON NOZZLE
A. Application
The Kennison nozzle is a proprietary product designed to
measure flow only in partially filled pipes. Kennison
nozzles are designed to flush through solids without
accumulation and are generally considered to have a maximum
to minimum measurement ratio of 10 to 1. A Kennison nozzle
would be selected for flow measurement only when all of the
following general conditions apply:
1, Pipe size is not smaller than 0.15 m (6 in) nor larger
than 0.9 m (36 in).
2. The pipe does not flow full.
3. High and low operating flows fall respectively within
the 100% and 10% capacity of the desired nozzle size.
4. The level of the liquid downstream will always be below
the bottom of the nozzle so that free discharge exists.
TABLE 4.1. FLOW MEASUREMENT, OPEN CHANNEL APPLICATION GUIDELINES
Recommended Not Recommended
Raw sewage Pilled lines
Partially filled lines
B. Principle of Operation
The Kennison nozzle is a constriction to liquid flow of known
geometry which will produce a hydraulic head at the area of
the constriction. When the nozzle is operating above minimum
flow rate (10% of the maximum nozzle flow) the head is
essentially linear to flow rate, providing that free
discharge exists. Figures 4.1 and 4.2 illustrate a Kennison
nozzle and the relationship between flow rate and head.
119
-------
C.
n
n>
to
o
•a
HEAD- INCHES
0 100 2OO 300 4OO 50O 6OO
FLOW- GALLONS PER MINUTE
>ical rating curve for a 10 in. Kennison nozz
3-MW*Oia>-J
-------
The Kennison nozzle is designed to measure flow in partially
filled pipes. If high flow conditions fill the pipe/ the
nozzle will overflow. Any measurements taken daring this
period will be inaccurate.
For low flow and low head applications, the manufacturer has
available a half section Kennison nozzle.
C. Accuracy and Repeatability
The accuracy of the Kennison nozzle is +_2% of actual flow
when properly installed and operated between 10 and 100%
capacity of the nozzle. At flows of less than 10% of maximum
flow accuracy will reduce to + 3 to +_5% of actual flow.
The accuracy of any flow indicator is dependent on the
combined accuracy of the nozzle (primary element) and the
hydraulic level measuring device (secondary element). An
accuracy of +_5% of flow is considered reasonable when
recording or indicating flow in the linear operating range of
the nozzle.
The manufacturer states that accuracies of _+0.1% can be
attained if the nozzle is flow tested in a hydraulics
laboratory.
D, Manufactured Options
The Kennison nozzle is a proprietary product of BIF, a unit
of General Signal. Available options include:
1. . A half section nozzle for measuring flow under low
hydraulic head conditions, or lower than standard flow
rates.
2. Cast iron or fiberglass material of construction.
3. Pressure tap connection on left or right side of nozzle.
4. Hydraulically operated vent cleaner for remotely or
automatically initiated cleaning of the pressure tap.
5. Flow indicator.
6. Stilling well, level float, and transmitter with an
output characterized for the nozzle.
7. Upstream spool piece and capacitance probe linearized to
the nozzle.
121
-------
Installation
1. Elevate the nozzle invert (bottom) above the level of
the downstream surface to ensure free discharge
conditions.
2. Level the nozzle in the horizontal plane (lengthwise and
crosswise).
3. Install a thick gasket between the nozzle flange and
pipe to facilitate leveling.
4. Ensure a uniformly distributed non-turbulent flow by
following the manufacturer's recommended approach
conditions. They are:
a. Eight pipe diameters of unobstructed straight pipe
upstream of the nozzle.
b. Slope of the pipe line should not exceed the
tabulated limits for the nozzle size selected as
indicated in Table 4.2.
c. Line velocity should not exceed tabulated approach
velocities for the nozzle size selected as
indicated in Table 4,3.
5. Set the invert of the nozzle flange and the mating pipe
flange at the same elevation.
6. Consult the manufacturer's engineering data bulletin for
details on the installation of a secondary level
measuring system.
TABLE 4.2. LIMITING SLOPE FOR APPROACH PIPING
Nozzle Size (In.) Slope (Meters Per Meter)
6 . 0.0070
8 0.0050
10 0.0040
12 0.0033
16 0.0027
20 0.0023
24 0,0021
30 0.0020
36 0.0020
122
-------
TABLS 4.3. MAXIMUM LINE VELOCITY FOR KENNISOM
:JOZ2L3 INSTALLATIONS
Maximum Line Velocity
Nozzle Size (In. ) (m/sec)
6 0.67
3 0.67
10 • 0.70
12 • 0.79
16 0.91
20 1.00
24 1.13
30 1.13
36 1.13
P. Designer Checklist
Use the following checklist when designing or reviewing
Kennison nozzle applications. All items should be answered
affirmative.
1. Have the manufacturer's rating curves been consulted to
assure the selected nozzle size meets the following
criteria?
a. Has the smallest practical size been selected for
the anticipated range of flows?
Refer to the manufacturer's rating curves for the
desired nozzle size.
b. Is the minimum anticipated flow greater than 10% of
the nozzle capacity?
c. Will the pipe be less than full under maximum flow
conditions?
2. Is the invert of the nozzle above the downstream liquid
level at all times?
3. Is the straight pipe upstream of the nozzle greater than
3 diameters?
4. Is slope of the pipe less than the manufacturer's limits?
5. Is approach velocity within the limits specified by the
manufacturer?
G. Acceptance and Performance Monitoring
Install either a pressure gauge or manometer on the zero
check tap. Use readings from this indicator to pick the flow
from the manufacturer's rating curve.
123
-------
Other methods for flow verification and testing are described
in Section 7.1.
H. Maintenance and Calibration
1. Weekly: Operate the vent cleaner to remove any buildup
or obstruction in the pressure tap.
2, Weekly: Measure the hydraulic head manually to
determine if calibration of the secondary system is
required.
3. Weekly: Inspect the secondary level system and maintain
as required.
4. Semiannually: Check for buildup of solids and remove as
required.
I. Common Deficiencies
Improper approach conditions constitute the most common
problem in the use of the Kennison nozzle; either the slope
of the approach piping is too great and/or the approach
velocity is too high.
J. References
1. BIF a unit of General Signal. Kennison Open Flow
Nozzle. Engineering Data Sheet No. 135.21-1. West
Warwick, Rhode Island.
2. Grant, D.M. Open Channel Flow Measurement Handbook.
ISCO, Inc., Lincoln, Nebraska, Second Edition, 1981.
124
-------
4.2 PALMER-BOWLUS FLUME
A. Application
Palmer-Bowlus flumes are normally installed in sewers between
sections of pipe. The following general conditions should
apply when selecting a Palmer-Bowlus flume.
1. Open channel with round bottom or partially filled pipes
(less than 90% filled) where fabrication of a flow
transition approach section to accommodate a parsnall
flume is not practical.
2. Hydraulic head loss must be minimized.
3. Sediment or solids in the measured stream (velocities in
the flume tend to flush away deposits).
4. variations in the flow rates are expected to be within a
10 to 1 range.
5. The flow entering the flume is subcritical (velocity is
less than in the flume throat), non-turbulent, and
uniformly distributed.
TABLE 4.4. PALMER-BOWLUS FLUME APPLICATION GUIDELINES
Recommended Not Recommended
Raw sewage Sludges
Chemicals
B. Principle of Operation
The Palmer-Bowlus flume is a restriction in the channel which
produces critical flow through the throat of the flume. This
restriction also causes the water to bacKup upstream of the
flume. Figure 4.3 shows the sections of a Palmer-Bowlus
flume. The throat cross-section is trapezoidal.
125
-------
TEXT
HJJ-fc
figure 4.3. Palmer-Bowlus £lume.
Flow rate is related to upstream depth. Thia relationship is
derived analytically from an energy balance between the point
of depth measurement and the flume throat. The point of
depth measurement is about 1/2 pipe diameter upstream from
the entrance to the flume (refer to Pigure 4.4).
uAir L!\E
Ur 7CX.T *
126 _
Reproduced from
best available copy.
-------
0/2 ,
FLOW-
\ A
20 •*• 2
Figure 4,4. Free flow/depth relation.
Palmer-Bowlus flumes are subject to free and submerged
discharge conditions. Free discharge will prevail as long as
the ratio of downstream to measured depth (H /H ) does not
exceed 0.90. One reference suggests that the H /H ratio
should not exceed 0.85. When the H /H exceeds 0.90,
submerged flow exists. Correction factors are not available
for submerged flow conditions.
The Palmer-Bowlus flume size is determined by its
cross-section diameter. Prefabricated flume liners are
available in sizes from 0,1 - 1.5 m (4-60 in). To avoid
selecting an oversized flume, care should be taken to base
flume size on actual flow rather than nominal pipe.
Two elements are required to measure flow with a
Palmer-Bowlus flume. The primary element is the
Palmer-Bowlus flume structure; the secondary element is a
level measuring device. For additional information on
equipment to measure level, refer to Section 5.0 of this
handbook.
C. Accuracy and Repeatability
When properly installed, the accuracy of the flume primary
element is +.3% of full scale. A given flow differential will
produce a relatively small head differential thus requiring a
sensitive, accurate level measurement for best results. In
operation, the Palmer-Bowlus flume and a level measuring
device should produce a combined accuracy of _+10% of full
scale.
127
-------
There are additional sources of arror which can add to the
inaccuracy of tne meter. The principal ones ara:
1. Longitudinal slope of the flume floor greater tnan 1.5%.
2. Any transverse slope of the flume floor.
3. Approach conditions which do not produce a smooth flow
with uniform velocity distribution parallel to the
center line of the flume.
4. Incorrect zero reference of the level measurement device.
5. Where stilling wells are used (see page 127) connector
hole is improperly sized.
D. Manufactured Options
Palmer-Bowlus flumes typically are purchased as a
prefabricated liner to be set in concrete or grouted into a
half-section of pipe. Some available options include:
1. Material of construction.
2. Alternative configurations:
a. Basic insert flume.
b. Flume with integral approach section.
c. Cutback flume for insertion into a manhole
discharge pipe.
3. Flanged ends.
4. End bulkheads to fit in a larger pipe.
5. For flumes with integral approach sections:
a. Depth gauge flush mounted in the sidewall.
b. Stilling well connection.
c. Attached stilling well.
d. Removable bubbler tube installed in the sidewall.
X
6. Nested flumes.
A large flume with a smaller one mounted internally.
The smaller flume is removed when flow exceeds its
capacity.
128
-------
Installation
1. Install the flume so that the floor of the flame is
level longitudinally and laterally.
2. Establish the flume floor elevation to prevent
submergence conditions at maximum flow.
3. Plan the flume installation to allow access for
inspection to ensure correct elevation and leveling of
the floor.
4. Use an approach channel long enough to create a
symmetrical, uniform velocity distribution and a
tranquil water surface at the flume entrance. A general
rule is the greater of either 20 throat widths or 3 pipe
diameters of straight run should exist upstream of the
flume inlet.
Designer Checklist
Use the following checklist when designing or reviewing
Palmer-Bowlus flume applications. All items should be
verified affirmative for correct application and installation.
1. Has the smallest practical size Palmer-Bowlus flume been
selected for the anticipated range of flows?
a. As a rule of thumb, the flume throat width is
one-third to one-half of the pipe diameter.
b. Another sizing guide is that the maximum flow
expected should fall within 70-100% of the maximum
capacity for the selected flume size (1, 3).
c. A minimum depth of 0.15 m (0.5 in) should exist at
the minimum actual flow.
2. Is the flume floor elevation sufficient to avert
submerged flow conditions?
3. Does a straight channel longer than 20 throat widths or
8 pipe diameters exist upstream of the flume?
4 Hill the anticipated upstream flow provide
non-turbulent, wave-free approach conditions?
5. Is the upstream approach velocity subcritical?
6. Do any downstream obstructions exist which could
restrict the discharge of the flume?
7. Is a reference gauge provided for measuring depth in the
flume?
129
-------
3. is the point of level measurement correctly located on
the flume?
9. Is level sensor zero referenced to the floor of flume at
the center line of the throat?
10. If required, is a stilling well provided for level
measurement?
The following items pertain to the stilling well:
a. Does the vertical height extend below and above the
anticipated operating depths in the flume?
b. Is the flume opening for the stilling well sized
large enough to avoid sensor lags and plugging?
c. Has the flume opening for the stilling well been
correctly located on the length of the converging
section wall?
d. is the flume opening for the stilling well
positioned below the operating level at minimum
flow?
e. Is a fresh water purge piped into the stilling well?
G. Acceptance and Performance Monitoring
It is recommended that a depth gauge be mounted upstream of
the flume entrance so that manual readings and flow
calculations can be made to check remote flow indicators or
recorders.
Other methods for flow verification and testing are described
in Section 7.1.
H. Maintenance and Calibration
1. Palmer-Bowlus flume.
a. Weekly, check the depth gauge with other level flow
indications for the flume to determine if
calibration of the secondary system is required.
b. Periodically, wipe down the flume walls to remove
slime or other buildup.
c. Check for bottom deposits and remove as required.
d. Quarterly, check the zero of the reference depth
gauge.
e. Semiannually, examine the flume surfaces for signs
of deterioration and wear.
130
-------
2. Stilling well (if used).
a. Periodically, check for solids accumulation and
clean as necessary (establish the interval between
checks by experience).
b. If purge water is used, check the flow rate daily
and adjust as needed.
I. Deficiencies
The following problems are encountered in existing
Palroer-Bowlus flume installations.
1. Plume sized too large or small for the operating flow
range.
2. Insufficient straight channel upstream resulting in
non-uniform velocity distribution through the throat.
3. Excessive transverse or longitudinal slope to the flume
floor.
4. Measuring the depth of the hydraulic head at the wrong
location.
5. Level sensor zero and the flume floor elevation not
. equal.
6. Using an incorrect equation for calculating flow from
level.
7. No provisions for purging the stilling well when
measuring a solids-bearing stream (buildup of deposits
in the stilling well renders the level sensor
inoperable).
8. The foundation of the flume is not water tight, allowing
leakage under or around the flume.
J. References
1. Grant, D.M. Open Channel Flow Measurement Manual.
ISCO, Inc., Lincoln, Nebraska, 1981, Second Edition.
2. Metcalf and Eddy, Inc. Wastewater Engineering:
Treatment/Disposal/Reuse. McGraw-Hill, 1979.
3. Kulin, G. Recommended practice For The Use of Parshall
Flumes and Palmer-Bowlus Flumes in Wastewater Treatment
Plants. EPA-600/2-84-186. Environmental Protection
Agency, Cincinnati, Ohio, November, 1984.
4. Flow: Its Measurement and Control in Science and
Industry, Volume Two. Instrument Society of America,
1981.
131
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4.3 PARSHALL FLOMS
A. Applications
Parshall flumes are suitable for metering flow under the
following general conditions:
1. Open channels.
2. Hydraulic head loss must be minimized. Typically, head
loss for a Parshall flume head loss is approximately 25%
of the head loss for a weir of equal capacity.
3. Sediment or solids in the measured stream (velocities in
the flume tend to scour and flush away deposits).
4. Anticipated flow rates will vary widely. Depending on
flume size and the accuracy of leva! measurement, a
"laximum to minimum flow range of 20 to 1 is reported for
Parshall flumes.
5. Approach conditions upstream of the flume will insure
that the entering flow is tranquil and uniformly
distributed.
TABLE 4,5. PA3SHALL FLUME APPLICATION GUIDELINES
Recommended Not Recommended
Raw sewage Sludges
Primary effluent Chemicals
Secondary effluent
Plant final effluent
Mixed liquor
B. Principle of Operation
The Parshall flume is a device for measuring liquid flow in
open channels. The Parshall flume is a constriction of the
channel that develops a hydraulic head which is proportional
to flow. Figure 4.5 illustrates the shape and sections of a
Par .snail flume.
132
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CONVERGING
SECTION
DIVERGING
SECTION
•WING
WALL
FLOW
Figure 4.5. Parshall flume flow element.
Parshall flume sizes refer to the width of the throat
section. Flumes are available in sizes from 0.025 m (1 in)
up to 15 m (50 ft). Large flumes are constructed on site,
but smaller flumes can be purchased as prefabricated
structures or as lightweight shells which are sst in
concrete. Dimensions for the fabrication of Parshall flumes
are contained in the Water Measurement Manual published by
the United States Department of the Interior.
If a Parshall flume has been constructed to standard
dimensions and properly set, it is possible to calculate flow
through the flume by measuring level at a single point. The
location for the level measurement is shown as H in Figure
4.6. Flow is approximately proportional to the three halves
power of the hydraulic head. Simplified equations can be
found in Reference 2.
133
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FLOW
THROAT WIDTH
Ha= HYDRAULIC HEAD
Figure 4.6. Head/width parameters.
Two flow conditions can exist in the Parshall flume: free
flow and submerged flow. Free flow exists when the only
restriction is the throat width and the water is not slowed
by downstream conditions. If the flow through the flume
increases sufficiently, this will cause a rising downstream
channel level which will impede the discharge from the flume,
thereby slowing the fluid velocity. This is known as
submerged flow.
It might be expected that the flume discharge would be
reduced as soon as the tailwater level exceeds the elevation
of the crest. Tests have shown that this is not the case.
Free flow conditions can still exist even with some degree of
submergence. Figure 4.7 shows the head relationship for free
flow. Table 4.6 lists flume sizes and the limits of free
flow submergence. For free flow conditions, the depth
measurement (H ) can be used to calculate the flume discharge
flow.
•<5O-ao%
Figure 4.7. Free flow submergence.
134
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TABLE 4.6, SUBMERGENCE LIMITS
Flume Size % Submergence
0.025, 0.05, 0.075 m (1, 2, 3 in) 50
0.15, 0.23 m (6, 9 in) 60
0.3 - 2.4 m (1-8 ft) 70
3 - 15 m (10-50 ft) 80
Although Parshall flumes can operate with a submergence
greater tnan those shown in Table 4.6, a second level
measurement and a correction factor is required to calculate
flow. There is a loss of accuracy at submergence, and it is
not a recommended design.
Two elements are involved in obtaining a flow measurement
with a Parshall flume. The primary element is the Parshall
flume structure and the secondary element is the level
measuring device. For additional information on methods to
measure level refer to Section 5.0 of this handbook.
C. Accuracy and Repeatability
The accuracy of a measurement derived using a Parshall flume
depends on the combination of the accuracies of the primary
(flume) and secondary (level measurement) elements. For a
correctly fabricated and installed flume the estimated
accuracy of the depth discharge equation is ^_3% of flow. A
combined flume and level measurement accuracy of +_5% of flow
is attainable with repeatability to _+l/2% of flow.
Additional sources of error, if uncorrected, can decrease the
accuracy of the flow measurement. These include:
1. Deviations of the throat width from standard dimensions.
2. Longitudinal slope of the floor in the converging
section. Tests on a 0.75 m (3 in) flume demonstrated
that a downward sloping floor produced added errors of 3
to 10% from low to high flow conditions.
3. Transverse slope of the flume floor.
4. Approach conditions which do not produce a smooth flow
with uniform velocity distribution parallel to the
center line of the flume.
5. Incorrect zero reference of the level measurement device
to the center line elevation of the crest.
6. If a stilling well is used, the connector hole is
improperly sized.
7, Incorrect zero reference of the level measurement device.
135
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D. Manufactured Options
Although Parshall flumes can be constructed on site, most
flumes used in wastewater applications are prefabricated
structures or liners for setting in concrete. Some available
options include:
1. Material of construction.
2. Stilling well connection.
3. Attached stilling well.
4. Depth gauge integrally mounted in the converging section
sidewall.
5. A cavity for a characterized capacitance level measuring
probe molded into the flume sidewall.
6. Removable bubbler tube installed in the sidewall.
7, A large flume with a smaller one mounted internally.
The smaller flume is removed when flow exceeds its
capacity.
E. Installation
1. Construct or install the flume so that the floor section
is level longitudinally and transversely.
2. Establish the flume floor's elevation to prevent
submergence conditions at maximum flow.
3. Plan the flume installation to allow access for
inspection of the flume to ensure correct elevation and
leveling of the floor.
4. Provide an approach channel long enough to create a
symmetrical, uniform velocity distribution and a
tranquil water surface at the flume entrance. A general
rule is that 10 channel widths of straight run should
exist upstream of the flume inlet.
P. Designer Checklist
Ose the following checklist when designing or reviewing
Parshall flume applications. All items should be verified
affirmative for a correct application and installation.
1. Has the smallest practical Parshall flume been selected
for the anticipated range of flows?
a. As a guide, the maximum flow expected should fall
within 70-100% of the maximum capacity for the
selected flume size (1, 3).
136
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b. A depth of at least 0.15 m (0.5 in) should exist at
the minimum actual flow,
2. Is the flume floor elevation high enough (relative to
'downstream conditions) to prevent submerged flow?
3. Does a sufficient straight run of pipe or channel exist
upstream?
4. Will the upstream flow be non-turbulent and wave free?
5. Do any downstream obstructions exist which could cause a
restriction to the discharge of the flume?
6. Is a depth gauge included for calibrating the flume?
7. Is the level sensor correctly located-on the flume?
8. Is the level sensor zero correctly referenced?
9. If required for the application, is a stilling well
provided?.
10. The following items pertain to the stilling well:
a. Is the vertical height extended below and above the
anticipated operating depths in the flume?
b. Is the flume opening for the stilling well sized
large enough to avoid sensor lags and plugging?
c. Has the flume opening been correctly located on the
length of the converging section wall?
d. Is the flume opening positioned below the lowest
flow operating level?
e. If the flume is used to measure raw sewage or mixed
liquor flow, has a water purge been piped to the
stilling well?
G. Acceptance and Performance Monitoring
A depth gauge mounted on the converging section is
recommended so manual readings and flow calculations can be
made to check remote flow indicators or recorders.
Other methods for flow verification and testing are described
in Section 7.1.
137
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H. Maintenance and Calibration
1. Parshall flume.
a. Weekly, check the depth gauge with other level flew
indications for the flume to determine if
calibration of the secondary system is required.
b. Periodically, wipe down the flume walls to remove
slime or other buildup.
c. Check for bottom deposits and remove as required.
d. Quarterly, check the zero of the reference depth
gauge.
e. Semiannually, examine the flume surfaces for signs
of deterioration and wear.
2. Stilling well (if used).
a. Periodically, check for solids accumulation and
clean as necessary (establish the interval between
checks by experience).
b. If a water purge is used, check it daily and adjust
the flow rate as needed.
I. Deficiencies
The following problems are encountered in existing Parshall
flume installations.
1. Insufficient straight channel upstream, resulting in
non-uniform velocity distribution through the throat.
2. Sloping floor in the converging section.
3. Low flume floor elevation (relative to downstream
channel level) resulting in submerged flow condition.
4. Measuring the depth of the hydraulic head at the wrong
location.
5. Using an incorrect equation for calculating flow from
level.
6. No provisions for purging the stilling well when
measuring a solids-bearing stream (buildup of deposits
in the stilling well renders the level sensor
inoperable).
138
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7. The foundation is not water tight, allowing leakage
under or around the flume.
8. Flume sized too large for the operating flow range.
J. References
1. Grant, D. M. Open Channel Plow Measurement Handbook.
ISCO, Inc., Lincoln, Nebraska, 1981, Second Sdition.
2, Liptak, 3. G. and K. Venczel. Instrument Engineers
Handbook of Process Measurement. Chilton Book Company,
Radnor, Pennsylvania, 1969, Revised, 1982.
3. Water Measurement Manual. U.S. Department of the
Interior, Bureau of Reclamation, U.S. Government
Printing Office, 1981, Second Edition."
4. Kulin, G. Recommended Practice For The Use of Parshall
Flumes and Palmer-Bowlus Flumes In Wastewater Treatment
Plants. EPA-600/2-84-186. Environmental Protection
Agency, Cincinnati, Ohio, November, 1984.
5, Instrumentation In Wastewater Treatment Plants. Manual
of Practice No. 21, Water Pollution Control Federation,
1978.
139
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4.4 WEIR
A. Application
A weir is used to measure flow in open channels where the
water is free of suspended solids. Consider the following
general conditions when selecting a weir for the primary flow
element.
1. Water quality should be equal to secondary effluent or
better.
2. Sufficient hydraulic head exists so a weir can be used.
Typically, the head loss of a rectangular weir is four
times that of a Parshall flume of equal size at the same
flow.
3. Flow rates vary over a large range. A range of flows of
20 to 1 can be tolerated by most weirs. For weirs
larger than 2.5 m (8 ft), rarges of 75 to 1 are
reported. These wide ranges are not recommended.
4, The approach conditions insure that at all flow rates
the flow is tranquil, free of eddies or surface
disturbance. Under maximum flow, approach velocities in
the upstream channel should not exceed 10 cm/s (4 in/s).
TABLE 4.7. WASTEWATER TREATMENT FACILITY APPLICATION GUIDELINES
Recommended Not Recommended
Secondary Effluent Raw Sewage
Primary Effluent (with Mixed Liquor
provisions for sluicing) Sludge
B. Principle of Operation
A weir is a dam or bulkhead placed across an open channel
with an opening on the top through which the measured liquid
flows. The opening is called the weir notch; its bottom edge
is called the crest. Normally the notch is cut from a metal
plate and attached to the upstream side of the bulkhead.
This is done to prevent the water from contacting the
bulkhead and is known as a sharp-chested weir. See
Figure 4.8. Weirs without a plate where the water contacts
the bulkhead are known as a broad-crested weir. This
presentation concentrates on sharp-crested weirs.
140
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CRESTS
WEIR
PLATE
RELIEVE TO
ASSURE FREE FALL
NAPPE
BULKHEAD
Figure 4.8. Sharp-crested weir.
The water depth measured at a prescribed distance upstream
can be used to determine the discharge through the weir.
Characteristic head versus flow relationships are governed by
the weir geometry. All level measurements are made relative
to the crest elevation.
The weir openings are normally fabricated in rectangular,
trapezoidal, or V-notch shapes. A trapezoidal weir with a
side slope of 4:1 is known as a Cipolletti weir. Figure 4.9
illustrates the weir shapes. Flow as a function of upstream
head (h) is expressed by empirical equations. See Figure
4.8. General equations for each weir shape are given below.
These are covered in more detail in the technical references
(1, 2).
141
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END
CONTRACTIONS
^.
CREST -v
•*>:
RECTANGULAR
(C1POLLETTI) TRAPEZOIDAL
END
-CONTRACTION
9- 60* OR 90*
TYPICALLY
V-NOTCH
Figure 4.9. Weir shapes.
The following equations show the relationship between flow and the
measured head.
For a rectangular weir:
Q = K(L-0.2H)H3/2
For a Cippoletti weir:
Q- KLHV2
For a V-notch weir:
Q = K tan 1/2 -6- H&/2
Where:
Q » Rate of flow.
L » Crest length.
H » Head of flowing liquid.
-*• » V-notch angle in degrees.
K » Constant dependent the units of flow.
142
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A special rectangular weir without end contractions can be
installed with the sidewalls of the channel forming the ends
of the weir. This is known as a suppressed weir and is shown
in Figure 4. 10. When this type weir is applied, an air vent
must be installed to allow free access of air beneath the
nappe for free flow.
AIR
VENT
Figure 4.10. Suppressed rectangular weir.
V-notch weirs are suitable for flows up to 17 kl/m (4500
gpm). Rectangular and Cipolletti weirs are capable of
measuring much higher flows than the V-notch wier.
C. Accuracy and Repeatability
Accuracy of .+2% for the head versus flow relationship is
attainable. However, the weir is a primary element, and the
accuracy of flow indicated or recorded flow is also dependent
on the secondary elements; i.e., level sensor and flow
converter. Therefore, for a properly installed weir and
level sensing secondary, it is reasonable to expect a _+_5%
flow measurement accuracy. Refer to Instrumentation Part 7.0
for more information on selecting and installing level
transmitters.
To obtain the best accuracy possible when designing and
installing a weir, observe the following factors:
1. The minimum head (see Figure 4.8) should be 6 cm (2.5
in) or greater.
2. The maximum head is less than one-half the height of the
weir.
3. When using rectangular or Cipolletti weirs, the maximum
head is less than one-half the crest length of the weir.
143
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4. Rectangular and Cipolletti weir crests snould be level.
5. Use a V-notch weir for low-flow measurement.
6. All edges and corners of the weir must oe sharp.
7. Weir edges should be straight, smooth, and free of burrs.
i
8. The approach channel should be straight and of uniform
cross-section for a length equal to at least fifteen
times the maximum head on the weir.
9. The channel should have a free fall of 15 cm (6 in)
downstream of the weir.
Manufactured Options
The weir is normally fabricated for each installation.
Nonetheless, it may be desirable to consider some optional
features when fabricating a weir such as:
1. Level sensor stilling well,
2, Depth gauge referenced to the crest elevation, and
3. A sluicing slit with cover located at the bottom of the
bulkhead for flushing out solids that may collect behind
the weir.
Installation
1, Make the upstream face of the bulkhead and weir plate
smooth, and install it in a vertical plane perpendicular
to the axis of the channel.
2. Insure the crest is level for rectangular and Cipolletti
weirs. For a V-notch weir, insure the bisecting line of
the V is vertical.
3. Cut the V-notch weir angle precisely and mount the plate
so the angle is bisected by a vertical line.
4. Machine or file the weir edges to be straight and free
of burrs. Chamfer the trailing edge to obtain a crest
thickness of 1-2 mm (0.03 - 0.08 in).
5, Install the weir so the distance from the weir crest to
the bottom of the approach channel is the greater of 30
cm (12 in) or two times the maximum head.
6. Design the weir so the end contractions on each side
(except suppressed weirs) will be a minimum of 30 cm (12
in) or two times the maximum head.
144
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7, Provide air vents under the nappe on both sides of a
suppressed rectangular weir.
8. Rectangular weir sides must be straight up and down.
9. Slope the side of a Cipolletti weir outward 1 horizontal
to 4 vertical.
10. Make the crest length of rectangular and Cipolletti
weirs at least three times the maximum upstream head.
11. Construct the bulkhead opening approximately 3 cm (3 in)
larger on all sides than the weir notch.
12. Slope the top of the bulkhead down to assure that the
nappe falls free without hitting the bulkhead.
13. Locate the level sensor next to the sidewall so it can
be easily reached,
14. Position the level sensor upstream of the weir at least
four times the maximum head to avoid the effect of the
drawdown.
15. Install the depth gauge and level sensor so the zero
reference elevation is the same as the weir crest
elevation.
F, Designer Checklist
Consider the following when designing or reviewing weir
applications. All checklist questions should be answered
"yes."
1. Is the level sensor located upstream at least four times
the maximum weir head?
2. Is the maximum downstream liquid level at least 6 on
(2.5 in) below the elevation of the crest?
3. Is the cross-sectional area of the approach channel at
least eight times the cross section of the water
overflowing the crest at maximum flow?
4. Is the approach channel straight and of uniform cross
section for a length at least fifteen times the maximum
head?
5. If a suppressed rectangular weir is being used, has an
air vent been provided under the nappe on both sides of
the channel?
6. At minimum flow, does the head above the crest exceed
6 cm (2.5 in)?
145
-------
7. Is the weir notch sized and shaped so the nappe will
clear the bulkhead and fall free?
8. Is the length of the bulkhead end contractions on each
side of the weir opening at least two times the maximum
head above the crest or 30 cm (12 in), whichever is
larger?
9. Is the height of the weir crest above the channel bottom
greater than twice the maximum head or 30 cm (12 in),
whichever is larger?
10, Is the length of the rectangular or Cippoletti wei~
crest at least three times the-maximum head?
11. Is the maximum velocity in che approach channel less
than 10 cm/s (4 in/s)?
12, Is a depth gauge installed so periodic checks can be
made on the level sensor?
G. Acceptance and Performance Monitoring
A depth gauge mounted adjacent to the level sensor is
recommended. Make periodic level readings and determine the
flow by calculation or from a lookup table. Compare remote
flow readings with the value determined from the visual
inspection as a conformance check on the calibration of the
level sensor and flow converter.
Other methods for flow verification and testing are described
in Section 7.1.
H. Maintenance and Calibration
1. Weekly check:
a. The level sensor with other level or flow
indicators to determine if calibration of the
secondary system is required.
b. Accumulation of bottom deposits and remove as
required.
c. Stilling well (if used) for solids accumulation and
clean as necessary.
2. Annual check:
a. Crest level,
b. Reference to zero on the depth gauge,
146
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c. Flush fitting of the weir plate to the bulkhead,
d. Leaks around the wier,
e. Weir notch for nicks, dents, and rounding of
upstream corners.
I. Deficiencies
The following problems have been encountered in existing weir
installation.
1. Insufficient head during low flow conditions so there is
no free air space under the nappe-
2. Suppressed rectangular weirs without air vents under the
nappe.
3. Insufficient relief on the bulkhead so the nappe strikes
the bulkhead interfering with the free fall.
4. Pool level downstream is too high so insufficient free
fall exists.
5. Weir notches cut from metal plate stock and installed
without the edges finished to proper thicknesses, shape,
or straightness.
6. Rectangular or Cipolletti weirs installed without
leveling the crest.
7. V-notch weirs with incorrectly cut angles.
8. Level sensor located too close to the weir.
9, Level sensors or gauges not zero referenced to the
bottom of the weir notch.
J. References
1. Water Measurement Manual. U.S. Department of the
Interior, Bureau of Reclamation, U.S. Government
Printing Office, 1981, Second Edition.
2. Grant, D. M. Open Channel Flow Measurement Handbook.
ISCO, Inc., Lincoln, NebrasKa, 1981.
147
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5.0 LEVEL MEASUREMENT
5.1 BUBBLER LEVEL MEASUREMENT
A. Applications
Bubbler level measurement instruments are used throughout
the wastewater treatment process for measuring both liquid
level and differential liquid level.
Bubblers are frequently used to sense the hydraulic head
created by flumes and weirs in open-channel flow
measurement. A special signal converter indicates flow
based on the level sensed by a bubbler. This section of
the manual addresses the use of bubblers in open tanks
which is applicable to open-channel flow measurement.
TABLE 5.1. BUBBLER LEVEL MEASUREMENT APPLICATION GUIDELINES
Recommended Not Recommended
Liquid treatment processes Digesters
Volatile chemical storage tanks
B. Principle of Operation
An open-ended pipe, called the bubbler tube or dip tube,
is connected to an air supply and positioned in the
process so that the open end is set at a reference level.
A constant air rate-of-flow regulator is used to maintain
air in the tube with enough excess to continually bubble
out the open end. Thus, the air pressure in the pipe is
equal to the head of the process liquid above the
reference level.
A pressure transmitter connected to the bubbler tube
measures the pressure of the dip tube. For water, the
level is equal to the pressure sensed by the transmitter.
For measuring other liquids, the transmitter must be
calibrated for that liquid's specific gravity. For closed
tanks, a differential pressure transmitter is used, with
the high-pressure port connected to the bubbler tube and
the low-pressure port connected to the gas space in the
top of the tank. A schematic of the bubbler application
is shown in Figure 5.1.
148
-------
LOW
PRESSURE
AIR
PURGE
AIR
PURGE
OPEN TANK OR
OP. CH. FLOW ELEMENT
CLOSED TANK
AIR
PURGE
BAR SCREEN
DIFFERENTIAL LEVEL
Figure 5.1. Typical bubbler applications.
Because of head loss caused by air flow in the tube and
connecting pipe, pressure at the transmitter will not be
exactly the same as at the open end of the bubbler tube.
This difference in pressure necessitates minimizing pipe
and fittings between the rate-of-flow air regulator and
the dip tube.
Airflow head is affected by bubble formation. To minimize
errors, the bottom of the tube usually has a notch or an
angular cut to produce a continuous stream of small
equally sized bubbles. Buildup of process solids on the
end of the tube will alter bubble formation; therefore,
the tube end must be kept clean.
149
-------
The air supply rate is controlled by a pressure regulator
and a flow control valve. Typical airflow rates are 8-30
cc/s (1-4 cfh). Use a purge-rotameter to adjust the airflow.
The air supply can be from instrument air, plant air,
compressed gas tanks, or dedicated bubbler compressor.
For applications requiring infrequent level readings, a
hand-operated pump can be used.
C, Accuracy
The accuracy of a level measured by a bubbler system is
dependent on the uncertainty of the pressure measuring
device (see Section 6.2), process fluid specific gravity,
head loss in the bubbler system, barometric pressure, and
the temperature of both the process fluid and the bubbler
system air. Whrle errors of less than +0.1% can be
achieved, ac<~.racy is typically _+Q.5-l% of full scale.
0. Repeatability
Repeatability is dependent on variances from standard
conditions of any of the uncertainties listed above in
paragraph C.
E. Manufacturer's Options
1. Automatic purge cycle.
A separate timer and valve control package for
periodic cleaning of the bubbler tube- A high
pressure air purge removes buildup of material at the
end of the tube when the pressure measuring device is
momentarily isolated from the system and full air
supply pressure is applied to the bubbler tube at
periodic intervals from 8 to 24 hours.
2. Air supply.
Bubbler systems can be furnished with a dedicated air
supply consisting of:
a. A compressor, or where extra reliability is
required, two compressors with automatic
failover. Intermittent duty compressors,
capable of producing the high pressure purge
required, range from 180 W (1/4 hp)-370 W (1/2
hp). Adequate purge pressure for most
wastewater applications is about 500 kPa (60
psig).
150
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b. An air dryer.
c. An air filter.
d. A pressure tank with a capacity of about Q
(2 gal.).
3. Enclosure.
a. NEMA 1, general purpose.
b. NEMA 4, watertight.
4. Purge gases.
Where the oxygen contained in an air supply system is
objectionable, nitrogen or another inert gas may be
substitute^. For explosive, volatile, or hazardous
atmospheres complete intrinsic safety can be achieved
by using a pneumatic pressure signal for remote
indication.
5. Weatherization.
Usually a thermostatically controlled heater.
6. Alarms.
Available alarms include low air flow, low air supply
pressure, purge-in-progress, and compressor failure.
P. Installation
The bubbler tube should be rigidly supported at a
convenient location in the tank. The opening of the tube
is the lowest level that can be detected, so set the tube
depth at or below the lowest level at which a measurement
is needed. Notch the tube opening to produce a continuous
flow of small bubbles.
Fabricate the bubbler tube from 1.25 cm (1/2 in.) diameter
stainless steel tubing o^ galvanized pipe. Properly
supported, this makes a rigid installation which can
withstand turbulence and wave action. A tee with one
branch plugged, when installed on top of the bubbler tube,
provides an opening for a cleaning rod when the high
pressure air purge cannot remove bubbler tip restrictions.
151
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The bottom of the tube should be at least 8 cm (3 in.)
from the cank bottom to avoid solids buildup on the tank
floor. This offset must be included in the zero reference
level for the liquid in the tank. An exception to this is
a bubbler installed in a flume or ahead of a weir. In
flume applications the bubbler tip must be at the same
elevation as the flume floor or if elevated/ the degree of
elevation must be compensated for in the flow
calculation. In weir applications the bubbler tip must be
at the same elevaf'on as the bottom of the notch, or if
below the notch, the degree of offset must be compensated
for in the flow calculation.
To minimize level measurement errors caused.by air flow
head loss, the air flow controller must be mounted as
close to the dip tube as possible and connected with a
minimum of fittings an^. tubing. For 1 cm (1/4 in.)
tubing, the distanc«=> from the air purge regulator to the
bubbler tube should not exceed 15 m (50 ft).
To ensure that the air purge tubing is free from traps
where moisture condensate can collect, install the tubing
with a continuous downward slope from the pressure
transmitter and the air flow controller to the bubbler
tube.
In open-tanks and flumes, for periodic reference checks
and to facilitate recalibration if the tube is removed for
cleaning or replacement, install a depth (staff) gauge in
the tank at a location visible from the dip tube. Zero on
this gauge must correspond to the bubbler tube's zero
reference.
A typical installation schematic is shown in Figure 5.2.
Maintenance access is needed for the clean-out tee and for
the bubbler system enclosure. Installation of the
differential pressure transmitter is addressed in Section
6.2.
TRANSMITTER /LENGTH LESS
THAN ISM (SOFT)
CLEANOUT
PLUG
INSTRUMENT
AIR SUPPLY
Figure 5.2. Open tank bubbler installation.
152
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G. Designer Checklist
Ask the following questions when designing or reviewing
bubbler level meter applications. All checklist questions
should be answered "yes."
1. Can air be passed through the process fluid? If not,
can another gas, such as nitrogen, be substituted for
air?
2. Is the tank open or vented? If not, is accumulation
of air acceptable?
3. Are head losses from the air flow regulator to the
bubble tube minimized?
4. Is the purge line length from the air flow regulator
to the bubbler tube less than 15 m (50 ft)?
5. For process streams containing more than about 100
mg/1 suspended solids, is automatic purging included?
6. Does the air supply reliability match the need for
level measurement reliability?
7. Is the bubbler tube mounted securely?
8. Is the clean-out tee accessible?
9. Is the bubbler enclosure suitable for its environment?
10. For open tanks, does the pressure transmitter
reference the same gas space? For example, a bubbler
tube mounted outside and a transmitter mounted inside
a building will be exposed to different atmospheric
conditions. Can this difference be tolerated?
H. Maintenance and Calibration
Maintaining and calibrating the differential pressure
transmitter is presented in Section 6.2.
Task Frequency
1. Check Air flow. Daily, or if the unit has a
low-flow
alarm detector, only at
calibration.
153
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2. Clean tube. Weekly purge foe solids-bearing
fluids. Manual cleanout at
calibration. Manual cleanout when
tube is depressurized and fluid
enters.
3. Check air filter. Weekly. If the unit has a
low-flow alarm detector, then
check filter only at calibration.
4. Calibrate. Every two months,
5. Inspect Depends on type and .size of
compressor. compressor. Follow the
manufacturer's recommendation.
I. Deficiencies
The following problems are commonly reported for bubbler
systems.
1. Compressor failure, and measurement loss,
2. Tube bubbler opening does not stay clean because no
purge or cleanout is available.
3. Condensate collection trapped in the purge tubing.
4. Purge tubing lines too long or too small, creating
excessive head losses.
5. The bubbler tube tip does not correspond with the
desired zero level reference.
J. References
1. Liptak, B. G, and K. Venczel. Instrument Engineers
Handbook. Chilton Book Company, Radnor,
Pennsylvania, 1969, Revised, 1982.
2. American Petroleum Institute. Manual On Installation
of Refinery Instruments and Control Systems, Part I,
Section 6 - Level. Washington, DC, 1974, Third
Edition.
154
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5.2 CAPACITANCE PROES
A. Applications
Capacitance probes are used to measure liquid levels throughout
wastewater treatment plants. For tiiis discussion, two kinds of
probes are identified: capacitance and capacitance with
compensation for coating. The compensated capacitance probes
nave additional electronics to offset material buildup on the
probe.
Special probes are available that produce a signal proportional
to flow in open channel flumes and weirs. These probes are
characterized to match the head/flow relationship of an open
channel primary element. Characterization is accomplished
either of two ways: one, by electronic calculation, or two, by
variation of probe insulation thickness in a manner that
produces a direct, linear relationship between capacitance and
flow. Consult with manufacturers on special capacitance probes
for direct flow measurement.
TABLE 5.2. UNCOMPENSATED CAPACITANCE PROBES
WASTEWATER TREATMENT FACILITY APPLICATION GUIDELINES
Recommended Not Recommended
Potable water Primary treatment
Non-coating chemicals Secondary treatment
Tertiary effluent Tertiary treatment
Solids handling
Polymer solutions
Lime slurry
TABLE 5.3. COMPENSATED CAPACITANCE PROBES
WASTEWATER TREATMENT FACILITY APPLICATION GUIDELINE
Recommended Not Recommended
Most aqueous solutions Liquids where heavy grease buildup
Raw sewage could occur
Secondary effluent
Capacitance probes can also be used to measure the level of dry
material. However, this discussion deals with only wastewater
treatment solution applications.
155
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B. Principle of Operation
A capacitor can be described as two electrically conductive
plates separated by a nonconductive material. A probe is
usually constructed to form one plate of a capacitor. The
other plate is the tank wall or the measured solution. Between
the probe and the tank wall is an air space above the liquid
surface and water below. As the water level rises, the
effective capacitance of the system increases. This
capacitance is linearly proportional to level and is measured
by a bridge circuit powered by a high frequency, 0.5-1.5 MHz,
oscillator. High frequency can reduce errors due to shorting
of the capacitor by conductive coatings. Sometimes capacitance
probes are referred to as radio frequency (RF) probes because
of this measurement technique.
Water in treatment processes is a good conductor. For this
reason, the probe must be insulated. The insulation's exterior
surface effectively becomes a third plate which complicates the
theory of operation. Figure 5.3 shows one way of illustrating
the system in electrical terms.
The situation becomes more complex if a conductive coating of
process solids accumulates on the probe. Through a combination
of capacitance and conductance effects in the coating, the
probe fails to respond to changes in level below the top of the
coating. The system's effective capacity remains constant
below this point. Thus, for most applications in wastewater
treatment some method of compensating for coatings is essential.
One method is based on assuming that the coating's capacitive
reactance is equal to its resistive reactance. The coating's
resistance is further assumed to be the greatest resistance in
an aqueous system. This resistance is measured and subtracted
from the effective capacitance. The result is proportional to
liquid level, although inaccuracies are introduced depending on
how well the system matches the assumptions made.
The probe is usually a cylindrical rod or cable inserted
perpendicular to the water surface, as illustrated in Figure
5.4.
Probes are available for use in open channel head loss type
flow meters. These probes are flat and shaped to provide a
signal proportional to flow. In either style probe, accuracy
decreases near the bottom because the submerged portion becomes
less and less line the ideal capacitor with plates of infinite
length.
C. Accuracy and Repeatability
For a clean probe, an accuracy of ±1% can be expected. Where
the probe becomes coated, accuracy will degrade to
approximately ±5%.
156
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D. Manufacturer's Options
1. probe type:
a. Rods of any length up to 5 m (20 feet),
b. Cables of any length up to about 50 m (150 feet) with
weight or anchor to Keep probe in place,
c. Plat probes for open channel flow meters, and
d. Proximity plate for non-contact with process fluids.
2. Transmitter enclosure:
a. NEMA 3 - weatherproof, or
D. NEMA 4 - watertight.
3. Indicating meter.
4. Output:
a. 4-20 mAdc, or
b. 10-50 mAdc.
5. Probe materials:
a. 304 stainless steel,
b. 316 stainless steel,
c. Teflon insulator,
d. polyvinylidene fluoride insulator, or
e. PolyvinyIchoride insulator.
6. Grounding rods.
7. Concentric probe. Required for non-conductive liquids and
for some installations to provide grounding or shielding
from process liquid turbulence.
8. Radio.frequency interference protection.
157
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STAINLESS STEEL ROD OR CABLE
PROBE INSULLAT1ON-
H
H
Figure 5.3. Illustration of probe/tank capacitive relationship.
158
-------
FLEXIBLE -x
PROBE
(ANCHORED)
-PROBE HEAD
STRAIGHT
PROBE
FLEXIBLE
PROSE
(WEIGHTED)
Figure 5,4. Capacitance level sensor.
159
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E. Installation
1. Install the probe as follows:
t
a. Isolate from vibration and possible physical damage.
b. Do not mount in the direct stream of process flow.
If necessary, install baffles or stilling well.
c. Mount vertically.
d. Mount at least 15 cm (6 in) from tan* wall to lessen
chances of material buildup.
e. Mount so that probe can readily be removed for
cleaning, inspection, or calibration.
2. Most manufacturers require the transmitter be within
75 m (250 ft) of the probe. The cable must b« furnished
by the capacitance probe manufacturer and snould not be
shortened in the field without consulting the manufac-
turer. Mount -the transmitter close enough to the
probe to see indicator change as level changes to allow
one person to calibrate the system. If the transmitter
is mounted further than this, provide a junction box
near tha probe. Allow enough cable between the junction
bc:c and probe for removing the probe. Provide storage
space for excess cable. Cable from the junction box to
tha transmitter should be in regid conduit and from the
junction box to the probe in flexible conduit.
3. Refer to tfte installation shown in Figure 5.5.
4. In an open tank, install a deptn (staff) gauge in the tank
at a location visible from the transmitter so periodic
checks can be made on the calibration, zero on this gauge
must correspond to the probe's zero reference.
-------
IIS VOLT 6O HZ
POWER SUPPLY
SIGNAL.
CABLE
IN RIGID
CONDUIT
MANUFACTURER'S RECOMMENDED
3-CONDUCTOR TWISTED.
SHIELDED CABLE
PROBE
HEAD
FLEXIBLE CABLE
TO EASE INSPEC-
TION a MAINTENANCE
INSERTION LENGTH
MOUNTING
BRACKET
TO SUIT
THE APP-
LICATION
COATING AROUND THE
L
NOTE(S):
I) MOUNT THE PROBE TO MINIMIZE
MOUNTING THREADS.
2) MOUNT THE PROBE SO IT IS NOT IN THE DIRECT STREAM
OP A FILLING CHUTE OR NOZZLE. IF REQUIRED. USE DE-
FLECTING BAFFLES.
3) USE CARE DURING INSTALLATION TO PREVENT ACCIDENTAL
DAMAGE TO THE PROBE INSULATION
Figure 5.5, Capacitance probe and transmitter installation.
161
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P. Designer Checklist
ASH the following questions when designing or reviewing
capacitance level measurement applications. All checklist
questions should be answered "yes."
1. is the process stream free from heavy grease? If not,
capacitance probes are not recommended.
2. Is coating of the probe likely? If yes, then coating
compensation is essential.
3. Is the tank or tank wall grounded? If not, then provide
grounding.
4. is the probe mounted securely, without providing potential
sites for solids buildup?
5. Can the probe be removed easily for inspection, cleaning,
and calibration?
6. Is the meter installation designed so that it can be
calibrated by one person?
7. Is the transmitter protected from the weather?
8. Has a depth gauge been installed for quick calibration
checks?
G. Maintenance and Calibration
Task Fr equency
^••MMHB |
1. Clean probe. Depends on application.
2. Calibrate probe. Once a month to once every two months.
H. Deficiencies
The following problems are often encountered in capacitance
probe applications.
1. Coating causes meter to measure level inaccurately. Clean
probe more frequently.
2. Tank not grounded signal noise and calibration drift.
I. References:
1. Liptak, B. G. and K. Venczel. Instrument Engineers
Handbook. Chilton Book Company, Radnor, Pennsylvania,
1969, Revised, 1982.
2. Schuler, E. A practical Guide to RP Level Controls.
Drexelbrook Engineering Co., Horsham, Pennsylvania, 1981.
162
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5.3 FLOAT LEVEL INSTRUMENTS
A. Application
1.
Float type level indicators are often used in wastewater
treatment plants if remote readout is not needed.
2. Float type level switches are generally used in wastewater
treatment processes for alarms and equipment on/off
control.
TABLE 5.4. FLOAT LEVEL INDICATORS
HASTEWATER TREATMENT FACILITY APPLICATION GUIDELINE
Recommended
potable water tanks
Fuel tanks
All process streams
Not Recommended
Process streams
B. Principle of Operation
1. Level indicator.
A float level indicator consists of a float, an attached
rod with pointer, float guide and indicator scale. These
components are shown in Figure 5.6. As the float rides up
or down on the liquid surface, the pointer indicates the
level.
ROD
GUIDES
FLOAT
INDICATOR SCALE
Figure 5.6. Simple float level indicator.
163
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Another type of float level indicator is shown in Figure
5.7. In tiiis case, float movement is indicated by the
counterweight position.
CONNECTING CABLE
STILLING WELL-
FLOAT
^- INDICATOR
X; SCALE
COUNTERWEIGHT
Figure 5.7. Counterweighted float level indicator.
Floats can also be connected to transmitters for remote
monitoring. However, this arrangement is seldom used.
Some other type of level meter such as bubbler,
capacitance, or sonic is used instead.
2. Level switch.
Float switches depend on the liquid's bouyant force to
activate the switch. In one switch position the float is
bouyed up by the liquid; in the other position the float
hangs down in the absence of liquid.
A, wide variety of float devices exist which translate the
float position into electrical on/off signals. The
Instrument Engineers' Handbook(D nas a summary of
them. Of these devices, the majority of wastewater
treatment plant applications use the tilt switch type
shown in Figure 5.8. Each switch is a bouyant bag with a
mercury switch inside. When the bag is tilted from one
position to the other, the mercury switch opens or closes
an electrical circuit. This circuit activates a relay to
provide the contacts necessary for local controls and
remote monitoring.
164
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B.
Figure 5.8. Float switches.
Accuracy and Repeatability
Accuracy for float devices monitoring quiescent liquids can be
+0.6 cm (1/4 in). Turbulence has an adverse affect on
accuracy. Most float switch (tilt type) installations leave
8-16 cm (3-6 in) of cable between the bag and the tie down to
allow freedom of movement of the bag for maximum sensitivity.
Turbulent conditions have been observed to cause inaccuracies
greater than 10 cm (4 in).
Solids buildup on guides and floats will degrade the accuracy
of float indicators. Therefore, applications having potential
for turbulence or solids buildup are not recommended.
Manufacturer's Options
1. Controller enclosure:
a. NEMA 12, dustproof, and
b. NEMA 4, weatherproof.
2. Output:
a. DPDT contacts rated at 5 amp, 120 VAC.
Installation
A recommended installation method for float switches is shown
in Figure 5.9. If turbulence is expected, also install a
stilling well.
The installation figure shows the floats permanently fixed to a
specific level. This is true of most float switch
installations. The switch setting cannot be easily changed, so
carefully design initial placement.
-------
JUNCTION BOX
r
CONDUIT RUN
RIGID CONDUIT
CONDUIT SEAL
STAND PIPE-RIGIDLY FASTENED
TO TANK WALL
FLOAT SWITCH ASSEM8LIES-
< LOCATED TO PROJECT RE-
QUIREMENTS
Figure 5.9. Float switch mounting installation.
F. Designer Checklist
The following questions snould be asked when designing or
reviewing applications of float level instruments. All
checklist questions should be answered "yes".
1. Will the float be bouyant at process pressure?
Temperature?
2. Is the float protected from turbulence?
3. Is the output matched to its intended uses?
4. Are switch settings accurately known?
5. Are switches sheltered from strong air currents?
166
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G. Maintenance and Calibration
1. Level indicators:
Task Frequency
a. Calibration. Depends on mechanical or electrical
linkage to indicator.
b. stilling well Weekly to bi-weekly.
cleaning.
2. Level switches:
Task Fr equency
a. Inspection and Every six months.
operational check.
b. Stilling well Weekly to bi-weekly.
cleaning.
H. Deficiencies
The following problems are commonly reported for float level
devices.
1. Stilling well cleaning requires too much attention.
2. Switches give false trips due to turbulence.
3. Solids buildup on the float changes the calibration.
I. References
1. Liptak, B. G. and Venczel, K. Instrument Engineers
Handbook. Chilton Book Company, Radnor, Pennsylvania,
1969, Revised, 1982.
2. Instrumentation in Wastewater Treatment Plants; Manual of
Practice No. 21. Water pollution Control Federation, 1978,
167
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5.4 SONIC AND ULTRASONIC LEVEL SENSORS
A. Application
Sonic and ultrasonic level sensors do not contact the process
fluid; therefore, they can be used in any wastewater treatment
process, provided that process vapors do not cause problems.
The two most common vapor problems are corrosion, which can be
lessened by choice of materials, and condensate or ice buildup
on cold sensors which can be prevented by heaters.
TABLE 5.5. SONIC AND ULTRASONIC LEVEL SENSORS APPLICATION GUIDELINES
Recommended
Open channel flow
Wet wells
Not Recommended
Foam
B. Operating Principle •
The sensor periodically generates a pulse of sonic or
ultrasonic waves that bounce off the liquids surface and echo
back. The echo is detected by a resonant metal disc. Based on
the speed of sound or ultrasound, the time between sending and
receiving is measured and converted into distance which is then
converted to level. See Figure 5.10.
SENDER a RECEIVER
y/////////////
Figure 5.10
168
-------
Sonic and ultrasonic wave velocity depends on air conditions:
temperature, pressure and humidity. Where changing conditions
are expected, automatic compensation can be provided.
Typically, only temperature compensation is used; temperature
errors are about 0.2% per °c (0.17% per °F).
Sensors are available with frequencies from about 9 kHz sonic
to about 50 kHz (ultrasonic). Also, the generator can have
different shapes such as wide angle cone, narrow angle cone or
parabolic. Selection of frequency and sensor shape are both
based on the amount of attenuation expected. ?or example, one
manufacturer provides:
• Wide angle cones for up to 3m (10 ft).
• Narrow angle cones for up to 10m (30 ft).
• Parabolic reflectors for up to 25m (80 ft).
Signal attenuation can be caused by absorption into the air,
reflection away from the receiver's sensing area, and
absorption by foam on the liquid surface. The cone shapes
listed above are selected to reduce attenuation by reflection.
Distance and wave frequency affect attenuation by absorption.
As distance from the sensor to the liquid level increases,
signal strength decreases in proportion to the distance
squared. Thus, if signal strength is 100% at distance "d" when
a tank is full, the signal strength will drop to 25% at a
distance of *2d".
Sonic waves attenuate less then ultrasonic. For example, foam
on liquid surfaces may completely absorb ultrasonic waves.
C. Accuracy and Repeatability
Accuracy of _+!% of span and repeatability of +0.1% of span can
be obtained. Air conditions, liquid turbulence, foam and
interfering echos from obstructions can reduce both accuracy
and repeatability of the sensor.
D. Manufacturers' Options
1. Sensor wave frequency:
a. Sonic.
b. Ultrasonic.
2. Sender shape:
a. Wide cone.
b. Narrow cone.
c. Parabaloid.
169
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3. Sensor temperature compensation.
4. Sensor thermostatically controlled heater.
5. Sensor air purge.
6. Sensor range selection.
7. Indicator on transmitter.
8. Transmitter output:
a. 4-20 mAdc into 750 ohms maximum, isolated.
b. 0-20 mAdc into 750 ohms maximum, isolated.
9. Transmitter enclosure:
a. NEMA 1,
b. NEMA 4, and
c. NEMA 12.
E. Installation
1. Range.
Determine the range of the meter from expected conditions
in the tank or channel. See Figure 5.11. The mounting
location of the sensor is then calculated from
restrictions established by the manufacturer. Generally,
the sensor must be above the maximum level by at least
some minimum distance, usually about 50-70 cm (18-24 in).
In Figure 5.11, this would correspond to an "A-MINIMCM"
value. The distance from the sensor to the lowest level
measured must be less than its maximum rating for "C*, and
the ratio of "A:C" must be less than the manufacturer's
specification.
The sensor must be mounted far enough from tank walls to
prevent false echoes. This distance, "D", depends on
sender shape. Calculations for correct sensor location
will differ for each mam-'acturer.
170
-------
.100% FULL-
0% FULL
A = Distance from generator/ceceiver to 100% full level.
B « Measured range, distance from 0 to 100% full.
C » A & B.
D * Distance from tank wall.
Manufacturer's set limits on dimensions of "A", "C", "D* and the ratio of "A"
to "a."
Figure 5.11. Installation dimensions.
2. Stilling well.
A stilling well is used with sonic/ultrasonic sensors to
dampen out liquid level turbulence, reduce foam, increase
signal strength (essentially producing a cylindrical-
shaped sensor), eliminate noise from icray echos or to
lessen condensate problems. When used, the stilling well
should be cut from a single length of PVC pipe 15-20cm
(6-8 in.) in diameter. The bottom end should be cut at a
450 angle.
Drill air relief holes near the top where the sensor is
attached.
171
-------
Keeping the stilling well clean is a must; accumulated
solids can cause echos that she transmitter will read as
liquid level. Therefore, provide for either manual or
automatic washdown of the well interior wall.
3. Transmitter.
The transmitter location depends on the intended method of
calibration. The transmitter may be remotely located as
much as 200m (700 ft) away provided that:
a. The sensor is equipped '• -th a calibration bar, or
b. The tank can be isolated from feed and exit streams
and the level manually raised (or lowered) , or
c. The liquid level in the tank can be manually observed
at the transmitter location by an independent method.
F. Designer Checklist
Ask the following questions when reviewing or designing a sonic
ultrasonic level meter application. All answers should be
"yes."
1. Is temperature compensation provided? If not, is the
degraded accuracy acceptable?
2. Is condensation unlikely to occur on the sender/receiver?
If it is likely, then is an air purge and/or heater
provided?
3. Can a stilling well be avoided?
4. Can the sensor be mounted so that the full range of
expected levels are within the manufacturer's
specifications for minimum and maximum distances?
G. Maintenance and Calibration
Task Frequency
1. Calibration. Every two months.
2. Check temperature Every two months.
compensation.
3. Stilling well Depends on process stream.
clearing.
172
-------
H. Deficiencies
The following problems have been reported for. sonic and
ultrasonic level meter installations.
1. Stilling well causes interfering echos from pipe joints or
deposited solids or grease. Maintaining a clean stilling
well is difficult.
2. Meter does not read on cold days because receiver is
covered with ice.
173
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6.0 ?S ES S UR S M E AS OR El S NT
S.I PRESSURE CELLS
A. Application
Pressure meters are applied to enclosed process lines such as
compressed air distribution systems, pump discharges, and
tanks. With the aid of isolation diaphragms, pressure
transmitters can be successfully applied to any wastewater
treatment process.
TABLE 6.1. PRESSURE MEASUREMENT APPLICATION GUIDELINE
Recommended Recommended with Isolation Diaphragm
Air Chlorine
Oxygen and ozone Wastewater with s»lids
Digester gas Sludge
Water
-Secondary effluent
B, Principle of Operation
1. Mechanical pressure elements.
The three most common elements used to indicate pressure
are: Bourdon tubes, bellows, and diaphragms. In each
case process pressure causes the element to move in
proportion to the pressure applied. This motion is
amplified by a mechanical linkage connected to a pointer
and dial or by electronics to a voltage or current
signal. Schematic, diagrams of each type are- shown in
Figure 6,1.
a. Bourdon tube.
A Bourdon tube is a curved tube sealed at the tip.
As process pressure increases inside the tube, the
tube will straighten causing the tip to deflect.
The deflection is transferred to a dial indicator
by mechanical linkage. Besides a C-shaped tube,
Bourdon tubes are available in spiral, twisted, and
helical forms, round, oval or rectangular in cross
section.
174
-------
BOURDON
C-TUBE-.
DIAL
MECHANICAL
LINKAGE
PROCESS
PRESSURE
MECHANICAL
LINKAGE
DIAL
MECHANICAL
LINKAGE
RESTORING
BELLOWS
PROCESS
PRESSURE
DIAL
DIAPHRAM
PROCESS
PRESSURE
Figure 6.1. Mechanical pressure elements.
b. Bellows.
Bellows elements are deeply corrugated metal
cylinders closed at one end. Process pressure
applied to the bellows causes it to expand.
Bellows expansion is transferred to a dial
indicator by mechanical linkage. Bellows are also
configured to contract on increasing pressure, in
some cases restoring springs are added to increase
operating range or to reduce element wear.
175
O
-------
c. Diaphragms.
Diaphragms are metal disks, either flat or
concentrically corrugated. Process pressure
applied to one side causes the diaphragm to deflect
outward. Diaphragm deflection is transferred to a
dial indicator by mechanical linkage. Corrugated
diaphragms are capable of greater deflection and
are more linear than flat diaphragms.
2. Electro-mechanical elements.
Electric signals proportional to pressure are obtained
by mechanically connecting an electrical component such
as a capacitor, strain gauge, or inductor to a
diaphragm. Deflection of the diaphragm will change the
associated electrical property, e.g., distance between
plates in a capacitor, piezoelectric response, and loop
reluctance.
In some elements a restoring force is applied to the
diaphragm to keep it undeflected. This eliminates
nonlinearity due to diaphragm deflection. The restoring
force is measured electrically and converted to
pressure. These elements are called force-balance
transducers.
3. Gauge, differential, and absolute pressures.
Gauge, differential, and absolute pressures may be
measured by any pressure element depending on the
reference pressure. Gauge pressure is measured using
atmospheric pressure as reference. In this case,
Bourdon tubes, bellows and diaphragms are constructed to
have access to the atmosphere on the side opposite the
process connection.
See Section 6.2 for information on differential pressure
and absolute pressure elements.
4. Vacuum measurement.
Vacuum pressures for slight vacuums, to -90 kPa, can be
measured with elements similar to those discussed
above. At low vacuums these elements become inadequate
because of gravitational interference in force
measurement near zero atmosphere. Usually, low vacuums
are measured indirectly from some other property of gas
such as thermal conductivity, viscosity, or the behavior
of gas during ionization or electrical discharge. Low
vacuums are not monitored in wastewater treatment
therefore, their measurement is not explained here.
176
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C. Accuracy and Repeatability
Quartz Bourdon tubes can be accurate to £0.01% of span, which
is as accurate as a quality manometer. On-line pressure
transmitters are accurate to better than +0.5% of span.
Pressure gauges are typically accurate to +1 to 2% of span.
Typically, repeatability of measurement is about one-fifth of
the stated accuracy.
D. Manufacturer's Options
1. Ranges - almost any range is available,
2. Materials (wetted):
a. Brass,
b. Bronze (or phosphor bronze),
c. Beryllium copper,
d. Stainless steel,
e. Monel, and
f. Hastelloy-C.
3. Transmitter output signals:
a. 4-20 mAdc into 650 ohms,
b. 10-50 made into 250 ohms, and
c. 1-5 VDC isolated.
4. Power:
a. Two-wire transmitters are the most common
configuration; they require external 12 Vdc or 24
Vdc power supplies, and
b. Four-wire transmitters which require line power
(115 VAC, 60 Hz).
5. Transmitter enclosure:
a. NEMA 4, and
b. Explosionproof.
6. Isolation diaphragms of the same wetted materials listed
for the elements are available from some manufacturers.
Diaphragm, transmitter, and connection line can be
furnished as a complete assembly.
177
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E. Installation
1. Install the transmitter in an environment that meets the
specifications listed by the specific manufacturer.
This is usually -20 to 65oc (0-15QO?) and 0-95%
relative humidity. Zero and span will shift with
ambient temperature, so avoid temperature extremes or
calibrate at conditions equal to the installed
environment,
2. Install the transmitter as close as possible to the
process measurement site. This will reduce response
time which can be important in flow control or level
control applications. The installation must allow good
maintenance access. In some cases it will not be
practicable to install the transmitter to meet both
nearness and maintenance criteria. In these situations,
control requirements must be given first priority.
3. Connect meter runs to liquid process lines
horizontally. This will minimize the amount of solids
and gas entering the connection. Entrapped gas will
decrease response time and solids may plug the meter
connection. Slope meter runs 8 on per meter (1 in. per
foot) of run so that gas bubbles bleed back into the
process line.
4. Connect meter runs to gaseous process lines at the top
of pipes or tanks to minimize the amount of solids and
moisture entering the connection. Slope meter runs at
least 8 cm per meter (1 in. per foot) of run so that
condensation will drain into the process line. Any low
spots in the meter run will require a condensate
collection pot. Heat trace meter runs on condensable
gases. Entrapped liquids may affect meter accuracy and
may cause accelerated corrosion.
5. For applications where the measured liquid contains
solids, flushing provisions or diaphragm isolation may
be needed. Diaphragm connections to the process should
be a minimum of 2.5 on (1 in.) for sludge lines and 1.3
cm (0.5 in.) diameter for other wastewater lines.
6. Install an isolation valve at the process measurement
connection on all meter runs. If this valve is not
readily accessible for maintenance, install another
isolation valve at the transmitter. See Section 6.2 for
manifold requirements.
7. Materials recommended for harsh environments are:
a. Chlorine - Hastelloy-C, and
b. Digester gas - 316 stainless steel.
178
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?. Designer Checklist
Ask the following questions when designing or reviewing
pressure meter applications. All checklist answers should be
"yes."
1. Is the meter situated for adequate response time and
good maintenance access?
2. Are meter runs installed to keep out interfering
substances?
3. Can the meter be calibrated in place?
4, Is the meter in a suitable environment?
G. Maintenance and Calibration
Task Frequency
1. Calibration. Every three to six months. Meters
used in critical control applications
may need more frequent calibration.
H. Deficiencies
•
The following problems are commonly reported for pressure
transmitters:
1. Meter installed in an inaccessible location,
2. Meter runs incorrectly installed, and
3, Diaphragms or flushing not provided on sludge lines.
I. References
1. Gillum, Donald R. Industrial Pressure Measurement. ISA
Publications, 1982,
2. Hewson, John E. Process Instrumentation Manifolds. ISA
Publications, 1981.
3. Measurement & Control Pressure/Force Handbook and Buyers
Guide, 1983. Measurements & Data Corporation, 1982.
179
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5.2 DIFFERENTIAL SRSSSURE
A. Application
Differential pressure transmitters, £p cells, are used with
primary elements to measure flows, gauge pressure, and liquid
level. With the aid of isolation diaphragms or purge
systems, -p cells can be successfully applied to any
wastewater treatment process.
TABLE 6,2. DIFFERENTIAL PRESSURE APPLICATION GUIDELINE
Recommended Recommended with Isolation Diaphragm
Air Chlorine
Oxygen and ozone Wastewater with solids
Digester gas Sludge
Water
Secondary effluent
B. Principle of Operation
1. Mechanical pressure elements.
The three most common elements used to indicate pressure
are: Bourdon tubes, bellows, and diaphragms. In each
case, the element moves in proportion to differential
pressure. This motion is amplified by mechanical
linkage to a pointer and dial. Schematic diagrams of
each type are shown in Figure 6.2.
a. Bourdon tube
A Bourdon tube is a curved tube sealed at the tip.
As process pressure increases inside the tube, the
tube straightens, causing the tip to deflect. The
deflection is indicated on a dial by mechanical
linkage. Besides a C-shaped tube, Bourdon tubes
are available in spiral, twisted, and helical
forms, round, oval, or rectangular in cross section.
b. Bellows.
Bellows elements are deeply corrugated metal
cylinders closed at one end. Process pressure
applied to the high side of the bellows causes it
to expand. Bellows expansion is converted to
pointer and dial indication. Bellows are also
configured to contract on increasing pressure. In
some cases restoring springs are added to increase
operating range or reduce element wear.
180
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HIGH
DIAL
DIAL-
HIGH
LOW
MECHANICAL
LINKAGE
DIAL
HIGH
LOW
Figure 6.2. Mechanical differential pressure elements.
c. Diaphragms
Diaphragms are metal discs, either flat or
concentrically corrugated. High and low process
pressures are applied to opposite sides of the
diaphragm. This causes the diaphragm to deflect.
A mechanical linkage connects the diaphragm to a
pointer for dial indication. Corrugated diaphragms
allow larger deflection and better linearity than
flat diaphragms.
181
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2. Electro-mechanical elements.
Electric signals proportional to differential pressure
are obtained by mechanically connecting an electrical
component such as a capacitorf strain gauge, or inductor
to a diaphragm. Deflection of the diaphragm will change
the associated electrical property, e.g., distance
between plates in a capacitor, piezoelectric response,
and loop reluctance.
In some elements a restoring force is applied to the
diaphragm to keep it undeflected. This eliminates
nonlinearity due to diaphragm deflection. The restoring
force is measured electrically and converted to
differential pressure. These elements are called
force-balance transducers.
3. Absolute pressure elements.
Absolute r-<2ssure elements are differential pressure
elements with the low pressure side evacuated to -101.3
kPa and sealed from atmosphere.
C. Accuracy and Repeatability
Quartz Bourdon tubes have reported accuracies of _+0.01% of
span, which is equal to the accuracy of a good quality
manometer. On-line differential pressure transmitter
accuracy is equal to or better than +_0.5% of span.
Differential pressure gauges typically have accuracies of
0,5% to 2% of span.
Repeatability of measurement for differential pressure gauges
and transmitters is commonly about one-fifth of the rated
accuracy.
D, Manufacturer's Options
1. Ranges - almost any range is available,
2. Materials (wetted):
a. Brass,
b. Bronze (or phosphor bronze),
c. Beryllium copper,
d. Stainless steel,
e. Monel, and
f. Hastelloy-C.
182
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3. Transmitter output signals:
a. 4-20 mAdc into 650 ohms,
b. 10-50 maCC into 250 ohms, and
c. 1-5 VDC isolated.
4. Power:
a. Two-wire transmitters are the most common; they
require a separated 12 Vdc or 24 Vdc power supply,
and
b. Four-wire transmitters which require line power
(115 VAC, 60 Hz).
5. Transmitter enclosure:
a. NEMA 4,
b. Explosionproof, and
c. Both available with insulated jackets or boxes.
6. Isolation diaphragms of the same wetted materials are
available from some manufacturers. Diaphragms,
transmitter, and connection line can be furnished as a
• complete assembly.
7. Square root extractor.
8. Scales:
a. Linear, and
b. Square.root.
E. Installation
1. Install the transmitter in an environment recommended by
the manufacturer. This is usually -20 to 65Oc
(0-150°F) and 0-95% relative humidity. Zero and span
will shift with changes in temperature, so avoid
temperature extremes.
2. Install the transmitter as close as possible to the
process measurement site to reduce response time which
can be important in flow control or level control
applications. The installation must allow easy access
for maintenance. In some cases it will not be practical
to install the transmitter to meet both nearness and
maintenance criteria. In these situations, control
requirements must be given first priority.
183
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3. For solids bearing liquid process lines connect meter
runs horizontally, do not connect meter runs to the
upper quadrant of the pipe. This will minimize the
amount of solids and gas entering the connection.
Entrapped gas will decrease response time, and solids
may plug the meter connection. Slope meter runs 8 cm
per meter (1 in. per foot) of run so that gas bubbles
bleed back into the process line.
4. Connect meter runs to gaseous process lines at the top
of pipes or tanks to minimize the amount of solids and
moisture enteri g the connection. Slope meter runs at
laast 8 cm per meter (1 in. per foot) of run so that
condensation will drain into the process line. Low
spots in meter runs should be avoided. If they cannot,
add drain pots to these low spots. Heat trace meter
runs on condensable gases. Entrapped liquids will
affect meter accuracy and may cause accelerated
corrosion.
5. Special precautions must be taken in steam applications
to prevent overheating of the manifold and transmitter.
Side mount pressure taps to allow steam into the tap
while still allowing drainage of excess condensate back
into the process pipe. A condensate pot should be
installed on each meter lead. To avoid overheating,
blowdown valves should not be incorporated in the
manifold.
In steam applications, a water seal is required between
the condensate pot and the manifold. This prevents
steam from reaching the manifold and transmitter and
prevents uncontrolled buildup of condensate in the meter
leads. The condensate pots must be identical in size,
the same height above the transmitter, and self-draining
to the process pipe.
6. For applications involving solids bearing liquids,
flushing provisions or diaphragm isolation may be
needed. Diaphragm connections to the process should be
a minimum of 2.5 cm (1 in.) for sludge lines.
7, Install an isolation valve on all meter runs at the
process measurement site (pressure tap), and except for
very short tap lines, at the transmitter.
8. Materials recommended for harsh environments are:
a. Chlorine - Hastelloy, and
b. Digester gas - 316 stainless steel.
184
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INSTRUMENT
EQUALIZE
IMPULSE PIPE
OR TUBING
CALIBRATION a
TEST
INSTRUMENT
TEST PORTS
PLUG
ISOLATION
PROCESS
EQUALIZE
IMPULSE PIPE
OR TUBING
ISOLATION
PROCESS
INSTRUMENT
EQUALIZE
PROCESS—^
TEST PORTS
PLUG
ISOLATION
IMPULSE PIPE
OR TUBING
CALIBRATION a
BLEED
INSTRUMENT
EQUALIZE
IMPULSE PIPE
OR TUBING
SLOWDOWN
ISOLATION
PROCESS
Figure 6.3. Manifolds.
185
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9. Differential pressure transmitters are usually installed
with valves that enhance some combination of instrument
calibration, blowdown of accumulated material in the
metec piping, and isolation of the transmitter. An
economic way to provide the desired functions is to use
factory-made valve manifolds. Some common manifolds are
shown in Figure 6.3. Available manifold options are:
a. Number and configuration of valves:
1) 3-valve: isolation and equalization,
2) 5-valve: isolation, equalization, and
calibration,
3) 5-valve: isolation, equalization, and
blowdown, and
4) 7-valve: isolation, equalization,
calibration, and blowdown.
b. Process connections:
1) Pipe: 1/2" NPT female and 3/8" MPT female, and
2) Tube: 3/8" and 1/2".
c. Transmitter connections:
1) Pipe: 1/2" NPT female,
2) Direct - flanged, and
3) Tube: 3/8" and 1/2".
d. Materials of construction:
Same selection as for transmitter.
e. Remote zeroing:
1) Motor operated 3-valve manifold.
Wastewater treatment applications typically use 3-valve
manifolds which implies that a check of meter zero is
sufficient and that a check of the span is not required
at a frequent enough interval to justify the expense of
a more complex manifold. Span calibration test
frequencies of three to six months for most applications
confirms this practice. Where Ap cells are part of a
flow measurement system used as a standard, or for
billing purposes, the frequency of calibration and
testing may be much more often. In such cases it is
recommended that 5-valve manifolds be installed to
reduce calibration setup time.
186
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10. A common use of -ip cells is with primary flow elements
to measure pressure-drop for flow calculation.
Special installation practices foe flow applications are
presented with the primary device, see orifice meters,
venturi meters.
11. Another common usage of Ap cells is to measure liquid
levels. Two general methods are used: hydrostatic head
and bubbler (dip tube). Bubbler installations are more
common in wastewater treatment processes; they are
discussed in Section 5.1. Figure 6.4 shows typical
hydrostatic level installations for an enclosed tank.
The tank is covered, so the A p cell must use the
pressure of the vapor phase as a reference. The Ap cell
is at the same level as the bottom pressure tap, and the
connection to the top tap is vapor filled or "dry."
Thus, the difference in pressure is proportional to
liquid level. If the tank was open, a plumbing
connection to the reference side of the dp cell would
not be necessary. The Ap cell would just need to be at
the same ambient pressure, i.e., in the same room with
the tank or both the Ap cell and the tank outdoors.
A "wet leg" configuration for level measurement is shown
in Figure 6.5. The reference side of the Ap cell is
filled with a liquid. It does not have to be the same
as the tank liquid. If the liquid is not the same, the
difference in specific gravity of the two liquids must
be used to correct the meter calibration. The liquid in
the wet leg prevents unwanted accumulation of condensate
at the reference side. In this configuration the
reference (wet leg) is connected to the low-pressure
side of the Ap cell, just as in a dry leg setup. As
shown, the meter would read -100% at '0' liquid level
and '0'% at 100% liquid level. This is corrected during
calibration by supressing zero to provide a correct tank
level indication.
F. Designer Checklist
Ask the following questions when designing or reviewing
differential pressure meter applications. All checklist
answers should be "yes,"
1. Is the meter situated for adequate response time and
good maintenance access?
2, Are meter runs installed to keep out interfering
i substances?
3, Can the meter be calibrated in place?
4. Is the meter in a suitable environment?
187
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VAPOR
LIQUID
VALVES IN OPERATING MODE
X OPEN >-^CLOSED
GAGE GLASS
AP
TRANS
n
-tx-
Figure 6.4. Dry leg.
VAPOR
LIQUID
VALVES IN OPERATING MODE
X OPEN >~4 CLOSED
GAGE GLASS
AP
TRANS
-cx-
FILL
WET LEG
LVENT
Figure 6.5. Wet leg.
188
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G. Maintenance and Calibration
Task Frequency
1. Calibration. Every three to six months. Meters
used in critical control applications
may need more frequent calibration.
H. Deficiencies
The following problems are commonly reported for pressure
transmitters:
1. Meter installed in an inaccessible location,
2. Meter runs incorrectly installed, and -
3, Diaphragms or flushing not provided on sludge lines.
I. References
1. Hewson, John E. Process Instrumentation Manifolds. ISA
Publications, 1981.
2. Manual on Installation of Refinery Instruments and
Control Systems, Part I - Process Instrumentation and
Control, Section 1 - Flow, and Section 2 - Level.
American Petroleum Institute, 1974.
3. Measurement & Control Pressure/Force Handbook and Buyers
Guide, 1983. Measurements & Data Corporation, 1982.
189
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PERFORMANCE TESTING
7.1 FLOW
A. General
Perform quality assurance testing to satisfy stipulated
acceptance requirements and to provide a cal* Cation
reference point for a particulai flow meter. Perform
quality control monitoring as an on-going practice to
maintain a continuous indication of meter reliability.
1. Even if a flow meter is factory calibrated prior to
delivery, perform an in-place calibration when the
meter is first installed to satisfy the stimulated
acceptance requirements and to establish a
calibration reference to use for quality control
monitoring and periodic maintenance calibration. Two
types of calibration procedures are presented:
a. Hydraulic calibration.
b. Non-hydraulic calibration.
2. Prior to calibration, be sure that the flow meter is
properly installed and that it is given a preliminary
checkout according to the manufacturer's instructions.
B. Non-Hydraulic Calibration
1. During the process design phase, layout equipment and
piping to allow in-place hydraulic calibration and
acceptance testing for all flow meter installations.
In certain situations this may not be feasible, and
calibration testing may be performed using
"simulator-type" calibration equipment offered as an
option by many meter/transmitter manufacturers.
2. Use the simulator method of calibration only when
complete hydraulic testing cannot be performed and
when there is no uncertainty about the proper
operation and performance of the primary metering
device. Obtain agreement from all parties involved
as to the validity of this type of testing before
acceptance.
3. With simulator-type calibration, the flow signal
produced by the primary metering element, is
simulated and input to the converter/transmitter to
set the zero and span.
190
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C. Hydraulic Calibration
1. Zero the meter and correct any offset according to
manufacturer's calibration instructions.
2. Set up a flowrate through the meter that is within
the anticipated working range (design range) of
flows. Allow enough time for flow to become steady,
then measure the reference or "true" flowrate (£%)
using one of the methods in Part C of this section.
During the time Q% is being measured, measure the
indicated flow rate through the meter. Use this
flowrate (Q) to calculate the percentage"of error
utilizing the following equation:
%Error = (Q - QO/& X 100
Utilize the above procedure for a minimum of three
flowrates, low, medium and high, within the design
range of the meter. Next, plot the calculated
percentage error against the reference flowrate, Cft
(additional test runs are recommended to ascertain
the repeatability of the meter) . Between each test
run, perform a zero check to determine any short term
drift.
Also, calculate and plot the percentage error in
measuring ^ vs. CR on the same plot. These
values will depend upon which reference method in
Part D is used and may be referenced elsewhere (2 -
9).
Next, draw a curve between the points of each of the
percentage error groups (Q and ^). if the
percentage difference between the curve for Q and
Ql is greater than the accuracy stipulated for that
type of meter (as noted in previous sections of the
handbook or alternative specifications), the
performance of the meter may be unsatisfactory and
the following options should be considered:
a. If the plot shows a constant percentage error
for Q, a span adjustment at the meter
transmitter is required following the
manufacturer's instructions. Repeat the testing
to verify correct span.
191
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b. If meter performance cannot be adjusted to meet
purchaser's specifications, repair or reject
under acceptance stipulations.
c. If meter data differences can be ascribed to
non-ideal installation conditions (e.g.,
inadequate approach piping for mag meters) and
the data is otherwise repeatable, the results of
the calibration tests can be used to develop a
new, in-place reference for the meter.
6. Regard the preceding error comparison method
described above as a suggested procedure, only. Other
comparison procedures agreed to by the involved
parties may be used to carry out initial calibration
and acceptance testing. The main consideration for
any comparison scheme, however, is to use the
reliability (percentage error) of the reference
measurement to determine the accuracy of the tested
meter.
D. Flow Measurement Methods for Hydraulic Calibrations
1. The following general methods for measuring reference
flow (Qjj) used in determining the measurement
accuracy of hydraulically tested flow meters are
briefly described:
a. Volumetric.
b. Comparison with a reference flow meter.
c. Dilution.
d. Salt velocity.
e. Velocity area.
These methods vary in difficulty and in accuracy.
The method selected will be determined by the type of
meter being calibrated, the type of liquid being
measured, and the resources available to conduct the
test.
2. Volumetric calibration.
The feasibility of the volumetric calibration
(drawdown) method depends primarily on the
availability of suitable tank space and connecting
conduits. Important considerations for this method
are:
192
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a. Conduct the test using the process liquid to be
measured under normal flow meter operation.
b. This method is very suitable for measuring
wastewater sludges.
c. The potential accuracy of this method is high.
d. The tank should be regularly shaped so its
volume can be calculated within acceptable
limits of accuracy.
e. The tank volume should be large enough to
provide a test run long enough to make start and
finish timing errors, negligible.
f. The change in liquid level in the tank should be
enough so that starting and finishing depths can
be measured without introducing significant
error.
g. The flow rate should remain relatively constant
during the test run.
Estimating the percentage error for this testing
method should include an estimate of errors
introduced due to physical mesurements of tank
volume, depth change, and the elapsed time of the
test.
3. Comparison with a reference meter (transfer meter).
a. A reference meter so described is a
flow-measuring device whose performance
characteristics can be referenced to published
standards or to recommended practices acceptable
to involved parties. Examples include:
1) Standard venturi tubes and venturi nozzles
(2,3,4).
2) Orifice plates (2,3).
3) Parshall flumes (4,5).
4) Thin plate weirs (5).
193
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b. Important considerations regarding this method
include:
1) The flow meter(s) used as reference devices
must meet all requirements of accepted
standard practices in fabrication
installation and use. In most wastewater
treatment plant applications, conformance
to these requirements, especially for
installation and use, is difficult.
2) Use of differential pressure type flow
meters requires pressure differential
measurement with a U-tube mercury manometer.
3) When standard weir or flume methods are
used, a point gauge must be used for head
measurement.
4. Dilution method.
a. With the dilution method the flow rate is
deduced from the dilution of measurable
properties of tracer chemicals added to the flow
(turbulent) in known amounts. The tracer can be
injected either in a constant rate or in a
one-shot slug. The constant-rate method is more
suitable in wastewater treatment plant
applications. Consult reference 7 for greater
detail of this method.
b. In the constant-rate injection method, a tracer
solution of accurately known concentration is
injected upstream at a constant, accurately
measurable, rate. At a downstream distance
sufficient to achieve complete mixing and a
steady-
state tracer concentration, the flow is
sampled. The tracer concentration is determined
and used to calculate the flow rate (6).
c. Important considerations regarding this method
include:
1) The tracer property measured must be
conservative. Rhodamine WT has been used
successfully in raw sewage, however, its
behavior in sludges is not known.
194
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2) Accurate measurement must be made of the
rate at which the tracer is added, and the
initial and final concentrations of the
tracer.
3) A high sensitivity spectrophotometec is
required for this method.
5. Salt-velocity method.
a. In the salt-velocity method, bcine is injected
suddenly at an upstream station in such a way
that it rapidly becomes distributed across the
pipe section. The time of passage of the salt
pulse between two downstream stations is
measured by conductivity-sensitive electrodes.
The flowrate may then be determined if the
volume of the conduit between the electrodes is
accurately known. Consult References 3 and 8
for details.
b. Important considerations regarding this method
include:
1) • The best attainable accuracy for this
method is 1%.
2) The process liquid being.measured must have
a significantly smaller conductivity than
the brine solution.
3) The method is not satisfactory for use with
raw sewage or sewage sludges because of
conductivity fluctuations; however, it is
suitable for treated effluent.
6. Velocity-area method.
a. This method is applied to a flow cross-section
by measuring a number of velocities over the
section, each representative of the average
velocity within an incremental area, and then
summing the resulting velocity-area products.
This method can be applied to both open and
closed conduit flows; but it is more
conveniently employed in accessible open
channels. Consult Reference 9 for further
information.
195
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b. Important aspects of this method include:
1) The individual velocity components can be
measured by point velocity measuring
instruments, e.g., current meters, pitot
tubes, or by acoustic velocity meters that
measure an average velocity component along
a line path.
2) The point velocity meters are intrusive and
may not work well with raw sewage and
sewage sludges.
3) The velocity sampling requirements are
lengthy and this method is suitable only
where long periods of steady flow are
available.
E. Performance Monitoring
1. In addition to carrying out acceptance and
calibration testing, conduct performance monitoring
on an on-going basis for those flow meters which
provide a flow rate value important to plant
operation (this would include almost every flow meter
in a wastewater treatment facility). Performance
monitoring can provide plant personnel with a quick
indication of meter performance by using a secondary
instrument which measures the flow to an intermediate
accuracy of 5 to 10% of the actual flow. Types of
measurements which can be used for flow meter
performance monitoring include:
a. Measuring pressure difference.
b. Manufacturer-prepared rating curves and tables.
2. Measuring pressure difference.
a. Measuring of a pressure difference at a location
where the flow rate has a unique and repeatable
relationship to the primary flow meter provides
a common and adequate means for monitoring meter
performance. The pressure difference may be
measured around any pressure differential-
causing hydraulic element, with 90° pipe
elbows being most commonly used.
196
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b. This type of monitoring should be available
prior to acceptance and calibration testing of
the primary flow meter, so that a relationship
between the pressure monitor and the flow rate
indicated by the meter can be established from
the testing results.
c. Consider the following for the use, placement,
and operation of pressure taps to monitor
pressure difference.
1) Mount the taps so they are flush with the
pipe inner surface and free of burrs.
2) Hole diameter is not extremely critical;
diameters from 1/64 inch (0.5 cm) to 3/8
inch (1.0 cm) for small to large pipes are
usually adequate.
3) Install the taps along the axial canter of
pipe elbows. For other locations, install
them both up- and downstream of the device
producing the pressure difference.
4) Multiple taps with piezometric rings are
not recommended for use with typical
wastewater treatment process streams; use
single taps, placed upstream and downstream.
5) Locate single taps in horizontal or
near-horizontal process lines in a
horizontal diametric plane to minimize gas
and/or solids entry into the measurement
lines.
d. Consider the following aspects of the
differential pressure sensing device and
connecting manometer tubing when setting up the
monitoring system:
1) When U-tube manometers are used the medium
selected should be appropriate for the
magnitude of the pressure difference
expected, e.g., water-air for small
differentials and water-mercury for large
differentials so that a minimum deflection
of 3 inches should be maintained.
197
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2) When commerical differential pressure cells
are used, they should be frequently
calibrated to a liquid column manometer.
3) Route manometer (tap line) connecting
tubing from the tap location to the sensing
device to avoid accumulation of gasses
and/or solids, «.g., sloping horizontal
lines, bleed valves at high points, etc.
4) The connecting tubing should be corrosion-
resistant with a nominal diameter of 3/8
inches. Provide it with valving to
facilitate periodic flushing.
3. Manufacturer prepared rating curves and tables.
Approximate flow measurements suitable for
performance monitoring may also be obtained from
manufacturer-prepared pump rating curves and flow vs.
angle of opening data for 'butterfly valves. These
measurements would be subject to inaccuracies caused
by installation and/or operational factors but none
the less provide an easy means of ascertaining gross
meter inaccuracies.
F. References
1. National Bureau of Standards. Recommenced Practice
'For The Use Of Electromagnetic Flow Meters In
Wastewater Treatment Plants. EPA, Municipal
Environmental Research Laboratory, Cincinnati, Ohio,
August, 1980.
2. International Standards Organization. Measurement of
Fluid Flow by Means of Orifice Plates, Nozzles and
Venturi Tubes Inserted in Circular Cross-Section
Conduits Running Full. ISO/DIS 5167, draft revision
of R781, 1976.
3. American Society for Testing and Materials. Standard
Methods of Flow Measurement of Water by the Venturi
Meter Tube. ASTM D2458-69.
4. American Society for Testing and Materials. Standard
Method for Open Channel Flow Measurement of
Industrial Water and Industrial Wastewater by the
Parshall Flume. ASTM D1941-67.
198
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5. British Standards Institution. Methods of
Measurement of Liquid Flow in Open Channels.
Standard No. 2680-4A, Part 4A, 1965.
6. International Standards Organization. Measurement of
Water Flow in Closed Conduits—Tracer Methods, Part
I: General. ISO No. 2975/1, 1974.
7. International Standards Organization. Part II:
Constant Rate Injection Method Using Non-radioactive
Tracers. ISO No. 2975/2, 1974.
8. Hydraulic Institute. Standards for Centrifugal,
Rotary and Reciprocating Pumps. 12th edition.
9. International Standards Organization. Liquid Flow
Measurement in Open Channels — Velocity Area
Methods. ISO 748, 1976.
199
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8.0 PUMP CONTROL METHODS
8.1 VARIABLE FLOW SERVIC3
A. Introduction
This section explains some ways pumps are used to control
wastewater treatment flows. Where pertinent to control,
principles of pump operation are included. Subjects such as
sizing, mechanical details, and manufacturer's options are
not included in this discussion. For further information on
these subjects, see the references listed at the end of the
section.
Three classes of pumps are presented here:" metering,
positive displacement, and centrifugal. Although metering
pumps use the positive displacement principles, they are
considered separately because they form a major subgroup of
^positive displacement pumps.
B. Metering Pumps
1. Applications
In wastewater treatment plants, metering pumps are used
to add polymer or other chemicals to clarifiers for aid
in settling or for precipitation of pollutants, to add
chemicals to boiler makeup water, to add chemicals for
dewatering of sludges, to add acid or base for pH
control, and in other similar ways.
In an open-loop control system, the pump is usually
either adjusted manually to tne desired rate of addition
or paced to the main process flow. Manual adjustments
are recommended only for processes that operate at a
constant or nearly constant flow. When addition is
paced to the process flow, the addition rate is
calculated as follows:
where:
D » dosage, mg/1
Q » main process flow, 1/min
QA » chemical addition rate, I/rain
p « density of chemical, g/1
200
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?or variable stroke pumps, the stroxe length is:
L = D • Q
SAL
where:
S = pump speed, stroKes/min
A = piston area, cm2
L = stroke length, cm
And for variable speed pumps, the speed is:
S - D • Q
LAP
Typically, the flow signal and the pump control signal
are both 4-20 ma, and the dosage is set at an electronic
ratio station. If both speed and stroke are variable,
the practice is to pace pump speed and manually adjust
stroke to set the dosage, thus eliminating the ratio
station. In open-loop applications, errors in stroke
and speed accuracy are not compensated by controller
action.
Accuracy of stroke adjustment is typically stated to be
+1% of full capacity, with linearity and repeatability
'also ±1%. In terms of absolute error, stroke adjustment
at constant discharge pressure would perform as follows:
Stroke Pump Speed
% Full Strone Absolute Error Absolute Error
100% +1% 0%
50% +2% +1%
25% +4% Tl%
10% +10% +1%
Error is also introduced by a change in motor speed
resulting from a change in load. Another 4-1% absolute
error is added to the stroke error for 50% stroke and
less. During the life of a pump, error will increase
due to wear on moving parts that allow hydraulic leaks
and metered liquid leaks. . Another source of error is
entrainment or buildup of gas that causes some piston
displacement to be negated by compressibility of the
gas. Mass flow rates will be affected by changes in
metered liquid density (e.g., due to temperature
changes).
201
-------
Closed-loop systems involve a process measurement
downstream of the chemical addition point. A controller
uses this measurament to vary the metering rate to match
the process aetpoint. Errors in speed or stroke
adjustment are corrected oy the controller. In
closed-loop applications the process requirements for
turndown of chemical addition can influence the choice
of variable drive. Variaole stroke pumps can turndown
100% to 0% of rated pump output, although strone
accuracy would normally limit turndown to 10:1.
Variable speed pumps on the other hand, are typically
limited by the type of drive from 3:1 turndowns to about
10:1 turndowns. Examples of open-looo and closed-loop
applications are shown in Figure 8.1.
PRESSURE
ROTARY
PUMPS
RECIPROCATING
PUMPS
FLAT
STEADY
RISE
STEEP
RISE
FLOW
Figure 8.1. Typical centrifugal pump curves, at constant speed.
202
-------
2. principle of Operation
Metering pumps are positive displacement type pumps of
either the reciprocating or rotary type. Reciprocating
metering pumps have two general methods to vary pump
output: variable speed or variable stroke. Stroke on a
variable speed pump is adjusted by either varying the
crank travel directly (amplitude modulation) or by
varying the amount of fixed-crank travel transmitted to
the piston.
Amplitude modulation can also be achieved by using a
slider-crank in which the length of a pivot arm (or
eccentric) is adjusted. Adjusting the pivot arm to zero
length results in zero piston travel. Design of
slider-crank mechanisms vary by manufacturer. (An
example is shown in Figure 8.2.)
Another method to modulate amplitude is by the
shift-ring drive in which the piston rotates in a ring
that can be positioned.
HIGH
SPEED
WORM
ROTATING
CRANK
PLUNGER
DRIVER
CONNECTING ROD
WORM?
GEAR
ZERO STROKE
FULL STROKE
ADJUSTABLE
LINK
CONNECTING
ROD
PLUNGER
DRIVER
ZERO STROKE
WORM
GEAR
FULL STROKE
Figure 8.2. Slider-crank stroke adjustment.
203
-------
Fixed-crank strode adjustments use a lost-motion drive
that limits the piston or diaphragm travel. Motion can
be lost in the transfer from crank to piston as shown in
Figure 3.3.
STROKE
ADJUSTMENT
KNOB
RECIPROCATING
PLUNGER
ECCENTRIC CAM
MECHANICALLY
ACTUATED
DIAPHRAGM
SUCTION
MECHANICAL LOST-MOTION DRIVE CHANGES THE DISCHARGE
FLOWRATE BY VARYING THE PLUNGER RETURN POSITION.
CRANK ECCENTRICITY REMAINS CONSTANT OVER THE ENTIRE
FLOW RANGE.
Figure 8.3 Sccentric cam lost-motion stroke adjustment.
The piston reciprocates as it follows the eccentric cam
until the piston is stopped by the stroxe adjustment
pin. piston travel is resumed when the eccentric
rotates enough to pass the position of the piston stop.
Another lost-motion drive is shown in Figure 8.3." Here
motion is lost between the piston and the diaphragm by
allowing some of the hydraulic fluid to escape to a
reservoir.
204
-------
CONTROL
PLUNGER
1?0TOLOL "^CHARGE
DIAPHRAGM 4
ECCENTRIC
DRIVE
A PORTION OF HYDRAULIC FLUID IS PERMITTED TO ESCAPE
THROUGH A BYPASS VALVE WITH EACH STROKE, THEREBY
CHANGING THE EFFECTIVE STROKE-LENGTH OF THE PLUNGER.
NOTE THE BALANCED-DIAPHRAGM LIQUID END ASSOCIATED
WITH THIS DESIGN
Figure 8.4. Hydraulic lost-motion stroke adjustment.
Variable speed drives on a metering pump can be
eddy-current, Silicon Controlled Rectifier (SCS),
variable frequency, or belt drives using conical pulleys
to vary the ratio of drive to pump speed, variable
speed drives are explained in Section 9.
Metering pumps can have just variable speed, just
variable stroke, or both variable speed and stroke
capabilities. The choice of variable drive depends on
the pump's application.
Another method of metering uses a recycle valve to vary
pump output. Variable rate pumps are more common
because in automatic control situations they can
eliminate the cost and maintenance of a control valve.
The variable rate pumps also provide energy savings.
205
-------
C. Positive Displacement pumps
Positive displacement pumps (p.d. pumps) used in wastewatec
treatment are usually diaphragm, piston, or progressive
cavity types. These pumps are used to pump liquids with high
solids content such as thickened sludges.
Diaphragm and piston pump are large scale versions of the
metering pumps discussed above. Variable speed and variable
stroke control capabilities also apply here, progressive
cavity pumps do not have variable stroke capability; the
rotor and stator are not adjustable.
Control methods for positive displacement pumps are either
open-loop or closed-loop. A typical open-loop system is
sludge withdrawal from a clarifier or thickener. Constant
rate p.d. pumps are controlled by a frequency and duration of
opertion basis (i.e., open-loop). Frequency and duration are
adjusted to give a desired average flow rate as follows:
Q = fxdxSxV
where:
Q = pumped flow, 1/min
f » frequency, starts/rain
d = duration of pumping, min/start
S = pump speed, strokes/min
V = stroke volume, I/stroke
Frequency needs to be often enough to scour sludge lines of
sludge that may have settled out while the pump was off.
Duration needs to be short enough to prevent short circuit or
vortex formation through, the sludge blanket. Frequency and
duration should not be so often and short as to cause
excessive wear and overheating on the motor starter and
windings.
Alterations to this control strategy include:
• Set frequency in terms of volume of clarifier influent
flow. This needs a clarifier influent flow meter.
• Stop pump based on duration or low-density cutoff,
whichever occurs first. This needs a suspended solids
analyzer.
• Start pump at high blanket level and stop at low blanket
level. This requires two blanket level detectors.
i • A ccmbintion of any of the above options.
206
-------
A variacle rate pump can be operated witii the same control
metiiod,-but augmented to ta!
-------
However, just because a pump has a curve that meets the
processes requirements at nigh efficiency, dees not mean it
is the best pump for the application. The designer must also
consider average and maximum horsepower (braise-horsepower)
requirements, net positive suction head, impeller design,
number of stages, volute design, diffuser design, and
mountings. These topics are beyond the scope of this
section, but are presented in the referenced literature.
PUMP
EFFICIENCY
CURVE
p,
PRESSURE
P,,
SYSTEM
HEAD LOSS
CURVES
EFFI-
CIENCY
FLOW
Figure 8.5. Sizing curves foe centrifugal pumps.
The most common flow control method for centrifugal pumps is
with a throttling valve. In design selection, a portion of
the pump head at maximum flow is used for loss at the
throttling valve. This portion may range from as low as 20%
up to 33%. It is set by desired throttling valve gain
characteristics, desired safety factors, and energy
conservation.
208
-------
For small process flows, the portion is often toward the
higher percentage because energy conservation is not as
important as gain and safety factors. The opposite is true
for large process flows. Here, lower percentages, are favored
oecause of lower energy costs. Thus, pump sizing is strongly
affected by the valve size. This js discussed in more detail
in the section on control valves.
The output of a centrifugal pump is also controlled with a
recycle valve. In this case, valve sizing and valve headloss
do not affect pump sizing. To allow total recycle, all the
pump head is lost or dissipated at the valve.
Pump output can also be changed by varying pump speed. A
variable speed drive, such as variaole frequencv drive, eddy
current, or SCR drive, is used to control gump speed. These
and other variable speed drives are explained in the
Section 3. These drives have less energy requirements than
control valves. Due to inefficiencies of a drive, energy
savings do not occur until average pump operation is under a
certain percentage of pump speed. Several sources estimate
this break-even point to be about 80-90% of full speed. A
comparison of variaole speed drives with throttling valves is
shown in Figure 8.6.
PRESSURE
^- PUMP WITH
Jf THROTTLING VALVE
SYSTEM
CURVE
V/S PUMP « • I-
MORE EFFICIENT |
I
L
C/S PUMP WITH
VALVE MORE
EFFICIENT
as
Q
AVE
IOO
Q
MAX
FLOW
Figure 8.6. Variable speed versus throttling of centrifugal pumps.
209
-------
These control methods also apply to multi-pump systems, in
addition, starting and stopping pumps is a way to alter trie
output of a pump station. When more than one pump is in
operation, station output can be calculated from the curves
of the individual pumps. In general, pumps in parallel need
similar curves and will have their capacities added together
which produces a longer, flatter curve, pumps in series will
have their heads added together which results in a higher,
steeper curve.
For example, consider a two pump system, with identical pumps
in parallel. The pump station output is controlled by a
throttling valve. A flow meter on the station discharge is
used to shut off one of the pumps when flow falls below 80%
of one pump's capacity and start a pump when flow rises above
100% of one pump's capacity. The system is shown on Figure
8.7. In this example, head losses at the' throttling valve
are a substantial portion of the pump curve. Could the use
of variable speed pumps save energy somewhere below 100% of
station output?
PRESSURE
100
FLOW.% 0
MAX
Figure 8.7. Two identical pumps in parallel with throttling valve.
210
-------
Now look at the sane pump curves wnere the pumps are variable
speed. Assume that pump turndown is limited to 70% of
maximum speed by the construction of the variable speed
drive. Speed is related to pump performance approximately as
follows:
Flow capacity
Head
Speed x Maximum capacity
Max Speed
Speed 2 x Head at maximum speed
Max Speed
Horsepower = Speed 3 x Horsepower at maximum speed
Max Speed
Therefore, the turndown limit on capacity is from 100% to 70%
of maximum capacity. This system is shown in Figure 8.8.
Using just variable speed and start/stop control is not
enough to provide a continuous range of control from one pump
at minimum speed to two pumps at maximum speed. A control
gap occurs from 50-70% of maximum station output. Either a
recycle valve or throttling valve is needed to eliminate this
gap. Energy savings should be expected if the average flow
is less than about 80-90% of maximum.
PRESSURE
TWO PUMPS.MAX
REGION
OF NO
FLOW
CONTROL
REGION OF
NO FLOW
CONTROL
35
5O 7O
FLOW. X MAX
IOO
Figure 8.8. Two identical variable speed pumps in parallel.
211
-------
When used for controlling wet well level in raw plant
influent, centrifugal pumps are normally constant speed;
started and stopped at selected level trip points. Usually
tight level control is not needed, and flow is allowed to
swing between wide limits. Once inside the plant, level
control is often more stringent, and variable rate pump
stations are more common.
E. References
1. Poynton, James, p. "Basics of Reciprocating Metering
Pumps." Chemical Engineering, May 21, 1979, p. 156.
2. Bristol, John M. "Diaphragm Metering Pumps." Chemical
Engineering, September 21, 1981, p. 124.
3. Coughlin, John L. "Control Valves and Pumps: partners
in Control." Instruments and Control Systems, January,
1983, p. 41.
4. Hilsdon, Charles W. "Using Pump Curves and Valving
Techniques for Efficient Pumping." Research and
Technology, March, 1982, p. 157.
5. "Energy Evaluation in Centrifugal pump Selection."
Plant Engineering, April 5, 1979, p. 71.
6. DeSantis, G. J. "How to Select a Centrifugal Pump."
Chemical Engineering, November 22, 1976, p. 163.
7. Burris, Bruce E. "Energy Conservation for Existing
Wastewater Treatment Plants." Journal WPCF, Volume 53,
No. 5, May, 1981, p. 537.
8. Yedidiah, S. "Make pumps with Drooping Curves Your
Servants, Not Your Enemies." Power, March, 1982, p. 51.
9. Zell, Blair. "Pumps and Pumping Systems: Specifying to
Save Energy." Specifying Engineer, October, 1981, p.
117.
212
-------
9.0 VARIABLE SPEED DRIVE
9.1 MAGNETIC COUPLING
A. Application
Variable speed drives are used for pumping or other
mechanical functions that cannot be properly accomplished
with a reasonable number of constant speed units. Variable
speed drives also offer increased flexibility in control.
A magnetic coupling variable speed drive system uses a
standard constant speed induction motor to drive a coupling
(clutch) that has an adjustable output speed. This drive
offers good flexibility, with such options as braking, and
accurate speed control by a feedback signal produced by a
tachometer generator on the output shaft.
Characteristics of a magnetic coupling variable speed drive
are:
1. Uses standard constant speed AC squirrel cage induction
motor.
2. Generally for treatment plant applications a wide range
of 10 - 750 KW (15 - 1,000 Hp) is available.
3. Minimum cost, simple variable speed drive.
4. Suitable for variable torque pumping or fan loads. Not
suitable for conveyors and piston pumps that are subject
to heavy vibrations.
5. Low efficiency at reduced speeds. At low speed a large
amount of power is dissipated as heat in the magnetic
coupling. This requires cooling the magnetic coupling
by air or liquid.
Don't use magnetic couplings at speeds below 75% of
nominal speed for an extended time because of lowered
efficiency.
6, Water cooling to improve heat dissipation is generally
required above 300 KW (400 HP).
7. Magnetic coupling reliability is good, and minimal
maintenance is normally required. It has brushes, but
unlike dc motors and wound rotor motors, they are on the
magnetic coupling and carry only small currents.
213
-------
8. Potential proalems exist in maintaining proper alignment:
between the motor, magnetic coupling, and pumps.
9. Additional floor space (for horizontal drives) or
additional ceiling height (for vertical drives) is
required, as compared to most other variable speed
drives.
B. Principle of Operation
Magnetic coupling variable speed drives use a constant speed
induction motor to drive a ferrous metal ring that rotates
around a dc excited magnetic rotor connected to the load. DC
current in the magnetic rotor is increased or decreased to
vary the degree of coupling force generated. Output speed of
the drive is a function of the strength of "the rotor magnetic
field and the load on the output shaft. Output speed is
automatically controlled by a speed controller that compares
the output speed signal from the speed transmitter to a
setpoint and adjusts the amount of dc current to the magnetic
rotor to maintain the desired speed. See Figure 9.1.
CONTENT SPEED
MAGNETIC
COUPLING
! L
1 SPEED FEEDBACK. _
Figure 9.1. 11a£net±c coupling functional block diagram.
214
-------
C. Speed Controllability
Remote speed control by a signal from an external source is
available. See Figure 9.2,
Controllability is good, and accurate speed control can be
obtained.
215
-------
*;—0^j_
MOTOR STARTER
MS AUX
a—i H—a
TO VARIABLE
Cf
•u 3
_J
ft
) UOC
L
ISAUX
AL
a
DttSlta OEC^TE2)
.— ^ 1 r< ^•oJ_Q— j
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RAMP OENERATOR a
Sjf. I «-20 MA OU
1 | TO v*ai*ai
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®
START
STOP
€>
SP€£0 (3)
TMANSMITTtR
START MOTOR
STOP MOTOR
MOTOR SUNNING
® LOCATED AT PUMP/MOTOR
® LOCATED AT MCC ^ (4.jo MA)
® LOCATED AT LOCAL PANEL
® LOCATED AT COMPUTER INTERMCC
® LOCATED AT VARIAM.E SPEED CONTROLLER
12 COMPUTER INTERFACE TERMINALS
SPEED ©
INDICATOR
NOTE(S)
11 SAFETY LOCKOUTS ANO/OR UNIQUE CONTROL
FUNCTION INTERLOCKS FOR THE PARTICULAR
APPLICATION
2) COMPUTES OUTPUT CONTACTS ARE
MAINTAINED CLOSED OUR I NO CONTROL
Mgure 9.^. Magnetic coupling variable speed pump.
216
-------
9.2 LIQUID RHEOSTAT
A. Application
Variable speed drives are used for pumping or other
mechanical functions that cannot be properly accomplished
with a reasonable number of constant speed units. Variable
speed drives offer increased flexibility in control.
Improved system efficiency can also be obtained where
periodic changes in the demand allows reduced horsepower,
consequently, offering an energy savings.
A liquid rheostat variable speed drive system uses a wound
rotor motor. The motor speed is adjusted by changing the
rotor current as determined by the depth in which the
rheostat plates are submerged in liquid.
Characteristics of a liquid rheostat variable speed drive are:
1. Uses wound rotor motor.
2. 'Available in all standard motor sizes from 10 - 750 + KW
(15 - 1,000 HP).
3. Not suitable for constant torque loads. A reasonably
good drive for variable torque pumping and fan loads.
4, In general, a 2 to 1 speed range is available.
5. Overall efficiencies range from about 85% at full speed
down to approximately 45% at half speed.
6, The power factor is poor at low speed.
7. A heat exchanger, with circulating pump, is required to
. dissipate from the liquid in the rheostat the heat
produced by the electrical slip in the motor used to
obtain variable speed operation.
8. Maximum speed, with minimum resistance between plates of
the liquid rheostat, is about 95% of full speed.
9, Motor starting current can be reduced to less than
full-load current by having maximum resistance between
plates of the liquid rheostat during startup.
10. Problems with brushes and slip rings on the wound rotor
motor occur with moderate frequency.
217
-------
3. Principle of Operation
Electrical resistance between the liquid rheostat plates
determines the wound rotor motor current which regulates the
motor speed. The motor speed is adjusted by a change in the
depth of the liquid in which the rheostat plates are
submerged. See Figure 9.3.
PNEUMATIC
SPEED
SIGNAL
ELECTROLYTE
PIPING
Figure 9.3. Liquid rheostat functional block diagram.
218
-------
C, Speed Controllability
Remote speed control by a signal from an external source is
available. See Figure 9.4.
Controllability is generally good, but accurate speed control
is difficult to obtain due to the slow response inherent with
the pneumatic interaction required to change the submergence
of the rheostat plates in the liquid.
219
-------
(—— — — _,
©
»
1ST
r
T 1
©
\-~/
I4.M M4,
(3)
0
UOCATCO AT PUMP/ MOTOR
UOC4T10 AT MCC
UOCATCO Ar LOCAI. PUKL
UOCATIO AT COMPUTER INTERFACE
UOCATEO AT VAMAILC 3PCEO CONTROLLER
COMPUTER INTERFACE TERMINALS
<^
START MOT5*
MOTOR HUNNMi
NOTEIS)
II SAFETY LOCKOUTS ANO/OR UNIOUE =
FUNCTION INTERLOCKS TOR THE ?*-~
APPL1CATION
Figure 9.4. Liquid reheostat variable speed pump.
220
-------
9.3 VARIABLE FREQUENCY
A. Application
Variable speed drives are used for pumping or other
mechanical functions that cannot be properly accomplished
with a reasonable number of constant speed units. Variable
speed drives offer increased flexibility in control.
Improved system efficiency can also be obtained where
periodic changes in the demand allow reduced horsepower,
consequently, offering an energy savings.
A variable frequency drive (VFD) system consists of an
induction motor where both the voltage and frequency supply
controlled by an electrical inverter to adjust the motor's
speed.
VFD's are available for a wide range of standard motor sizes.
Characteristics of a VFD system are:
1. Other than the addition of thermostats in two phases of
the stator winding, a standard squirrel cage ac
induction motor can be used.
2. All standard motor sizes available from 4 - 400 KW (5 -
500 HP).
3. Suitable for variable torque pumping or fan loads.
Normally suitable for piston pumps and conveyors.
4. Continuous operation at constant torque is available
over a 3 to 1 speed range.
5. Overall efficiencies are about 83% at full speed down to
approximately 75% at half speed.
6. Multiple motors can operate off one common VFD
simultaneously. Also, one VFD can control more than one
motor where it is switched between motors to give a
combination fixed speed and variable speed system.
7. The VPD can convert existing constant speed motors to
variable speed operation when retrofitting existing
installations.
8. Starting current can be limited to less than full-load
current.
9. Requires little maintenance; however, complex components
and circuitry require an expert technician when problems
do occur.
221
-------
Principle of Operation
The variable speed controller/power converter changes
constant frequency, constant voltage line power to variable
frequency, variable voltage power to vary the drive motor's
speed. Raising the frequency of the power applied to the
drive motor increases its speed. Lowering the frequency
decreases its speed. Voltage applied to the drive motor is
adjusted to control the motor's output power. The variable
speed controller compares the drive motor's speed to an
adjustable setpoint value and outputs the required frequency
and voltage to maintain the desired speed. See Figure 9.5.
Speed Controllability
Remote speed control by a signal from an external source is
available and is probably the most advanced" speed control
method available today. See Figure 9.6. Controllability is
excellent, and accurate speed control can be maintained.
AC
POWER
PUMP ) C PUMP ) C PUMP ) CPUMP j C PUMP
>
PUMP
L-SIMPLEX-1
UNIT
Figure 9.5. Variable frequency drive functional diagram.
222
-------
CONSTANT
SPEED
MOTOR
STARTER
• I2O VAC SO HZ •
STlRT
HS—i r—3
STOP
i—a—i MS
0-4-
u •
a
y
®
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i
_J
Kl '
©LOfLATFQ AT PimP
LOCATEO AT PUMF
©LOCATED AT MCC
LOCATED AT MCC
© LOCATED AT LOCAL PANEL
W
0 LOCATED AT COMPUTER INTERFACE
© LOCATED AT VFO PANEL
^ COMPUTER INTERFACE TERMINALS
NOT El SI
|( S4FETT LOCKOUTS 4NO/OR UH.QUJ
FUNCTION NTERLOCXS FOR TH£
PARTICULAR APPLICATION
*l INCREMENTAL SETPOINT CONTROL
SOLID STATE RAMP OENeRATOR,
MOTORIZED POT.OR EOUIVALEMf
MOTOR REOL-a
ON VARIABLE Sa
MOTOR RUNMXS
I AT CONSTANT 5=
Figure 9.6. Simplex VFD variable speed pump.
223
-------
9.4 VARIABLE PULLEY
A. Application
Variable speed drives are used for pumping or other
mechanical functions that cannot be properly accomplished
with a reasonable number of constant speed units. Variable
speed drives also offer increased flexibility in control.
Electromechanical variable speed pulley belt drives use a
variable sheave ratio principle to change the speed of the
final element.
Characteristics of a variable pulley variable speed drive are:
1. Available in all standard motor sizes from 0.2 - 75 KW
(1/4 -100 HP), but are normally used only in the lower
ratings of 0.4 - 20 KW (1/2 - 25 HP).
2. This is a constant torque drive.
3. In general, a 10 to 1 speed range is available.
4. Overall efficiencies are about 70% at maximum speed down
to approximately 45% at half speed.
5. Additional floor space or ceiling height is required as
compared to most other drives.
6. Belt life is typically 18 months.
B. Principle of Operation
Variable pulley or electromechanical drives use a constant
speed motor coupled to a variable belt drive system for speed
control. Output speed is varied by changing the diameter of
the drive sheave or pulley. Drive sheave diameter is
adjusted using a reversing gear motor connected mechanically
to the drive system. When the gear motor operates in one
direction, the sheave diameter is increased and the drive
speed increases. When the gear motor operates in the other
direction, the sheave diameter decreases and the drive speed
decreases.
224
-------
C. Speed Controllability
Remote speed control by a signal from an external source is
available. See Figure 9.7.
Potential operational problems holding a speed setting can be
numerous due to belt wear, belt stretching, and groove-worn
pulleys. Small stepless speed adjustments are difficult co
obtain (and maintain).
225
-------
I2O VAC SO HZ
I2O VAC SO HZ
-C B
-r _
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START
MOTOR
STOP
MOTOR
INCREASE
SPEED
TOMOTOS
CONTROL
CIRCUITS
> MOTOR RUNMNS
Q LOCATED AT PUMP/MOTOR
(|) LOCATED AT MCC
(j) LOCATED AT LOCAL PANEL
0 LOCATED AT COMPUTER NTERRCE
(3) LOCATED AT VARIABLE SPEED (4"20
CONTROLLER
^ CCNVUTER INTERFACE TERMINALS
NOTE (SI'
I) SAFETY LOCKOUTS */O« UNIQUE
CONTROL FUNCTION INTERLOCKS AS
PER THE PARTICULAR APPLICATION.
Figure 9.7. Variable pulley drive control circuit.
226
-------
9.5 DIRECT CURRENT SILICON CONTROLLED "ECTIFIEH (SCI)
A. Application
Variable speed drives are used for pumping or other
mechanical functions that cannot be properly accomplished
with a reasonable number of constant speed units. Variable
speed drives offer increased flexibility in control.
Improved system efficiency can also be obtained where
periodic changes in the demand allows reduced horsepower,
consequently, offering an energy savings.
Variable speed control of dc motors is achieved by varying
the armature or field voltage to the motor, or both.
Characteristics of dc variable speed drives are:
1. DC motors are considerably more expensive than
comparable ac motors.
2. In general, for treatment plant applications, drives are
available from 4 - 100 KW (5 -150 HP).
3. Can be used for variable torque operation. Usually
suitable for piston pumps and conveyors.
4. In general, a speed range of 60 to 100% is available.
5. Reduced speed efficiency is very good.
6. The dc motor has a commutator and brushes, both
potential maintenance problems,
7. Commutators on dc motors can cause problems if they are
not well ventilated and properly maintained or if they
are subject to a corrosive environment.
8. Drive requires little maintenance; however, complex
components and circuitry require an expert technician
when problems do occur.
9. Starting current can be limited to less than full-load
current.
B. Principle of Operation
The term SCR drives is defined as variable speed drives that
are based on the use of a dc motor. 3CR drives include a
speed controller that uses an SCR controlled bridge circuit
to rectify constant voltage ac line power to variable voltage
dc power. Variable voltage dc power is applied to the dc
motor to regulate its speed. As the applied voltage
increases, the motor speed increases as the voltage
decreases, the speed decreases, SCR speed controllers accept
an input signal representative of the desired motor speed and
provide a dc output sufficient to operate the motor at the
required speed.
227
-------
C. Speed Controllability
Remote speed control by a signal from an external source is
available. See Figure 9.8.
Controllability is good, and accurate speed control can be
maintained.
228
-------
c
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(?) LOCATED AT LOCAL PANEL
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® LOCATED AT VARIABLE SPEED CONTROLLER
12 COMPUTER NTERFACE TERMINALS
I) COMPUTER OUTPUT CONTACTS ARE
MAINTAINED CLOSED DURING CONT93..
Figure 9.8. DC variable speed drive.
229
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10.0 CONTROL VALVES
10.1 MODULATING SERVICE
A. Application
Control valves for modulating service are used in all
wastewater treatment processes. Excluded from this
discussion are valves used in isolation or routing service in
two states; open or closed. Commonly used control valves are
compared in Table 10.1. Their applications are presented
later in the text and in Tsl^e 10.2.
TABLE 10.1. COMPARISON OF CONTROL VALVES IN WASTEWATER
TREATMENT PROCESSES
Valve Type
Ball (Rotary)
Butterfly
(Rotary)
Gate (Linear)
Globe (Linear)
Eccentric plug
(Rotary)
High pressure recovery
Tight shutoff
Equal percentage flow
characteristic
Available with flangeless
connection
1-24 inch sizes
High pressure recovery
Equal percentage flow
character is tic
Good shutoff
Usually a flangeless connection
2-144 inch sizes
Cost
Low cost
Lowest cost
Low pressure recovery
Choice of flow characteristics
Good shutoff
Usually small size applications
1/2 - 16 inch sizes
Equal percentage and linear
flow characteristics available
Good shutoff
Moderate pressure recovery
2-24 inch sizes
Not usually
used for
modulating
service
High cost
Moderate cost
230
-------
BALL VALVE
Figure 10.1. Ball valve.
231
-------
B. Principle of Operation
Control valves (modulating service is implied in this term)
are made in a wide variety of designs. Five designs used in
wastewater control applications are presented here. There
are many, many other designs, but this discussion will make
you aware of some of the problems encountered in control
valves.
1. Ball valves.
Valves are classified as either "rotary" or "linear."
In a rotary valve, the ball, disk, or plug is rotated to
open or close the flow stream. A linear valve lifts the
gate, disk or plug up or down to open or close the flow
stream. Ball valves are rotary. Flow goes through a
port in the ball. To shut off flow, the ball is rotated
until the port is closed. The ball may be a complete
sphere, full ball, as shown in Figure 10.1, or a partial
sphere. Standard port diameters range from 80% of
inside pipe diameter to 100% or full port.
Ball valves have high friction resistance to rotation
due to valve body and trim contact with the ball
surface. This gives a tight shutoff at the expense of
needing larger actuators. In some designs the ball is
allowed to "float" in its seat so that line pressure
will assist keeping a tight shutoff by pushing the ball
against the downstream seal ring. This freedom of
movement introduces a deadband between the actuator and
the ball. The deadband will make this design unusable
for many control applications. For control valves, a
rigid call to actuator connection is preferred.
232
-------
2. Butterfly.
Butterfly valves are rotary type valves that use an
axially pivoted disk to restrict or open the flow
stream. The disK may be flat or contoured in shape and
mounted to the atem (pivot) in several ways. An example
is profiled in Figure 10.2.
DISK
iASKET
BOLT OR STUD
Figure 10.2. Butterfly valve, swing-through type with
flangeless pipe connection.
One problem with butterfly valves is obtaining a tight
shutoff. The seal must be made along the entire inner
circumference of the valve body. Upper and lower half
seats must be matched and the pivot point must be
sealed. Liners and seals used for good shutoff increase
the opening and closing torque requirements of the
actuator.
Another problem encountered in butterfly operation is
torque applied by the fluid. During rotation of the
disn, torque reaches a maximum at about 70 degrees (from
full open). This is shown in Figure 10.3. In some
applications this pea* may exceed the opening or closing
torques. To overcome this problem, disks are contoured
for low torque in the near closed positions.
233
-------
TORQUE
BRcAK AWAY
TORQUE
CLOSED
VALVE POSITION (DEGREES) "
Figure 10.3. Butterfly torque.
3. Gate.
Gate valves are linear valves in which the gate's disk
is raised or lowered past a port by the actuator. The
disk is a flat or wedged-shaped plate. The valve may
have one or several ports for flow which can be sealed
by the plate. A gate valve is shown in Figure 10.4. in
knife-gate valves, plates are made with a sharp edge for
service with solids bearing streams including dry
solids. Gate valves are not usually used for modulating
service.
234
-------
Figure 10.4. Gate valve, multi-orifice.
4. Globe.
Globe valves are linear valves in which the plug moves
up or down into a port. Some valves have two sets of
plugs and ports, these are called double ported. In
order to reduce stem size and to obtain better seating
of the plug on the port, the stem or plug is
mechanically guided. An example of a globe valve is
shown in Figure 10.5.
235
-------
Figure 10.5. Globe valve.
Actuator size can also be reduced for double ported
valves with balanced flow. The ports and plugs are
aligned such that the flowing stream tends to close one
port and open the other, thus balancing the fluid forces.
5. Plug.
Plug valves ara rotary type valves with a conical or
cylindrical shaped plug wnich has an orifice. Rotating
the plug at a right angle to flow causes tight shutoff
while full flow occurs when the orifice of the plug is
parallel to the flow axis. A simple plug valve is shown
236
-------
in Figure 10.6. In the same figure is a popular version
of the plug valve, the eccentric spherical plug or
"camflex" valve. Just as a traditional plug valve is
similar to a traditional ball valve, the eccentric
spherical plug is similar to the segmented ball valve.
Both newer designs provide large flow for the valv« size
and high pressure recovery yet have reduced friction
through most of the valve travel.
TAPERED
PLUG
STEM
BODY
AN ECCENTRIC ROTARY VALVE PLUG
TRADITIONAL PLUG VALVE
Figure 10.6. Plug valves.
C. Valve Size
Control valves are rated by a "Cv" factor which is the amount
of water at 15°C (60°F) or any liquid with a specific
gravity of 1.0, that goes through a full open valve at a
pressure differential of 6.89 KPa (1 psi). The Cv factor is
found in the general flow equation below.
Cv
A P
S.G.
Q = flow, gpra
Cv = valve factor gpm/(psi) V2
P = pressure differential, psi
S.G. = specific gravity
While this flow equation is not restricted to English units
of measure, the Cv values available for valves in this
country are only for English units. To use kpa for pressure
multiply manufacturer's English Cv values by conversions in
Table 10.2.
237
-------
TABLE 10.2 Cv CONVERSION FACTORS
Cv x conversion factor = flow in units
1.44 = 1pm
0.0862 = m 3/h
0.00144 = m 3/ra
0.0000239 = m 3/s
The metric conversion factors also convert to a specific
gravity at 4°C. For most wastewater treatment processes
tne liquid is water at 5-25°C (41-77°F) so the specific
gravity factor will cause less than 0.5% correction and wi 11
be deleted from further discussion hers. Additional
correction factors for viscosity and critical flow and
compressibility for gas flow may De needed. These factors
and how to apply them are found in manufacturer ' s
literature. For a complete explanation of valve sizing see
the ISA text books referenced at the end of this section.
In sizing a valve for wastewater treatment processes,
the range of flows is known. A safety factor of 110% of
maximum flow, or 130% of average flow is commonly used in the
sizing calculations. Another way to add a. safety factor is
to make calculations using 100% of maximum flow, but
selecting a valve that will do the job at 90% of its rated
capacity (i.e., 0.9 Cv) . Once the pressure drop over the
valve is determined, the valve size can be calculated by
rearranging the above equation. Traditional sizing rules for
control valves in pump systems call for 33% of system
pressure loss to be at the valve. This and other rules of
thumb are shown in Figure 10.7. The lost energy is not
wasted, but is used to reduce flows to the control setpoint.
238
-------
FOR APD< 1000 kpo (ISOpsi)
PUMPS
FLOW < 750 gpm
FLOW > 750 gpm
-Apn
•^
AP £ 0.25 APQ a > 100 kpa (ISpsi)
AP > 0.2 APn
GRAVITY FEED
-APD-
-AP-H
AP> 0.33AP 8 2 O.I
pa
pa
-APO-
COMPRESSORS
AP > 0.5 AI a > o.os pa
Figure 10.7. Traditional pressure drops for control valves.
239
-------
This rule of thumb is not particularly energy efficient and
should be replaced for large sized flow streams where the
energy cost is significant. Allotting less head loss to the
control valve will tend to increase the valve size up to full
line size, and will restrict the choice of valves to those
with high capacity (e.g., butterfly or ball). Reducing head
loss at the valve calls for more careful design than
otherwise because there is less safety factor. The risk is
that the valve will be oversized for the application. In a
severe case, the valve may have to operate nearly closed to
achieve the setpoint flow, which may make satisfactory flow
control impossible. To help understand sizing of valves,
lets review two fundamentals: pressure drop and valve gain
and characteristic.
Pressure differential or drop across a valve is proportional
to the square of flow. This is the same relationship that
occurs in differential pressure flow meters: orifices and
Venturis. The pressure profile across a valve is similar to
an orifice or venturi, and is shown in Figure 10.8.
Similarly the flow is dependent on the pressure difference
measured from upstream to the vena contracta. Due to the
impracticality of measuring vena contracta pressure, control
valve flow equations and sizing principles are established
based on upstream and downstream pressures. The relationship
between vena contracta and downstream pressure is normally a
constant proportion expressed as:
- pvc)
where:
A P = Difference between upstream and downstream pressures,
kpa (psi)
Pj_ = Upstream pressure, kpa (psi)
Pvc = Pressure at vena contracta, kpa (psi)
FL = Pressure recovery factor, unitless
This relationship does not hold during conditions of
cavitation or flashing in liquids or at sonic velocities in
the vena contracta in gases. In these cases choked or
partially choked flow occur. When sizing valves for low head
loss, choked flow and related phenomena are not likely
problems except where the vapor pressure of a liquid
approaches the vena contracta pressure as in hot water for
example. See the reference publications for further
information.
240
-------
pvc
pi
PRESSURE
PROFILE
Pi
J I
I I
pvc» PRESSURE AT VENA CONTRACTA
FL IS THE PRESSURE RECOVERY FACTOR
Figure 10.8. Pressure drop across a valve.
The recovery factor, FL, is combined with other factors to
form the Cv numbers reported in manufacturer's literature.
Therefore, the pressure difference of most use in sizing
valves is the permanent pressure loss, A p.
The second fundamental for review is valve gain and
characteristic. Valve gain is defined as the change in flow
caused by a change in valve position and may be dependent on
valve position. The way in which valve gain changes with
valve position falls into three basic characteristics:
• Equal percentage - equal changes of valve position cause
equal percentage changes in flow.
241
-------
• Linear - flow changes linearly with valve position.
• Quick opening - flow changes rapidly with valve position
at low valve position, but only slightly at higher valve
positions. Most of the valve capacity is reached after
opening just a small amount.
These three characteristics are shown in Figure 10.9.
100%
FLOW
CAPACITY
IOO% OPEN
VALVE POSITION
Figure 10.9. Intrinsic valve characteristics.
When these terms are used to describe a valve they are called
intrinsic or inherent flow characteristics. They represent
the gain dependence on position in cases where the total
dynamic head loss of a system occurs at the valve. As less
proportion of system head loss occurs at the valve, the flow
characteristic shifts to a relationship called the installed
characteristic. This shift is shown in Figure 10.10 for
linear and equal percentage valves. Shift in gain is shown
in Figure 10.11.
At control valve head losses traditionally used for valve
sizing, p/ PD - 0.2 to 0.33, "linear" valves approach
quick open response, and "equal percentage" valves approach
linear response. In cases where head loss at the valve is to
be reduced to a minimum, the shift in valve response is even
greater. When viewing Figures 10.10 and 10.11 Keep in mind
that the ratio of valve head loss to total dynamic head loss
will not be the same at all flows. This is shown in Figure
10.11. For most of the flow range a valve will be close to
its intrinsic characteristic or gain, but near maximum flow
the installed characteristic or gain will shift.
242
-------
loor.
FLOW
CAPACITY
%CV
IOO%
VALVE POSITION
100%
FLOW
CAPACITY
%cu
IOO%
VALVE POSITION
Figure 10.10. Valve character is tic shift witii
decreasing ratio of valve pressure loss to system
dynamic pressure.
243
-------
G
GAIN
EQUAL PERCETAGE
VALVE
IOO%
VALVE POSITION
G
GAIN
AP
*Apn
0.33
050~
I.OO
VALVE POSITION
LINEAR
VALVE
100 %
G
GAIN
VALVE POSITION
QUICK OPENING
VALVE
100%
Figure 10.11, Installed valve gain.
244
-------
100
80--
60--
PRESSURE
40--
20--
0
I
I
20
40
I
60
FLOW I
I
.PUMP
CURVE
-DYNAMIC
SYSTEM
-LOSSES
STATIC
LOSSES
80
100
VALVE
CONDITION
— AP —
Figure 10.12. Valve pressure drop in a pumping system.
245
-------
Gain analysis is further complicated by the behavior of real
valves which although they may exhibit a general type of
characteristic, the actual installed characteristic will
deviate from the ideal. This is specially true at the limits
of valve travel. Generally, a valve will only provide
acceptable control over a range of performance. This range
may be expressed as a ratio of high to low flows, high to low
valve position, or high to low valve capacity, Cv. The last
measure of rangeability is the most useful in selecting a
valve. Using the sizing equation, rangeability requirements
may be calculated by:
Rangeability
CVL QL APH
Where the subscrips H and L indicate high and low flow rate
conditions.
Putting together all valve gain considerations discussed so
far results in a plot of gain versus valve capacity as shown
in Figure 10.13. The valve is a cage-guided, equal
percentage globe valve with flow tending to open which is
used to throttle a centrifugal pump. The application called
for two thirds of the total system dynamic pressure losses at
maximum flow to occur at the valve. Therefore, the valve
should be close to its intrinsic characteristic throughout
its operation. Note that the installed characteristic is
only equal percentage from about 7 to 70% of valve capacity.
If the change in characteristic outside of this region of
equal percentage is detrimental to control of the process,
the rangeability is 70 to 7 or 10 to 1.
The concern for predicting valve gain centers around
obtaining stable control. That is, the system should respond
to'load and setpoint changes with desired dampening and
response time. A general rule to follow is that the valve
gain should act opposite to the process gain to produce as
linear a combination as possible. For example, consider a
flow control situation where the flow rate is measured by a
differential pressure element. This follows the previously
stated principle that flow is proportional to the square root
of pressure difference. Therefore its gain increases with
flow. A quick opening valve has a gain that decreases with
flow and would be a suitable valve choice. A linear valve
could also be used if the system operated near the maximum
flow, in which case the installed linear valve characteristic
would be similar to quick opening. If the flow transmitter
signal was sent to a square root extractor and then to the
controller, the meter's gain would be constant. In this
case, select a linear valve for operation near the intrinsic
characteristic, or select an equal percentage valve for
operation near the maximum flow.
246
-------
2--
G
GAIN
EQUAL-PERCENTAGE
CHARACTERISTICS
100
VALVE CAPACITY, % cv
Figure 10.13. Gain of a single ported globe valve. •
Table 10.3 contains a recommendation of valVe characteristic
for several control applications. If there is doubt about
how much installed characteristic to allow for, or which
condition overrides choose equal percentage over linear/ and
linear over quick-opening.
247
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TABLE 10.3. VALVE CHARACTERISTIC SELECTION GUIDE
CONTROL APPLICATION
Flow
Level
pressure
D.
CONDITIONS OF APPLICATION
o Linear flow signal
o Differential pressure
signal
o Small flow range, large
pressure drop changes
o Most applications
o Liquid
o Gas with large pressure
drop
o Gas in fast responding
system
o Gas in slow responding
system
VALVE CHARACTERISTIC
Linear
Quick-opening
Equal percentage
Linear
Equal percentage
Equal percentage
Equal percentage
Linear
Manufacturer's Options
1. Body styles are available as presented above and in a wide
selection of variations. Consult manufacturer's
literature for available valve bodies.
2. Materials of body construction:
a. Iron,
b. Carbon steel,
c. Stainless steel, and
d. Hastelloy.
3. Materials of trim and plug construction!
a. Carbon steel,
b. Stainless steel,
c. Brass,
248
-------
d. Copper,
e. Monel, and
f. Other speciality materials are available.
4. Stem packing materials:
a. TEE, and
b. Graphite at high temperatures.
5. Pipe connections:
a. Threaded,
b. Flanged, and
c. Plangeless (wafer).
6. Body liner material:
a. Teflon,
b. Buna-N, and
c. Viton.
7. Trim:
Trim is available in many configurations for each valve
type and varies by manufacturer.
8. Bonnet:
Bonnet designs are also varied according to manufacturer.
E. References
1. "Advances in Water and Wastewater Valves." Consulting
Engineer, September, 1982, p. 105.
2. Boger, Henry W. "The Effect of Installed Flow
Characteristic on Control-Valve Gain." ISA Transactions,
Vol. 3, Ho, 4, 1969, p. 265.
3. Ramsey, John R. and Charles D. Fournier. "Matching Valves
with Process Requirements." Instrument and Control
Systems, June 1973, p. 65.
4. Hammit, Donn. "Choosing a Control Valve is Easy."
Instrument and Control Systems, December 1976, p. 28.
249
-------
5. Moore, Ralph W. "Allocating Pressure Drops to Control
Valves." Instrumentation Technology, October 1970, p. 102.
6. Baumann, Hans D. "How to Estimate Pressure Drop Across
Liquid - Control Valves." Chemical Engineering, April 29,
1974, p. 137.
7. Kern, Robert. "Control Valves in process plants."
Chemical Engineering, April 14, 1975, p. 85.
8. Rezac, Mark A. and Kevin P. Bornhoft. "Choose the Right
Control Valve for the Job." In-Tech, October, 1980, p. 59.
9. Vendor Literature from: Fisher Controls, Masoneilan
International and Pratt Valve.
10. Driskell, Les, Control Valve Sizing, ISA, 1982.
11. Driskell, Les, Introduction to Control Valves, ISA, 1981.
12. Driskell, Les, Selection of Control Valves, ISA, 1982.
13. Chaflin, Sanford. "Specifying Control Valves." Chemical
Engineering, October 14, 1974, p. 105.
14. Wolter, D. G. "Control valve Selection." Instrumentation
Technology. October, 1977, p. 55.
15. Wing, Paul. "Plain Talk on Valve Rangeability."
Instrumentation Technology, April, 1978, p. 53.
250
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11.0 CONTROL VALVE OPERATORS
11.1 ELECTRIC
A. Application
Characteristics of an electric control valve operator are:
1. Normally economical only in relatively small torque
requirements.
2. Large units requiring high torque generally operate
slowly.
3. Large units weigh considerably more than their pneumatic
counterparts.
4. Normal fail-safe position action is loclc-in-last-state.
5. Major advantage is in remote installations where no
other power source (e.g., air supply for pneumatic
operators) is available.
TABLE 11.1. ELECTRIC VALVE OPERATORS
APPLICATION GUIDELINES
Recommended Not Recommended
In remote, nonhazardous In explosion-hazard locations
installations - Digesters
- Pump stations - Oxygen plants
In nonexplosion-hazard - Incinerators
processes For not environment
- Primary treatment
- Most secondary treatment
- Tertiary treatment
- Chlorination
For cold or damp environment
B. principle of Operation
1. On/off operation.
Electrically actuated valves use a. reversing electric
motor to drive a gear box or other mechanical
positioning mechanism to open or close a valve. The
motor is controlled by relay contacts or a reversing
motor starter that is interlocked to stop the valve
motor once the valve reaches a fully open or closed
position. Adjustable limit switches detect the fully
open or closed position of the valve.
251
-------
Electrically operated valves generally require
high-torque cutout switches in series with the reversing
starter to prevent damage to the motor and operating
mechanism if the valve becomes obstructed or jammed.
2. Modulating operation.
Electrically actuated modulating valves use a reversing
electric motor to operate the valve. The motor is
controlled by a solid-state reversing motor starter that
starts the valve positioning motor in the required
direction when a contact closure is initiated. The
motor will continue to operate and change the valve
position as long as the contact closure is maintained.
When the operating contact is opened, the valve
positioning motor will stop. A valve position
transmitter provides signal condition-ing to generate an
output signal proportional to the valve position.
C. Valve Operation
Valves can be operated using optional pushbuttons mounted in
the operator or from a remote position control.
Controllability is good, and accurate position control can be
obtained.
Figure 11.1 provides an example of a typical electrically
actuated modulating valve interface.
D. Manufacturer's Options
1. Open/Close limit switches:
a. Cam-operated, or
b. Snap action.
2. Position potentiometer typically 0-1000 OHMS to
correspond to 0-100% open.
3. Housings:
a. NEMA 12, dustproof,
b. NEMA 4, weatherproof, and
c. NEMA 7, explosionproof.
252
-------
©
SOLID STATE
REVERSING MOTOR STARTER
TRIGGER CIRCUIT
(NOTE 2)
©
LS
CSC
lg—1 p— 13
©
'©x
® POSITION j(D
TRANSMITTER /^>
, INDICATOR
.. L
14-20 MA)
NOTE(S)
I) COMPUTER OUTPUT CONTACTS ARE MAINTAINED
CLOSED QURINO CONTROL.
2) POWER WIRING FOR VALVE MOTOR MAS
MOT BEEN SHOWN
Q LOCATED »r «LVC
(f) LOCATIO AT LOCAL PANEL
d) UJCATCO AT COMPUTER INTERFACE
g COMPUTER INTERFACE TERMINALS
Figure 11.1. Electric operator control circuit-modulating service,
253
-------
4. power supply:
a. 115 VAC, two phase, 60 Hz,
b. 208 VAC, three phase, 60 Hz,
c. 240 VAC, three phase, 60 Hz,
d. 480 VAC, three phase, 60Hz, or
e. 48 VDC.
5. Mechanical brakes to lock a valve in position..
6. A solid state position controller with an analog
(usually 4-20 mA) remote setpoint.
7. Housing heaters to prevent condensation, reduce relative
humidity, and keep lubricants at proper viscosity.
8. Motor rated for continuous modulating duty.
E. Installation
1. Size operator for the corresponding valve and service
conditions. Operators are rated by torque which is
determined for each application. Reference the sections
on control valves.
2. The operator torque rating must exceed the highest
expected valve torque, and the motor must not overheat
in the maximum stroke time under the average expected
torque. Furthermore, the motor must perform adequately
at ^10% of rated voltage. Most operator manufacturers
have tables that build in safety factors for the various
types of valves. These tables let you select operator
size directly from valve size or from valve torque data.
3. Frequently, placement of the valve in piping galleries
or new process equipment restricts the operator
dimensions and weight. Most often, the smallest and
lightest operator with sufficient torque is selected for
the application.
F. Designer Checklist
Ask the following questions when reviewing or planning an
electric valve operator installation. If an answer is "no,"
an electric operator may not be appropriate.
1. Is there a reason pneumatic operators are not suitable
for the application?
2. Is the highest valve torque in the range of 200 - 5500 J
(150 - 4000 in-lbs)?
254
-------
3. Is a positioner required? Note: Many manual loading
stations provide similar functions with contact closure
or triac interfaces, so the positioner is not required.
4. Is adequate power available?
5. Are limit switches or position feedback specified for
remote monitoring?
6. Is the gear train between the actuator motor and valve
stem adequate?
7. Is the fail-safe condition to hold the last position?
If not, backup power such as batteries is required.
Some spring return electric operators are available, but
review the application to check on merits of electric
operators versus other types of opera-tors. Also
consider plumbing changes in the valve's location so a
maintained position is a fail-safe position.
G. Deficiencies
1. Modulating service operators overheat due to too small
operator size.
2. Modulating service operators fail frequently.
255
-------
11.2 HYDRAULIC
A. Application
Characteristics of a hydraulically actuated valve operator
are:
1. High-pressure hydraulic fluid is generally supplied from
a common pumping unit.
2. Actuator control is accomplished through a valve
positioner or a system of hydraulic pilot valves.
3. Excellent throttling control features due to high
stiffness (e.g., resistance to changing valve body
forces).
4. Capable of high-torque outputs.
5. High initial cost.
6. Normal fail-position action of hold, fail open or fail
close.
B. Principle of Operation
Hydraulically actuated valves use a piston operator to open
or close the valve. Separate solenoid pilot valves direct
hydraulic fluid to one side of the piston to operate the
valve. Adjustable limit switches on the valve actuator close
the pilot valve when the valve reaches its desired position.
Hydraulic pressure in the cylinder is maintained or vented
mechanically to hold the desired valve position. SEE FIGURE
11.2,
256
-------
RETURN
SPRING
SUPPLY
PORT
STEM
•PISTON
0-RING
SPRING RETURN
SUPPLY
PORT
GUIDE ROD
DRIVE
SHAFT
PISTON
SCOTCH
YOKE
SUPPLY
PORT
DOUBLE ACTING
Figure 11.2, Hydraulic piston actuator.
Hydraulically actuated modulating valves are typically
butterfly, ball, and plug type valves. Solenoid control
valves direct regulated hydraulic fluid to either side of
piston operator or a valve positioner to position the valve.
When a contact closure is initiated, the appropriate solenoid
valve port will open, and the cylinder will move the valve
operator until the limit contact is opened or the desired
position is reached.
257
-------
Adjustable limit switches detect the fully open or closed
position of the valve. A valve position potentiometer
provides a resistance output signal proportional to the valve
position for remote monitoring.
C. Valve Operation
Operation is normally from a local or remote panel external
of the valve.
Controllability is good, and accurate position control can be
obtained.
For interfacing, see Figure 11.3 for example of valve actuators
that use solenoid control valves. Contact closure signals would
be connected directly to the solenoid valves.located on or near
the modulated valve.
258
-------
HYDRAULIC OR <«-»> «•*'
"PNEUMATIC SUPPLY
O LOCATED AT VALVE
(D LOCATED AT LOCAL PANEL
© LOCATED AT COMPUTER INTERFACE
fxl COMPUTER INTERFACE TERMINALS
NOTE(S)
I) COMPUTER OUTPUT MAINTA!N£L
Figure 11.3. Piston actuator control circuit -
incremental modulating service.
259
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11.3 PNEUMATIC
A. Application
Pneumatic valve operators are available in two types. First
is the piston actuator, shown in Figure 11.5, which is the
same as the hydraulic actuator (Figure 11.5). Second is a
diaphragm actuator as shown in Figure 11.6.
Cnaracteristics of a pneumatic control valve operator are:
1. pneumatic piston.
a. Used open/close or modulating duty.
b. Used when thrust or torque requirements exceed the
capaoility of spring-and-diaphra
-------
• SUPPLY PORT
SPRING
%3 Y///////77/77,
STEM
O-RING
SPRING RETURN
PISTON
-------
UPPER CASING
DIAPHRAM
SPRING
SUPPLY PORT
LOWER CASING
STEM
LINEAR ACTUATOR
SUPPLY PORT
SPRING
LOWER CASING
DIAPHRAM
DRIVE ARM
DRIVE SHAFT-
ROTARY OR QUARTER-TURN ACTUATOR
Figure 11.5. Diaphragm actuator.
Adjustable limit switches detect the fully open or closed
valve position. A valve position transmitter with
appropriate signal conditioning provides a 4 to 20 ma output
signal, proportional to the valve position, for control
purposes.
Pneumatically actuated modulati.^ valves are typically
mechanically operated butterfly, ball, and plug type valves
or bladder type pinch valves. Separate solenoid control
valves direct regulated compressed air to the valve
positioner to position the valve. When a contact closure is
initiated, the appropriate solenoid valve opens and the
cylinder moves the valve operator until the contact is opened.
C. Valve Operation
Remote position control from an external source is available.
Controllability is good, and accurate position control can be
obtained.
-------
APPENDIX A
TITLE 3Y SUBJECT
REFERENCE LIST
Activated Sludge Wastewater Treatment Plants, Design Handbook for
Automation of. Manning, A,W., Dobs, D.M. EPA-600/8-80-028 1980.
Reference Order 120, Appendix B
Analog & Digital Control, Combined. Murray, J.J. Chemical Engineering,
June 1976.
Reference Order 16, Appendix B
Automatic Control—Reprint from Chemical Engineering. Soule, L.M.
Foxboro Company, 1970.
Reference Order #13, Appendix B
Automatic Process Control. Johnson, E.F. McGraw-Hill, 1967.
Reference Order 118, Appendix B
Automatic Process Control, Principles of. Instrument Society of America.
ISBN 87664-108-7, 1968,
Reference Order #17, Appendix 3
Closed Loop Control System: Operation of a Direct-Digital On-Line Closed
Loop Control System for Wastewater Treatment. Nelson, J.K.,
Mishra, B.B. Conference Presentation, Oct. 1978.
Reference Order #26, Appendix B
Control Theory Notebook. Wilson, H.S., Zoss, L.M. Reprinted from ISA
Journal.
Reference Order #19, Appendix B
263
-------
TITLE BY SUBJECT
Control Valves, Allocating Pressure Drops to. Moore, Ralph W.
Instrumentation Technology, Oct. 1970, p. 102.
Reference Order #17, Appendix B
Control-Valve Gain, The Effect of Installed Flow Characteristic on.
Boger, Henry w, ISA Transactions Vol. 8, No. 4, 1969, p. 265.
Reference Order 117, Appendix B
Control Valves and Pumps: Partners In Control. Coughlin, -John L.
Instruments and Control Systems, Jan. 1983, p. 41.
Reference Order 116, Appendix B
Control Valve: Choosing a Control Valve is Easy. Hanunit, Donn. Instrument
and Control Systems, Dec. 1976, p. 28.
Reference Order #16, Appendix B
Control Valve, Choosing the Right Control Valve for the Job. Rezac, Mark A.,
Bornhoft, Kevin F. In-Tecn, Oct. 1980, p. 59.
Reference Order 117, Appendix B
Control Valves In Process Plants. Kern, Robert. Chemical Engineering,
April 1975, p. 85.
Reference Order 16, Appendix B
Control Valves, Introduction to. Driskell, Les. Instrument Society of
America, 1981.
Reference Order #17, Appendix B
Control Valve Selection. Wolter, D.G. Instrumentation Technology,
Oct. 1977, p. 55.
Reference Order #17, Appendix B
264
-------
TITLE BY SUBJECT
Control Valves, Selection of. Driskell, Les. Instrument Society of America,
1982.
Reference Order #17, Appendix 3
Control Valves, Sizing. Driskell, Les. Instrument Society of America, 1982.
Reference Order #17, Appendix B
Control Valves, Specifying. Chaflin, Sanford. Chemical Engineering,
Oct. 1974, p. 105.
Reference Order #6, Appendix 3
Digital Computer Process Control. Smith, C.L. In-Tech, 1972.
Reference Order #17, Appendix B
Dissolved Oxygen Field Test Protocol, Evaluation of. Kulin, G.,
Schuk, W.W. EPA 78-D-X0024-1, 1978.
Reference Order #25, Appendix B
DO Analyzers, Comparison of Field Testing on. APWA Research Foundation,
Sept. 1982.
Reference Order #1, Appendix B
Effluent Specs: Tight Effluent Specs Met With Digital Control.
Robbins, M.H. Water/Engineering & Management, Oct. 1983,
Reference Order #22, Appendix B
Electromagnetic Flow Meters In Wastewater Treatment Plants, Recommended
Practice For The Use of.
Kulin, G. EPA-600/2-34-187, November, 1984
Reference Order #20, Appendix B
265
-------
TITLE BY SUBJECT
Energy Conservation for Existing Wastewater Treatment Plants.
Burns, Bruce E. Journal WPCF, Vol. 53, No. 5, May 1981, p. 5.
Reference Order #26, Appendix B
Flow; Its Measurement and Control in Science and Industry, Vol. 2.
Instrument Society of America, 1981.
Reference Order f!7, Appendix B
Flow Measurement HandbooK, Open Channel. Grant, D.M. ISCO, Inc.
2nd Edition, 1981.
Reference Order #15, Appendix B
Flow Measurement; Parsnall Plume, Standard Method for Open Channel Flow
Measurement of Industrial Water and Industrial Wastewater.
ASTM D1941-67.
Reference Order #4, Appendix B
Flow Measurement In Wastewater Treatment Plants with venturi Tubes and
Venturi Nozzles, Recommended practice For.
Kulin, G. EPA-600/2-84-185, November, 1984.
Reference Order #20, Appendix B.
Flow Meter Engineering Handbook. Honeywell, Inc., 5th Edition, 1977.
Reference Order #14, Appendix B
Flow Meter Engineering, Principles and Practice of. Spink, L.K.
Foxboro Co., 9th Edition, 1967.
Reference Order #13, Appendix B
Flow Meters In Wastewater Treatment plants, Recommended Practice For The Use
of Electromagnetic. ——-
Kulin, G. EPA-600/2-84-187, November, 1984.
Reference Order #20, Appendix B
266
-------
TITLE BY SUBJECT
Flow Monitor Device, Choosing. Hall, J. Instruments and Control Systems,
June 1978.
Reference Order #16, Appendix 3
Flcwmeter Accuracy, piping Arrangements for Acceptable. Sprenkle, R.E.
ASME Transactions 67:345, 1945.
Reference Order #10, Appendix B
Flowmeters to Water Management Systems, Application of. Brown, A.E.
Instrument Society of America, Oct. 1981.
Reference Order |17, Appendix B
Flowmeters, Ultrasonic; Basic Design, Operation and Criteria Application.
Powell, D.J. Plant Engineering, May 1979.
Reference Order #21, Appendix B
Fluid Meters, Their Theory and Application. ASME Report on Fluid Meters.
6th Edition, 1971.
Reference Order 13, Appendix B
Flumes In Wastewater Treatment plants, Recommended Practice For The Use of
Parsnail Flumes and Palroer-Blowlus.
Kulin, G. EPA-600/2-84-186, November, 1984.
Reference Order #20, Appendix B
Industrial Pressure Measurement. 'Gullum, R. Instrument Society of America
Publications, 1982.
Reference Order 117, Appendix B
Industrial Process Control. Lloyd, S.G., Anderson, G.D. Fisher Controls,
1971.
Reference Order 112, Appendix B
267
-------
TITLE BY SUBJECT
Instrument Engineers Handbook, Process Measurement. Liptak, B.C.,
Venczel, K. Chilton Book Co., 1969, Revised 1982.
Reference Order 17, Appendix B
Instrumentation In Wastewater Treatment Plants. Manual of Practice No. 21.
Water Pollution Control Federation, 1978.
Reference Order 126, Appendix B
Liquid Plow Instruments, Meter Runs. Foxboro Technical Bulletin 8-110,
Feb. 1943,
Reference Order #13, Appendix B
Liquid Flow Measurement. Foxboro Technical Bulletin 8-251, Aug. 1965.
Reference Order #13, Appendix B
Magnetic Flow Meter. Fischer & Porter Instruction Mo. 10D1435 A,,
Revision 1, 1969.
Reference Order til, Appendix B
Magnetic Flow Meter—Application. Sybron/Taylor Corp. Product Data,
PDS-15E002, Issue 3.
Reference Order #23, Appendix B
Magnetic Flow Meter—Basic Theory. Sybron/Taylor Corp. Product Data,
PDS-15E001, Issue 3.
Reference Order #23, Appendix B
Magnetic Flow Meter—Installation. Sybron/Taylor Corp. Product Data,
PDS-15E003, Issue 2,
Reference Order #23, Appendix B
268
-------
TITLE BY SUBJECT
Minicomputers in Industrial Control. Harrison, T.V. Instrument Society of
America, 1978.
Reference Order $17, Appendix B
Orifice and Venturi Meters, Survey of Information Concerning the Effects
of Nonstandard Approach Conditions. Starret, P.S., Halfpenny, P.F.,
Noltage, H.B. ASME 1965.
Reference Order flO, Appendix B
Orifice Plates for Flow Measurement, Flange Mounted snarp Edged, instrument
Society of America, RP 3.2-1978.
Reference Order $17, Appendix B
Parshall Plumes and Palmer-Bowlus Flumes in Wastewater Treatment plants,
Recommended Practice For The Use Of.
Kulin, G. EPA-600/2-84-186, November, 1984.
Reference Order 120, Appendix 3
Process Control; Basic Concepts, Terminology and Techniques for.
Gordon, Lewis M. Reprinted from Chemical Engineering, May 1983, p. 58.
Reference Order $13, Appendix B
Process Control Computer Systems Guide for Managers. Stire, T.G.
ISBN 250-40488-5.
Reference Order #5, 9, Appendix 3
Process Control Systems. ShinsXy, F.M. McGraw-Hill, 2nd Edition, 1979,
Reference Order $18, Appendix B
Process Instrumentation Manifolds. Benson, J.E. Instrument Society of
America, 1978.
Reference Order |17, Appendix B
269
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TITLE BY SUBJECT
Process Instruments and Control Handbook. Considine, D.M. McGraw-Hill,
2nd Edition, 1974.
Reference Order 118, Appendix B
Refinery Instruments and Control Systems; Manual on Installation of.
Part I—Process Instrumentation and Control, Section 1—Flow.
API RP550, 3rd Edition, 1977.
Reference Order 12, Appendix B
Refinery Instruments and Control Systems; Manual on Installation of.
Part I—Process Instrumentation and Control, Section 2—Level.
API 1974.
Reference Order #2, Appendix B
Refinery Instruments and Control Systems: Manual on Installation of.
Part I, Section 6—Control Valves and Accessories. API, 3rd Edition,
1976.
Reference Order 12, Appendix B
Refinery Instruments and Control Systems: Manual on Installation of.
Part II—Process Stream Analyzers. API RP550, 3rd Edition, 1977.
Reference Order #2, Appendix B
Three Mode Controllers, Fundamentals of. Jury, P.O. Technical Monograph 28,
Fisher Controls, 1973.
Reference Order *12, Appendix B
Tuning Controllers for Noisy Processes. Fertik, H.A. ISA Transactions,
Vol. 14, No. 4, 292-304, 1975.
Reference Order 117, Appendix B
Ultrasonic Flowmeters; Basic Design, Operation and Criteria Application.
Powell, D.J. Plant Engineering, May 1979.
Reference Order 121, Appendix B
270
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TITLE BY SUBJECT
Valve; Advances In Water and Wastewater Valves. Consulting Engineer,
Sept. 1982, p. 105.
Reference Order #8, Appendix B
Valve Rangeability, Plain Talk on. Wing, Paul. Instrumentation Technology,
April 1978, p. 53.
Reference Order |17, Appendix B
Valves With process Requirements, Matching. Ramsey, Jonn.R.,
Fournier, Charles D. Instrument and Control Systems, June 1973, p. 65.
Reference |16, Appendix B
Venturi Meter Tube, Standard Methods of Flow Measurement of Water.
ASTM D2458-69.
Reference Order $4, Appendix 3
Venturi Tubes and Venturi Nozzles, Recommended practice.For Flow Measurement
In Wastewater Treatment Plants With.
Kulin, G. EPA-600/2-84-185. November, 1984.
Reference Order #20, Appendix B
Water Measurement Manual. U.S. Department of the Interior, Bureau of
Reclamation, 2nd Edition, 1981.
Reference Order |24, Appendix B
271
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APPENDIX B
REFERENCE LIST SOURCES
Reference f Available From;
1 American Public Works Association
1313 East 60th Street
Chicago, IL 60637
(312) 667-2200
American Petroleum Institute
Publication and Distribution Section
2101 L Street NW
Washington, DC 20037
(202) 457-7160
American Society of Mechanical Engineers
Order Department
United Engineering Center
P.O. Box 3199
New York, NY 10163
American Society for Testing and Materials
1916 Race Street
Philadelphia, PA 19103
(215) 299-5400
Butterworth Publishers
c/o EMA, Inc.
Hanover Building
480 Cedar Street
St. Paul, MN 55101
(612) 298-1992
Chemical Engineering, 43 PL
1221 Avenue of the Americas
New York, NY 10020
272
-------
Reference t Available From:
7 Chilton Book Company
Chilton Way
Radnor, PA 19039
(215) 964-4729
Consulting Engineer
1301 S. Grove Avenue
P.O. Box 1030
Harrington, IL 60010
(312) 331-1840
EMA, Inc.
Hanover Building
480 Cedar Street
St. Paul, MN 55101
(612) 298-1992
10 Engineering Societies Library
345th East 47th Street
New York, NY 10017
(212) 705-7606
11 Fischer & Porter
25 Jacksonville Road
Warminster, PA 18974
12 Fisher Controls Company
205 South Center Street
Marshalltown, IA 50158
13 Foxboro Company
Literature Stock Department 544
P.O. Box 568
Foxboro, MA 02035
(617) 543-8750
273
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Reference f Available From:
14 Honeywell, Inc.
Publications
Fort Washington, PA 19034
(215) 641-3000 '
15 ISCO, Inc.
P.O. Box 82531
Lincoln, ME 68528
1-800-228-4373
(402) 464-0231
16 Instrument & Control Systems
(Chilton's I&CS)
•-nilton Co.
Radnor, PA 19089
(215) 964-4496
17 Instrument Society of America
67 Alexander Drive
P.O. Box 12277
Research Triangle Park, NC 27709
(919) 549-8411
18 McGraw-Hill Publications Co.
1221 Avenue of the Americas
New York, NY 10020
(212) 997-1221
19 Moore Products Co,
Sumney Town Pike
Springhouse, PA 19477
(215) 646-7400
20 National Technical Information Service
5285 Port Royal Rd.
Springfield, VA 22161
(703) 487-4650
274
-------
Reference # Available From:
21 Plant Engineering
1301 S. Grove Avenue
P.O. Box 1030
Harrington, IL 60010
(312) 381-1840
22 Water, Engineering & Management
Scranton Gillette Communications, Inc.
380 Northwest Highway
Des Plaines, IL 60016
23 Sybron/Taylor Corp.
100 Midtown Tower
Rochester, NY 14604
(716) 546-4040
24 Superintendent of Documents
U.S. Government Printing Office
Washington, DC 20402
25 United States Environmental Protection Agency
26 West St. Clair Street
Cincinnati, OH 45268
(513) 684-2621
26 WPCF
2626 Pennsylvania Avenue NW
Washington, DC 20037
(202) 337-2500
275
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GLOSSARY
access time: 1. The time it takes a computer to locate data or an
instruction word in its storage section and transfer it to its
arithmetic unit where the required computations are performed. 2. The
time it takes to transfer information which has been operated on from
the arithmetic unit to the location in storage where the information is
stored.
accuracy: The maximum error in the measurement of a physical quantity in
terms of the output of an instrument when referred to the individual
instrument calibration. Usually given as a percentage of full scale.
actuator: A mechanism for translating an electronic or pneumatic signal
into a corresponding movement or control. Example: an actuator moves
a valve based on a control signal. See also final control element.
address: A numerical expression which designates a specific location in
stored memory (software), or a specific card and pin number (hardware).
alarms: Devices which signal the existence of abnormal conditions. See
also annunciator.
algorithm: A prescribed set of rules or procedures for tiie solution of a
problem in a finite number of steps.
alphanumeric: Pertaining to a character set that contains both letters and
numerals and other characters such as punctuation marks.
amplifier: A device that enables an input signal to control power from a
source independent of the input signal and thus be capable of
delivering an output that bears a relationship to, but is generally
greater than, the input signal.
analog: Pertaining to representation of numerical quantities by means of
continuously variable physical characteristics. Contrast with digital.
analog control: Implementation of automatic control loops with analog
(pneumatic or electronic) equipment.
analog device: A mechanism which represents numbers by physical
quantities, i.e., by lengths, as in a slide rule, or by voltage or
currents as in a differential analyzer or a computer of the analog type.
analog signal: A continuously varying representation of a physical
quantity, property, or condition such as pressure, flow, or
temperature. The signal may be transmitted as pneumatic, mechanical,
or electrical energy.
276
-------
analog-to-digital converter (ADC): A device used to convert an analog
signal to approximate corresponding digital data.
annunciator: A visual or audible signaling device and the associated
circuits used for indication of alarm conditions.
application software: Programs which are unique to a specific process
control system installation or other specific installations, rather
than general purpose and of broad applicability.
assembler: A software program that translates symbolic language into
machine language by the substitution of operation codes for symbolic
operation codes and absolute or relocatable addresses for symbolic
addresses.
asynchronous transmission: Transmission in which each information
character, word, or block of data is individually synchronized,
usually by the use of start and stop elements. Time between
transmission of characters can vary.
automatic: Pertaining to a process or device that under specified
conditions, functions without intervention by a human operator.
background processing: A processing method whereby some computer programs
with a low priority are executed only when the computer is not busy
with execution of higher priority programs.
backup: provisions for an alternate means of operation in case of
unavailability of the primary means of operation.
batch processing: 1. pertaining to the technique of executing a set of
programs, such that each is completed before the next program of the
set is started. 2. Loosely, the execution of programs serially.
baud rate: A unit of signaling speed indicating the number of signal
changes per second. Most signal schemes have two states representing
a bit equal to 1 or 0. In this case bit rate equals baud rate. Some
signaling schemes have multiple states. In these, baud rate is less
than bit rate.
binary coded decimal (BCD): Describing a decimal notation in which
individual decimal digits are represented by a group of binary bits,
e.g., in the 8-4-2-1 coded decimal notation each decimal digit is
represented by a group of four binary bits. The number twelve is
represented as 0001 0010 for 1 and 2, respectively, whereas in binary
notation it is represented as 1100.
binary number: A number usually consisting of more than one figure,
representing a sum, in which the individual quantity represented by
each figure is based on a radix of two. The figures used are 0 and 1.
277
-------
bit: 1. An abbreviation of binary digit. 2. A single character in a
binary number. 3. A single pulse in a group of pulses. 4. A unit of
information capacity of a storage device. The capacity in bits is the
logarithm to the base two of the number of possible states of the
device. Related to storage capacity. 5. One binary digit, the
smallest piece of information in a computer system. A bit can be
either 1 or 0.
bit rate: Rate at which binary digits are transmitted over a communications
link. (See baud rate.)
block transfer: The process of transmitting one or more groups of data.
boiler plate: Common expression referring to standard written requirements
or regulations, usually inserted in the front of a contract or
specification.
Boolean: An algebraic system formulated by George Boole for formal
operations on true/false logic.
buffer: 1. An internal unit of a computing device which serves as
intermediate storage between two storage or data handling operations
with different access times or formats; usually to connect an input or
output device with the main or internal high-speed storage. 2. An
isolating component designed to eliminate the reaction of a driven
circuit on the circuits driving it, e.g., a buffer amplifier.
bumpless transfer: A characteristic of a controller which permits control
mode changes (such as automatic or manual selection) to be made
without producing a discontinuity in the controller output.
byte: 1. A generic term to indicate a measurable portion of a string of
two or more binary digits, e.g., an 8-bit byte. 2. A group of binary
digits usually operated upon as a unit.
cascade control: The use of two conventional feedback controllers in series
such that two loops are formed, one within the other. The output of
the controller in the outer loop modifies the setpoint of the
controller in the inner loop.
cathode ray tube (CRT): 1. An electronic vacuum tube containing a screen
on which information may be stored for visible display by means of a
multigrid modulated beam of electrons from the thermionic emitter. 2.
Loosely, a computer terminal using a cathode ray tube as a display
device.
central processing unit (CPU): A unit of a computer that includes circuits
controlling the interpretation and execution of instructions.
character: One alphanumeric symbol, e.g., letter, figure, number,
punctuation or other sign. Characters are usually represented by a
code of binary digits, e.g., ASCII.
278
-------
closed-loop: A signal path which includes a forward path, a feedback path,
and a summing point, and forms a closed circuit.
common mode rejection (CMR): The ability of a circuit to discriminate
against a common mode voltage. CMR may be expressed as a
dimensionless ratio, a scalor ratio, or in decibels.
common mode voltage: A voltage relative to ground of the same polarity on
both sides of a differential input.
compiler: A program that translates a problem-oriented language to a
machine oriented language, such as FORTRAN, and substitutes
subroutines and single machine instructions for symbolic inputs.
computer: 1. A data processor that can perform substantial computation,
including numerous arithmetic or logic operations, without
intervention by a human operator during the run. 2. A device capable
of solving problems by accepting data, performing described operations
on the data, and supplying the results of these operations. Various
types of computers are calculators, digital computers and programmable
controllers.
control: 1. in process control, this refers to actions taken to achieve a
desired result in the process. 2. In some applications, a mathematic
check. 3. In computer programs, instructions which determine
conditional jumps are often referred to as control instructions, and
the time sequence of execution of instructions is called the flow of
control.
control loop: Several control devices connected in series to perform a
specific control function.
control mode: A specific type of control action such as proportional,
integral or derivative.
control sequence: See sequence control program.
control strategy: A sequence of instructions (program) executed by a
computer to achieve a desired control objective.
control system: 1. A system in which deliberate guidance or manipulation is
used to achieve a prescribed value of a variable. 2. Refers to a
system of hardware and software components including computers, disks,
printers, instruments, control panels, operator facilities,
communications channels, systems programs, development programs, and
applications programs.
controller: A device which operates automatically to regulate a controlled
variable by comparing a measurement of the variable with a reference
value representing the desired level of operation.
279
-------
cursor: A marker which moves over a surface to delineate position. A dot
on a CRT screen, for example.
OAS: Data acquisition system.
DEAC: Digitally emulated analog control.
data base: A collection of stored data in memory that is application
specific for use by the control software - for example, remote station
files that contain input/output point description data for each remote
station.
data highway: Refers to a coax cable data link connecting remote processors,
operator facilities and the central computer to provide information
exchange.
deadband: 1. A specific range of values within which the incoming signal
can be altered without also changing the outgoing response. 2. The
range of values of a process variable where no control action is
taken, if the process variable exceeds the deadband high or low
limits, control action is started,
deadman control: Continuous manual action (e.g., depressing a pushbutton)
is required to modulate device position or speed. Device maintains
status quo when control action is absent.
dead time: The interval of time between initiation of an input change or
stimulus and the start of the resulting observable response.
derivative action: A controller mode which contributes an output
proportional to the rate of change of the error.
development software: Software which provides a means for creation of
application software, such as a data base generator.
diagnostic: A computer routine designed to test a hardware or software
function and identify malfunction or error.
digital: Pertaining to representation of numerical quantities by discrete
levels or digits conforming to a prescribed scale of notation.
direct acting controller: A controller in which the value of the output
signal increases as the value of the input (measured variable)
increases.
direct digital control (DDC): 1. A control technique in which a digital
computer is the controller and its output is connected directly to the
final control element. Used to distinguish from analog control. 2.
Implementation of analog PID control using computer software rather
than conventional analog controllers (DEAC).
disk (or disc): A memory storage device using one or more rotating plastic
discs coated with a magnetic material for recording information.
280
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distributed control: Location of controlling equipment (either computer or
conventional controllers) at remote locations throughout the process.
disturbance: A change in the operating condition of a process, most
commonly a change in input or output loading.
download: a process by which a CPU in a computer control system network
transfers a program (task) image to another CPU in the network and
causes it to be executed.
duplex: See full duplex.
dynamic data: Data whose value depends on conditions or parameters that
change with time. Contrast with fixed or static data.
engineering unit (EGU): Units of measure of process variables.
error: The difference between the setpoint reference value and the value of
the measured signal.
expendables: Items expected to be consumed such as print paper, lubrication
fluids, and air filters. Distinguished from spare parts used for
replacement of failed components such as printed circuit boards, power
supplies, and fuses.
failure: Loss of ability to perform a specified function.
feedback: The signal in a closed-loop system representing the condition of
the controlled variable.
feedback control: Control in which a measure variable is compared to its
desired value to produce an actuating error signal which acts upon the
process to reduce the magnitude of the error.
feedforward control: Control in which information concerning one or more
conditions that can disturb the controlled variable is converted,
outside of any feedback loop, into corrective action to minimize
deviations of the controlled variable.
final control element: The device used to directly change the value of the
manipulated variable.
firmware: A series of computer instructions (programs) permanently stored
in BOM (read only memory).
floppy disk: A small flexible inexpensive magnetic disk commonly used for
data storage in small computer systems.
flowchart: 1. A system analysis tool that provides a graphical presentation
of a procedure. Includes block diagrams, routine sequence diagrams,
and data flow symbols. 2. A chart to represent for a problem, the
flow of data, procedures, growth, equipment, methods, documents, and
machine instructions. 3. A graphical representation of a sequence of
operations by using symbols to represent the operations such as
compute, substitute, compare, jump, copy, read, and write.
281
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foreground processing: A nigh-priority processing method where real-time
control programs and process inputs are given preference (tnrough the
use of priority interrupts) over other programs being executed by the
computer system. See bac leg round processing.
full duplex: A communications channel with separate circuits for
transmission and reception, so that both can occur simultaneously.
Also referred to as duplex communications.
graphic: Pertaining to representational or pictorial material, usually
legible to humans and applied to the printed or written form of data
such as curves, alphabetic characters, and radar scope displays.
naif duplex: A communications channel where transmission and reception
share the same circuit so that both cannot occur simultaneously.
hand/off/automatic (HDA): Refers to a 3-position selector switch on a
control panel. In AUTOMATIC a computer or logic in the panel control
the associated device. In HAND, the device is turned on from the
local panel. In OFF, the device state is turned off from the local
panel.
hard coded: A programming practice whereby application-related information,
such as point names, and ranges, are not represented symbolically
within a program such that recompilation is necessary in order to
change the information.
Hertz: Abbreviated Hz. A unit of frequency equal to one cycle per second.
hierarchy: Refers to levels of supervision and control responsibility
within a centralized control system.
hierarchical network: A computer network in which processing and control
functions are performed at different levels tyy several computers
especially suited to the functions performed.
incremental control: Use of short pulses to increase or decrease the value
of the controlled variable. Contrast with positional control.
input/output list: A list that describes remote station and satellite input
and output signal points containing information such as point label,
type, address, and limits.
instruction set: A listing of the instructions or operations a particular
computer can execute.
instrumentation: The application of devices for the measuring, recording,
or controlling of physical properties and movements.
interlock: A mechanical or electrical device or wiring which is arranged in
such a manner as to allow or prevent operation of equipment only in a
pre-arranged sequence.
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integral action: A controller mode which contributes an output proportional
to the integral of the error.
integral time: The time required after a step input is applied for the
output of a proportional plus integral mode controller to change by an
amount equal to the output duo to proportional action alone.
isolation: a physical and electrical arrangement of the parts of a
receiving instrument to prevent interference currents within or
between input and output of the instrument.
lead-lag compensation: An electronic network or software used to influcence
the response of a control loop.
line conditioning: The addition of equipment to a leased voice-grade
channel to provide minimum line characteristics necessary for data
transmission.
linearity: Ability to achieve a straight-line response to an input signal.
load shedding: starting and stopping of equipment to reduce electrical
power demand.
local control: Control operations performed either manually or automatically
at a control panel located near the process or equipment.
logging: Recording values of process variables for later use in trending,
report compilation, or historical records.
loop: See control loop.
loop gain: The ratio of the change in the return signal to the change in
its corresponding error signal at a specified frequency. Note: the
gain of the loop elements is frequently measured oy opening the loop,
with appropriate termination. The gain so measured is often called
the open loop gain.
machine language: Coding in the numeric language form acceptable to the
computer arithmetic and control unit.
main frame: 1. The central processor of the computer system. It contains
the main storage, arithmetic unit and special register groups.
Synonymous with CPU and central processing unit. 2. All that portion
of a computer exclusive of the input, output, peripheral and in some
instances, storage units.
manipulated variable: The process variable that is changed by the
controller to reduce or eliminate error.
manual control: Control operations are performed directly by a human
operator and not by computer control algorithms. Two levels of manual
control are possible; 1. local manual - the process is controlled
manually from the local panel; 2. computer manual - the process is
controlled manually through computer system interactive CRT displays.
manual loading station: A manual electronic controller. Used here to refer
to a controller whose output is adjusted manually by an operator from
the front of a panel.
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menu: A CRT screen which lists operator options available related to a
given area. For example, a graphic menu lists all graphic displays
which may be chosen.
modem: A contraction of modulator/demodulator. This is a device that
converts digital data into a form suitable for transmission ovar the
communications media. For example, phone line modems convert digital
data to audio signals.
monitoring: The information on the conditions of various water control
processes, operations, levels, and security obtained by electronic
devices.
multi-drop network: A type of communications system where lines are
terminated (dropped) at intermediate points between the end terminals
of the system.
multiplexer (MUX): A device which samples input and/or output channels and
interleaves signals in frequency or time.
multiplexing: The process of combining several measurements for
transmission over a pair of wires or link.
multi-processing: Pertaining to the apparent simultaneous execution of two
or more programs or sequences of instructions by a computer or
computer network.
multi-programming: pertaining to the apparent simultaneous execution of two
or.more programs by a single computer.
multi-tasking: The facility that allows the programmer to make use of the
multi-programming capability of a system.
normal mode voltage: A voltage induced across the input terminals of a
device.
OOC: Optimizing digital control.
offset: The steady-state deviation of the controlled variable from the
setpoint caused by a change in load.
on-line: Pertaining to a computer that is actively monitoring or
controlling a process or operation, or pertaining to a capability of
the user to interact with a computer.
on/off control: A system of regulation in which the manipulated variable
has only two possible values, on and off.
open-loop: A signal path without feedback.
operating limits: High and low limits set for a process variable. A value
of process variable between these limits is considered normal and no
control action is taken. When either of the limits are exceeded an
alarm or control action is initiated.
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operating system: Software that controls the carrying out of computer
operations such as scheduling, compilation, storage assignment, and
data management.
operator interface (operator process interface): The means through which an
operator accesses tne computer system to affect process control
actions. Usually this consists of a CRT and Keyboard arrangement
along with appropriate display software.
optimization: A process whose object is to make one or more variables
assume, in the best possible manner, the value best suited to the
operation at hand, dependent on the values of certain other variables
which may be either predetermined or sensed during the operation.
package system: Equipment which is supplied as a system including controls.
parallel transmission: A mode of transmission whereby each bit of a data
word is transmitted simultaneously over separate communications
circuits.
•
parameters: The limits or context within which the problem is considered.
parity: One method of error checking in data communications. As a bit
string is transmitted, an extra bit is added to make the total number
of bits either odd or even (called odd or even parity). The receiving
machine checks each bit string for correct parity.
peripheral: A machine that enables a computer to communicate with the
outside world or aids the operation of the computer.
permissive: A signal which permits the placing of equipment into operation.
piping and instrumentation drawing (P&ID): A schematic drawing of tne
process showing, all liquid flow paths, location of all sensors and
instruments, and location of backup conventional control equipment.
point: A single signal to or from a field device, points are either analog
in, contact (digital) in, modulating (analog) out, or contact
(digital) out.
polling: Periodic interrogation of each of the terminals that share a
communications channel to determine whether it needs to use the
channel to transmit. An alternate to a contention system of
communication channel control.
positional control: Use of a maintained signal output value to modulate
device position or speed. Contrasts with incremental control.
power management: Making most efficient use of operational periods for
equipment and using tne lower cost rate non-peak electrical energy
periods to the greatest extent possible. See load shedding.
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precision: The quality of being exactly defined. This is sometimes
represented by the number of significant bits in the digital
representation. Also related to repeatability instruments.
primary element: The device which converts a portion of the energy of the
variable to be measured to a form suitable for amplification and
retransmission by other devices.
priority: Level of importance of a program or device.
priority interrupt: The temporary suspension of a program currently being
executed in order to execute a program of higher priority. Priority
interrupt functions usually include distinguishing the highest
priority interrupt active, remembering lower priority interrupts
which are active, selectively enabling or disabling priority
interrupts, executing a jump instruction to a specific memory
location, and storing the program counter register in a specific
location.
process: 1. The collective functions performed in and by industrial
equipment, exclusive of computer and/or analog control and monitoring
equipment. 2. A general term covering such items as assemble,
compile, generate, interpret and compute.
process control: Descriptive of systems in which controls are used for
automatic regulation of operations or processes.
process I/O: Input and output operations directly associated with a
process, as contrasted with I/O operations not associated with the
process. For example, in a process control system, analog and digital
inputs and outputs would be considered process I/O whereas inputs and
outputs to bulk storage would not be process I/O.
process variable: 1. In a control loop, the variable being controlled to
the setpoint. 2. Any parameter within the process that is of interest
from an operations or control standpoint.
programmable controller: A control machine using solid-state digital
logic. Used primarily for replacement of electromechanical relay
panels.
programmable Read Only Memory (PROM): A non-volatile (data won't be lost
when power is turned off) memory that is used to store unchanging
information such as programs. The computer system cannot write
information tc. a PPOM. PROM's are loaded at the factory and can only
be changed using special equipment.
proportional action: A controller mode which contributes an output
proportional to the error.
proportional band: The range of the controlled variable that corresponds to
the full range of the final control element.
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protocol: A set of rules governing communications between computers that
insures messages are correctly sent, received, and understood.
R/C filter: An electronic filter network made up of passive components such
as resistors and capacitors.
Random Access Memory (RAM): Tnat portion of computer memory that can be
both written to and read from.
range: The region between the limits within which a quantity is measured,
received or transmitted, expressed by stating the lower and upper
range values.
rate time: For a linearly changing input to a proportional plus derivative
mode controller, the time interval oy which derivative action advances
the effect of proportional action.
ratio control: Control in which a secondary input to a process is regulated
to maintain a preset ratio between the secondary input and an
unregulated primary input.
real time: Tne performance of a computation during the actual time that the
related physical process transpires in order that the results can be
useful in guiding the process.
regulatory control: Maintaining the outputs of a process as closely as
possible to tneir respective setpoint values despite the influence of
setpoint changes and disturbances.
remote: Referring to an operations point located at some distance from the
field device. »
remote/local (R/L): Refers to a switch setting at local panel controls.
The switch in local position means that control can be exercised at
the local panel only, locking out control from a remote location. The
switch in remote position means that control is exercised frcm a
remote location and local controls are locked out.
repeataoility: The ability of an instrument to produce the same output
reading when a given condition is applied repeatedly.
repeats per minute: Controller integral mode adjustment units. The inverse
of integral time.
reset windup: In a controller containing integral action, the saturation ot
tne controller output at a high or low limit due to integration of a
sustained deviation of the controlled variable from the setpoint.
reverse action controller: A controller in which the value of the output
signal decreases as the value of the input (measured variable)
increases.
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screen refresh: A Hardware function which maintains an image on a CRT
screen by the continuous generation of a composite video signal from
data stored in the memory of a CRT controller.
screen update: A software function which periodically replaces the dynamic
data portion of a display with current real-time data.
select-before-operate: A supervisory control system in which a point is
first selected and a return displayed proving the selection after
which one of several operations can be performed on that point.
self-configuring: A method of programming which eliminates the need to
reassemble or recompile programs after a change in configuration by
the dynamic use of parameters external to the program which define the
particular configuration.
sensor: See transducer.
sequence control program: A high-level program whose primary function is to
cause a sequence of events to happen based on current process
requirements or operator requests.
serial transmission: A mode of transmission whereby bits of a data word are
sent sequentially (starting with either the most or least significant
bit) over a single communications channel.
setpoint: in a control loop, refers to the desired value of the process
variable being controlled.
signal: 1. The event or phenomenon that conveys data from one point to
another. 2. A time dependent value attached to a physical phenomenon
and conveying data.
simulation: The representation of certain features of the behavior of a
physical or abstract system by the behavior of another system, for
example, the representation of physical phenomena by means of
operations performed by a computer or the representation of operations
of a computer by those of another computer.
software: 1. A set of programs, procedures, rules and associated
documentation concerned with the operation of a computer system, for
example, compilers, library routines, and manuals. 2. A program
package containing instructions for tne computer hardware.
software development system: See development software.
software packages: Various computer programs or sets of problems used in a
particular application.
software subsystems: Refers to major segments of the software wnich
performs a unique, identifiable function. This includes such
subsystems as operating system, logging, scanning, graphic displays,
alarming, DDC and each separate control function.
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source code: Code used as input to a translation program, such as an
assembler or a compiler.
SCADA: Supervisory control and data acquisition.
span: The algebraic difference between the upper and lower range values.
steady-state: A characteristic of a condition, such as value, rate,
periodicity, or amplitude, exhibiting only negligible change over an
arbitrary long period of time. It may describe a condition in which
some characteristics are static, others dynamic.
supervisory control: 1. A control technique in which a digital computer is
used to determine and fix setpoints for conventional analog
controllers. Used to distinguish from direct digital control. 2. A
high-level program whose primary function is to oversee an on-going
process and alter the general parameters of a contcol strategy based
upon mathematical relationships.
synchronous transmission: 1. Transmission in which the sending and
receiving instruments are operating continuously at substantially the
same frequency and are maintained, by means of correction, in a
desired phase relationship. 2. A mode of data transmission whereby
the message is sent in a continuous bit string.
system regeneration: The complete reconfiguration of a computer's main
memory and mass memory. This function will typically require talcing
the computer control subsystem off line.
system software: Software which includes the operating system, resources
management functions, and processing functions for data acquisition
and control.
table driven: A programming method whereby the paths of execution through a
computer program are controlled by the contents of a data table
containing logical or numeric data.
telecommunication: The use of leased telephone lines for sending
informative data and performing operational controls at great
distances.
telemetering: The transmission of a measurement over long distances,
usually by electromagnetic means.
time constant: The time required for the output of a single capacity
element to change 63.2 percent of the amount of total response when a
step change is made in its input.
time slicing: A technique for allocating CPU time between multiple programs
within a multi-programming environment. This technique allocates a
given segment of time to each task in a round-t;obin fashion without
regard to task priority or importance.
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transducer: An element or device which receives information in the form of
one ptiysical quantity and converts it for transmission, usually in
analog form. This is a general definition and applies to specific
classes of devices such as primary element, signal transducer and
transmitter.
transmitter: A transducer which responds to a measured variable by means of
a sensing element, and converts it to a standardized transmission
signal which is a function only of the measured variable.
uninterruptible power supply (UPS): A power supply having backup battery
storage. A UPS is used to insure operation of critical computer
equipment during power failures.
utility program: A program providing basic conveniences such as loading and
saving programs, and initiating program execution.
watchdog timer: An electronic internal timer which will generate a priority
interrupt unless periodically recycled by a computer. It is used to
detect program stall or hardware failure conditions.
word: 1. A character string or a bit string considered as an entity. 2. A
group of binary digits treated as one unit of information and stored
in a single memory location.
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REFERENCES
American National Standards institute. American National standard - process
Instrumentation Terminology. ANSI/ISA - S51.1. 1979.
American National Standards Institute. American National Directory for
Information Processing. ANSI X3/TR-1-77.
American National Standards Institute. IEEE Standard Dictionary of
Electrical and Electronic Terms ANSI/IEEE 100-1977.
SKrolcov, M. R. ED. Mini - and Microcomputer Control in Industrial
Processes. Van Nostrand Reinhold Co. New York. 1980.
Graf, Rudolf F., Modern Dictionary of Electronics. Howard W. Sams & Co.,
Inc., 1977.
Reproduced from
best available copy.
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