Draft
ENFORCEMENT WORKSHOP ON
PLANT INSPECTION AND
EVALUATION PROCEDURES
VOLUME VII
g CONTROL EQUIPMENT OPERATION
0 AND MAINTENANCE - WET SCRUBBERS
CO
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532
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF ENFORCEMENT
OFFICE OF GENERAL ENFORCEMENT
WASHINGTON, D.C. 20460
07-OO-7&.
-------
REFERENCE MATERIAL FOR TECHNICAL WORKSHOP
ON EVALUATION OF INDUSTRIAL AIR POLLUTION
CONTROL EQUIPMENT OPERATION AND MAINTENANCE
PRACTICES
Volume VII
Operation and Maintenance of
Wet Scrubbers
Compiled by
PEDCo Environmental, inc.
505 S. Duke Street
Durham, North Carolina 27701
Contract No. 68-01-4147
PN 3470-2-O
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Enforcement
Division of Stationary Source Enforcement
Washington, D.C. 20460
May, 1979
-------
FOREWORD
The following document is a compilation of selected
technical information and publications on the evaluation of indus-
trial air pollution control equipment operation and maintenance
practices. The reference manual is intended to be an instructional
aid for persons attending workshops sponsored by the U.S. Environ-
mental Protection Agency Regional Offices.
-------
TABLE OF CONTENTS
Page No.
VOLUME VII: Operation and Maintenance of Wet Scrubbers
VII-1. Wet Collectors: Section VII, Control of
Particulate Emissions Training Course Manual
in Air Pollution, U.S. Public Health Service,
March 1977. 1-1
VII-2. Venturi Scrubber Operation and Maintenance,
K. C. Schifftner, prepared for Environmental
Research Information Center Seminar on Operation
and Maintenance of Air Pollution Equipment
for Particulate Control, U.S. EPA, Cincinnati,
Ohio, April 1979 2-1
VII-3. Maintaining Venturi-Tray Scrubbers, William J.
Kelly, Chemical Engineering, December 4, 1978,
pp. 133-137. 3-1
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WET COLLECTORS:
SECTION VII, CONTROL OF PARTICULATE EMISSIONS
TRAINING COURSE MANUAL IN AIR POLLUTION
U.S. Public Health Service
March 1977
1-1
-------
WET COLLECTORS: INTRODUCTION
I Wet collectors increase particle removal
efficiency by two mechanisms.
A Re-cntrainment of the collected particles
is prevented by trapping them in a liquid
film or stream and then washing the liquid
(and trapped particles) away.
B Fine particles are "conditioned" so that
their effective size is increased, thus
enabling them to be collected more
efficiently.
addition of wetting agents does not
significantly increase removal
efficiency.
3 Effect of solubility of particles
Solubility of the particles in the
droplets is not a factor in effectiveness.
(An exception is the case of concentrated
mist droplets, such as sulfuric acid.
These droplets may grow in size by
absorption of moisture when passing
through a humid chamber).
II PARTICLE CONDITIONING
Particle conditioning in wet collectors
involves the process of increasing the
effective size of the fine particles so that
they may be more readily precipitated. The
effective size may be increased by:
Forcing precipitation of fine particles on
liquid droplets, or
Promoting condensation upon fine particles
(which act as nuclei) when the water vapor in
a gas passes through its dewpoint.
Conditioning by Forcing Precipitation of
Particles on Liquid Droplets
1 An example:
An example is the attachment of a
5-micron dust particle to a liquid
droplet 50-microns in diameter thereby
increasing its apparent mass 1000 fold
for collection purposes.
2 Effect of wetting agents in resisting
redispcrsion
Collision of solid particles with liquid
droplets is inelastic and because of
Van iler Waal's forces, the agglomerates
resist redispcrsion. Therefore, the
B Conditioning by Promoting Condensation
upon the Particle Surface
If the liquid spray causes the gas to pass
through its dewpoint, condensation will
take place upon the surface of the particles
when the particles act as nuclei. Thus,
the effective size of the particles is
increased under such conditions. This
mechanism is important for initially hot
gases containing relatively small dust
concentrations (say less than 1-grain/cf).
Ill OPERATING PROBLEMS OF WET
COLLECTORS
A Corrosion
All water scrubbers have the inherent
problem of corrosion.
a Even when no chemically corrosive
constituent may be contained in the
carrier gas stream, the carbon
dioxide present contributes to
corrosion.
b When corrosive agents are contained
in the gas stream (SO2, chlorides,
fluorides, nitric acid, etc.),
will occur on wet metallic surfaces.
PA.C.pm.7!>.!> (JO
1-2
-------
Wet Collectors: Introduction
B Erosion
Wet collectors that remove insoluble.
abrasive materials have troubles due
to erosion especially if removal is
dependent upon impingement velocities
or centrifugal action.
C Wet-Dry
Scrubbers are faced with problems at
wet-dry junctions, particularly at the
entrance of an installation.
a When dust concentrations and gas
temperature' are high, there may be a
zone where dust build-up can occur
(by reason of moist dust layers).
D Mist Elimination
1 In all scrubbers, entrainment eliminators
are important to prevent carry-over of
droplets.
2 Many scrubbers have mist eliminators
built into their design.
3 When not incorporated in the design,
mist elimination is accomplished by
means of additional separators.
E Slurry Handling
1 For all scrubbers, a method must
be provided for handling the liquid
effluent. Slurries may be treated by
means of:
t
a Settling tanks
b Filters
c Liquid cyclones
d Further chemical or recovery
methods
e Disposal to sumps, streams, rivers
f Others
2 All these effluent handling methods have
their own unique engineering problems.
1-3
-------
COLLECTION OF PARTICLES ON CYLINDRICAL
AND SPHERICAL OBSTACLES
Participates transported by a carrier gas
through a depth of cylindrical (fibers) or
spherical (granules) obstacles tend to be
precipitated upon the surface of the obstacles.
Van dcr Waal's and electrical forces cause
the particulates to adhere to the surfaces of
the obstacles resulting in the removal of the
particulates from the gas stream.
I MECHANISMS OF PARTICULATE
REMOVAL
A Screening (or sieving) is not the principal
mechanism
It can be shown that the sizes of the gas
passages through the depth of obstacles
are very much larger than the sizes of the
particulates collected.
B The principal mechanisms by which par-
ticulates are brought into contact with
the obstacles include:
1 Interception
2 Gravitation
3 Impingement
4 Diffusion
5 Electrostatic
6 Thermal
II INTERCEPTION
Particulates being carried by a flow of gas
tend to follow the streamlines around an
obstacle. By chance, a particle on one of
the streamlines may make contact with the
obstacle if the streamline passes the obstacle
at a distance less than the radius of the
particle. This typo of removal is called
diri'i.-t interception, and depends solely on
tin- position that a particle has in the gas
stream.
Ill GRAVITATION
As a particle passes by an obstacle, it may
fall (under the influence of gravitational
force) from the streamline along which it is
being carried and settle upon the surface of
the obstacle.
IV ELECTROSTATIC
Since a force of attraction exists between
bodies possessing electrostatic charges of
opposite polarity, it is possible for a
charged particle to be removed from the
gas stream by an oppositely charged obstacle.
However, when only the particle or obstacle
is charged, a charge may be induced upon
the uncharged component resulting in a
polarization force that can also effect
particle removal.
The effect of the electrostatic mechanism
of particle removal from a gas stream may
be significant when the charge on the particle
or obstacle is high, and when gas velocity
is low. The significance of particle size and
obstacle size varies, depending on whether
the electrical attraction originates from
Coulomb or polarization forces.
The mechanism involved in a bed of fibers
or granules depends principally on the charac-
teristics of the particulates and obstacles in
the bed, and on the gas velocity.
V IMPINGEMENT TARGET EFFICIENCY
A The meaning of impingement target
efficiency
When an obstacle is placed in the path of
a particulate-ladcn gas stream (Figure 1)
the streamlines will diverge and pass
around the obstacle. The particles, how
ever, tend to leave the streamlines (along
PA. C. pm. GOa. 5.61
1-4
-------
Collection of Particles on Cylindrical and Spherical Obstacles
which they are being carried) at the be-
ginning of the curvature and may impinge
upon the obstacle.
If like particles, initially within a cross-
section of the carrier-gas stream having
a radius of IV (measured from the central
2
streamline) strike a cylindrical obstacle
of diameter Do, then D1 is termed the
"impingement target diameter" of the
obstacle for the particular particles being
considered.
The ratio D1 is called the "impingement
F°J
target efficiency" and is symbolized rjj.
In other words, it is the ratio of the cross-
sectional area of the gas stream cleaned
of particles (all of which are alike) to the
projected area of the obstacle.
If it is assumed that all particles are alike
and equally dispersed throughout the gas
stream, the 'impingement target efficiency"
is the ratio of the weight of partioulatc
collected by the obstacle to the weight of
particulate that would pass on if tht: ob-
stacle were not there. Therefore, "im-
pingement target efficiency" is the effi-
ciency of removal by weight of like
particles by one obstacle.
B The Mathematical Expression
Impingement target efficiency (rjj) is a
function of the dimcnsionless ratio,
Cross-Section of Air
Stream Cleaned of
Particles
D0g
where:
vp/ofp(s)
impingement target efficiency for
uniformly dispersed like particles
and for one obstacle.
DQ = diameter of the obstacle
vp/o = relative velocity of the particle
(in the approaching gas stream)
to the obstacle
'p(s) = Stokes' settling velocity
C Impingement Target Efficiency Curves
(Figure 2)
Figure 2 demonstrates the relationship
between impingement targe efficiency
and the dimensionless ratio
vP/ofp(s)
Note that there are two curves; one for
spheres and one for cylinders. The im-
pingement target efficiency for spheres is
higher than that for cylinders because the
streamlines diverge more sharply around
spheres.
Impingement on a Spherical Obstacle
Figure 1
1-5
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Collection of Particles on Cylindrical and Spherical Obstacles
W
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.01
.02 .03 0.1 0.2 0.3
Gas Velocity (Ft/Sec)
Impingement and Diffusional Target Efficiencies at Various Air Velocities for
Different Diameter Dust Particles (Ref. 2)
-4
(p ,20) (Fibre Diameter * 10u) (Viscosity = 1.8(10) poises)
P
Figure 3
Figure 2 shows that small obstacles (Do)
and high relative velocities (vp/0) are
essential if high impingement target ef-
ficiencies (HJ) are to be achieved for a
given particle.
VI DIFKUSIONAL TARGET EFFICIENCY
FOR CYLINDRICAL OBSTACLES
In addition to impingement, another important
mechanism of particle precipitation on ob-
stacles in a gas stream is diffusion. However,
the diffusion mechanism plays little part in
the separation of particles from a gas stream
except for the very finest ones. It is of
special interest where very liifjii overall ef-
ficiencies of removal are required, and
where there are low velocities.
A The Mathematical Relationship^2)
1 The general equation
8K
(2)
where:
r)D = diffusional target efficiency for
uniformly dispersed like particles
and for one obstacle.
D0 = obstacle diameter (cm)
vp/o= relative velocity of the particle (in
the approaching gas stream) to the
obstacle (cm/see)
1-7
-------
Collection of Particles on Cylindrical and Spherical Obstacles
.01 .02 .03
0.1 0.2 0.3
Gas Velocity (Ft/Sec)
1.0 2.0 3.0
Target Efficiency at Various Air Velocities for Different Diameter Dust
Particles (Ref. 2)
-4
(p s 2 0) (Fibre diameter 10u) (Viscosity r 1.8(10) poises)
P
K = a constant depending on the temper-
ature, viscosity, and particle
diameter (cm2/sec)
K = 1.45(10)
-17
where:
T » absolute temperature (°K)
p = absolute viscosity of the gas (poise)
. DD = particle diameter (cm)
2 For air below 100°C
2.45
Dp DO
where:
(3)
rjD = diffusional target efficiency for like
particles and one obstacle when gas
stream is air below 100°C (%)
vi- relative velocity of the particle (in
P the approaching gas stream) to the
obstacle (ft/sec)
1-8
-------
Collection of Particles on Cylindrical and Spherical Obstacles
D = diameter of the particle (microns)
D0 = obstacle diameter (microns)
B Remarks
The above equations show that if target
efficiency due to diffusion (HQ) is to be
high, then the relative velocity (vp/o)
must be low. This is opposite to the re-
quirements for high impingement target
efficiency (rjj) which demands high relative
velocity. This leads to a velocity zone
for a given obstacle where the efficiency
of removal will be low for a given particle-
si'/.e; that is, where conditions are poor
for both impingement and diffusion.
This is evident by observing the low por-
tions of the curves in Figure 4.
VII SIZE-EFFICIENCY
A The equations for target efficiencies pro-
vide information on the removal of like
particles by only one obstacle, or, let's
say, removal of like particles by one
"treatment" of the gas stream.1 Since in
the actual course of filtration through a
bed of fibers or granules the- gas stream
meets a number of obstacles and is there-
fore "treated" a number of times before
it exits, an efficiency equation taking all
"treatments" into account is necessary.
B The general equation for size-efficiency
E = 1 - (1 - n)S°
where So is small [as in spray devices
(S0 * 5) and old cloth (So ~ 2)] (4)
E = 1 - e'nS°
where SQ is large [_as in packed fiber or
granular beds and new cloth filters
(S0 * 5 Of] (5)
where:
E = efficiency of removal of a given
particle-size (size-efficiency).
The particle size is identified in
' e = natural logarithmic base = 2. 718
number of "treatm
by the gas stream
S0 = number of "treatments" received
_ Total projected area of all obstacles in the filter
° Cross-section of filter normal to the gas flow
r\ - target efficiency of the individual
obstacles
C Size-efficiency for a bed of spherical
granules
E = 1 - e
E = 1 -e'"So
(6)
(7)
where:
E = efficiency of removal of a given
particle-size by a bed of spherical
obstacles. The particle-size is
identified in 1.
e = natural logarithmic base = 2. 718
n - target efficiency of the individual
shperical granules in the bed
L = depth of the bed
« = volume of the spherical granules
per unit volume of bed
Do = diameter of the spherical granules
So = number of "treatments" received
by the gas stream as it passes
through the bed
s _ Total projected area of all obstacles in the bed
0 Cross-section of the bed normal to the- gas flow
1-9
-------
Collection of Particles on Cylindrical and Spherical Obstacles
D Size-efficiency for a bed of cylindrical
fibers
Impingement"
DpPpvP/o
1 - e
E = l-e'nS°
(9)
where:
E = efficiency of removal of a given
parade-size by a bed of cylindrical
obstacles. The particle size is
identified in rj.
« = volume of fibers per unit volume
of bed
e = natural logarithmic base = 2. 718
rj = target efficiency of the individual
fibers in the bed
L = depth of bed
DO = diameter of the fibers
S0 = number of "treatments" received
by the gas stream as it passes
through the bed
_ Total projected area of all obstacles in the bed
0 " Cross-section of the filter normal to the gas flow
Diffusion
where:
Dp = diameter of the particle removed
from the gas stream
Do = diameter of the obstacle
p_ = density (mass) of the particle
g - local acceleration due to gravity
M. = viscosity of the gas
vp/o= velocity of the approaching gas
stream with respect to the obstacle
R = universal gas constant
T = absolute temperature of the gas
REFERENCES
1 Lapplc. C. E. Fluid and Particle
Mechanics. U. of Delaware. 1956.
VIII PARAMETERS OF COLLECTION
EFFICIENCY
Using the following theoretical relationships.
a qualitative evaluation of the relative im-
portance of the various collection mechanisms
may be derived,
Mechanisfn
Direct interception
Gravitation-T
Parameter
Dp-/
2 Stairmand. C. J. Dust Collection by Im-
pingement and Diffusion. Paper read
at Midland Branch of A. Inst. P.
Birmingham. England. Oct. 14, 1950.
3 Dallavalle, J. M. Micromeritics. Pit-
man Publishing Corp. N. Y. 1948.
4 Miller. J. S. and Traxler, R. N.
Annual Asphalt Paving Conference.
The Asphalt Institute, pp 315 - 23.
(Discusses physical properties of
mineral filters).
1-10
-------
THE GRAVITY SPRAY TOWER
I IMPINGEMENT OF GRAVITATIONAL
SPRAY DROPS
A In a gravitational spray unit, there are a
number of liquid spherical obstacles
(droplets) falling in an empty tower by
the action of gravity in the path of rising
particles.
The relationship between_impingement
target efficiency (nj) and' Dog "| is
shown in Figure 1.
vp/o fp(S)j
It is seen that the maximum efficiency
for the smaller particle sizes (say
less than 5n) occurs for droplet size
of about 800|j.: and that for larger
particle sizes, the efficiency varies
little over the range of droplet sizes
500 to 1000^.
Thus in gravitational spray towers,
there is little point in using very
fine spray sizes even if such were
available.
Vc
in gravitational spray towers.
is the difference in the free-falling
velocities (Stokes1) of the droplets and
the particle.
In practice, since the free-falling
velocity of the particle is small com-
pared to the droplet, [~Vp/o i may be
taken as the free-falling velocity of the
droplet.
C From Figure 1, it is evident that for high
collection efficiency by impingement
there must be a small obstacle (Do) and
a high relative velocity I V" between
I Vp/g
the obstacle and particle.
1 In gravitational spray towers, these
conditions tend to be mutually in-
compatible. (Small droplets have
small free-falling velocities)
2 Therefore, there is an optimum drop-
let size (for a given particle size) for
maximum impingement target efficiency.
(See Figure 2. )
a Inspection of Figure 2 shows that as
droplet size diminishes to the range
500- 100(V, the target efficiency
im-reases. However, a further de-
crease in droplet size, decreases
the impingement target efficiency.
II EFFICIENCY
A Inspection of Figure 2 shows that the ef-
ficiency of a gravitational spray tower is
very low for particles below 1-2 microns.
B Figure 3 illustrates a size-efficiency curve
for a large industrial spray tower handling
70, 000 cfm. The tower is 22 ft in diameter,
66 ft high. Pressure drop is less than
1" water.
Ill DUST CONCENTRATIONS
A There are no fine clearances for the pas-
sage of dust-laden gases. Therefore,
it can handle relatively high dust con-
centrations without fear of chokage.
IV GAS VOLUME
A It is capable of handling large quantities
of gas.
B It is often used as a pre-cooler where
large quantities of gas are involved (as
in blast furnaces).
V RECIRCULATION OF WATER
A Since very fine droplets are not employed,
the spray generators need not have fine
jets.
PA. C. pin. 7-Ja. 5. (il
1-11
-------
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Particle Size, Microns
Size-efficiency Curve for Spray Tower
Figure 3
-------
. The Gravity Spray Tower
Hence, the dirty water may be re-
circulated until it contains quite a high
concentration of trapped dust particles.
a Therefore, there is a saving of
water, and perhaps a simplification
of effluent treatment and ultimate
waste disposal.
VI PRESSURE DROP
A The pressure drop is very small (less
than 1 in. w. g.)
Draft loss-
Efficiency-
less than l" w. g.
very low for below
l-2n
Particle concentrations- relatively high (over
5 gr/cu ft)
Particle composition solid, liquid. Some
problems with corrosion.
Water usage
about 18 gal/lOOOcuft
VII PERFORMANCE DATA
Gas flow
Gas temperature
-over 70, 000 cfm
-often used as pre-
cooler. Gas tem-
perature over 2000°F
may be reduced to
275°F.
Gas velocity about 3-5 fps
Treatment time about 20-30 seconds
REFERENCES
1 Stairmand, C. J. Dust Collection by Im-
pingement and Diffusion. Paper read
at the Inaugural Meeting of the Midland
Branch of A. Inst. P. Birmingham,
England. October 14, 1950.
2 Stairmand, C. J. The Design and Per-
formance of Modern Gas-Cleaning Equip-
ment. Paper read before the A. Inst.
P. London. November, 1955.
L-15
-------
VENTURI SCRUBBERS
I MECHANISM OF PARTICLE REMOVAL
A Since for high collection efficiency of fine
particles by impingement there must be a
small obstacle (Do) and high velocity of
approach of the gas stream relative to the
obstacle (v_/0). attempt is made to ap-
proach this ideal by:
1 An arrangement in which very small
water droplets (upon which the particles
impinge) are formed by the gas flow so
that the droplets are initially at rest
at the time of impact with the particles.
Even during the period of acceleration
of the droplet, high relative velocities
will be maintained since the particles
move at the velocity of the gas stream.
a Such an arrangement is incorporated
in the Venturi scrubber. See
Figure 1.
II OPERATION (Figure 1)
A Collection of Particles upon the Droplets
1 In the Venturi scrubber, the particulate-
laclen gas passes through a duct which
incorporates a Venturi scrubber.
2 At the throat, high gas velocities of the
order of 200-600 fps are attained.
3 Coarse water spray is injected into the
throat by way of radial jets in quantities
of 5 to 7 gpm per M cfm of gas.
4 The high gas velocities at the throat
immediately atomize the coarse water
spray to fine droplets (about 50 microns).
5 Since, at their genesis, these fine drop-
lets arc initially at rest relative to the
particles in the gas stream, it is at this
moment that collection efficiency is at
its maximum. (vp/o) is maximum.
a The atomized droplets, being fine,
rapidly accelerate to the velocity
of the carrier gas; but even during
this short period, relative velocities
will be high and effective collision
between droplet and particle will
take place.
It is during the period before the
droplets attain the same velocity
as the gas stream that any relative
velocity between the droplets and
particle is obtained. For example,
a 100-micron droplet introduced
into a gas stream moving at 100
fps would accelerate to 90% of the
gas velocity in 16 inches; a 20-
micron droplet would reach 90%
of the gas velocity in 2 inches.
B Removal of the Dirty Droplets
1 As the gas decelerates after passing
through the throat, agglomeration of
the particle-laden droplets takes
place.
2 The large agglomerates arc readily
removed by a cyclonic separator.
EFFICIENCY
A Effect of Pressure Drop on Efficiency
1 The higher the pressure drop, the
higher the removal efficiency of
particles. See Figures 2 and 3.
2 Pressure drops across the Venturi
of 25-30 inches of water gage may be
expected.
3 Pressure drop can be increased (and
hence efficiency can be increased)
simply by increasing the gas velocity
and/or the water injection rate. See
Figure 4.
PA.C. pm. 68a.5. 61
1-16
-------
Venturi Scrubbers
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Venturi Scrubbers
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Venturi Pressure Drop (in. w. g.)
Curve A: Rotary iron powder kiln
B: Lime kiln, asphalt plant
C: Iron cupola
D: Phosphoric acid plant (acid mist)
E: Incinerator (sodium oxide fumes)
Figure 2(5)
C
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tn
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30
40
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Venturi Pressure Drop (in.w. g.)
Curve A: Cupola gases
B: Blast furnace gases
Figure 3(5>
Water/Gas Ratio, Gal/1000 cu. f*.
Reja'ior Be-weer. Pressure- loss
and Wa'.er Usage in Ver'uri
Scrubber
Figure 4
4 When gas cleaning requirements change,
the only adjustment necessary to the
Venturi scrubber, in most cases, is
in the flow of scrubbing liquid to in-
crease1 the pressure drop. Thus higher
cleaning efficiency is accomplished
without modification or addition.
B Effect of Particle Concentration on
Efficiency
1 If the number of water droplets is held
constant and the number of particles
(concentration) is increased, the number
of collisions would be expected to in-
crease. In other words, collection
efficiency should increase as loading
increases.
2 This increase, however, is due not
only to the increased chances of particle
collision with droplets, but also due to
collisions between the particles
themselves.
C Size-Efficiency
1 The Venturi scrubber approaches
100% for all particles larger than 1. 5
to 2 microns.
2 Figure 5 shows a size-efficiency curve
for a Venturi scrubber^). Sizes above
2-microns were obtained on special
1-18
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IT 1 L. " rr
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8
Particle Sixe. Microns
Size-efficiency Curve for Venturi Scrubber (6-in. throat)(3. 500 cfm gas)
Figure 5
n
£
c
C/}
o
a-
ro
-------
Venturi Scrubbers
TABLE 1
TYPICAL PERFORMANCE DATA FOR VENTURI SCRUBBER
(5)
Source of Gas
IRON 1 STEfl INDUSTRY
Guy liun Cupula
()>)>!«" Miti Cuiiicitti
ten upi-ii H. i, Hi 1 urn..-, t !>. i.iln
Slcrl 0|.u. Hi-jilh (JTI.JII
0. Mist
lead Compounds
lead 1 Im Compound!
Aluminum Chloride
2mc t Lead Ondt Dusts
line 0»de Fume
Lime Dust
Soda Fumt
Limestone i Rock Dull
Cement Dust
Catalyst Dust
H.SO. Mist
Oil Fumes
Ammonium Chloride Fumes
Fluorine Compounds
lime Oust
Soda Fume
Salt Cake
HCI Fumes
Fly Ash
Sodium Oiirh Fumw
pproximat
>ize Range
(Microns)
1 10
i?
oa-i
M
.520
.M
.1-1
.550
.5-100
^
«
_
_
_
_
^
*l
_
.M
.1-1
.1-J
.1-.9
.1-1
.05-.$
1-50
J-l
ISO
.MS
5-50
.K
.
.05-1
.1-SO
.1-2
_
.1-3
A.I
s Load
(Grain
Inlet
17
810
.515
l<
374
10-12
15
3-10
525
JOS'
408*
136*
198*
756'
25'
1080*
1-5
1-1.5
192'
24
1-2
35
1-5
1-8
5-10
.2-5
5-15
1-2
.09
138*
756'
.1-.5
309'
S-10
25
4-6
25'
1-2
±1
ing
s/ cf)
Exit
.05.15
05 08
.03. Ot
.01 -.0?
.008. Ob
.04- .08
.1-.3
.1-.3
.005-01
1.7*
2.8*
3J'
2.0'
7.8'
2.0'
58.0*
.05-.!
.OS-.08
3.8*
.05-15
.12
.02-.05
.05-.!
.1-.5
.05-.15
.01-.05
.05-15
.05-.!
.005
3.3*
8.0'
.05
5.5'
.05-.1S
.01-.05
.4-.E
2.3'
.05-.08
.02
Average
Removal
Efficiency (%)
95
98I>
35
99
99
99
92
99
99.9
99.4
99.3
97.5
99
98.9
90+
95
95
95
95+
9«+
99
91
95
98
95
994.
99
98+
97+
95+
97.5
98+
85+
98+
99+
99
90
90+
98
98
Nott: Tht tffielrnetn (kotvn atom art avtrafft vatutl for a particular
plant or group of tiuladaffont optrating muttr a iptetff *tt of eonditiom.
Milllrtnwper cubic It
1-20
-------
Venturi Scrubbers
silica dust powders and those for the
smaller sizes on dispersed non-patho-
genic bacteria. This size-efficiency
curve suggests a very high efficiency
for a comparatively simple piece of
equipmi-nt.
D Overall Efficiency
Table I shows some efficiencies of collec-
tion experienced by various installations.
IV ENERGY USAGE
A Since pressure drops of 30 inches water
gage correspond to 120 kwh per million
cubic feet of gas cleaned, efforts have
been made to reduce the pressure drop.
However, if pressure drop is reduced,
there is a tendency to reduce the efficiency
also.
B Additional high energy usage results from
the method of injecting water into the
Venturi throat. See Figure 6.
TUB TVPK 8-F VX>mtmX
The Chemico Type S-F Venturi Scrubber is particularly
recommended for these hard-to-handle situations: re-
moval of "sticky" solids from gases; recycling of
heavy slurries where water supplies are limited; and
recovery of process materials in concentrated form.
In the SF Venturi, scrubbing liquid is introduced
through troughs at the top of the unit. The liquid
flows downwardly in a continuous film along the slop-
ing walls to the deflecting lips, which direct it across
the throat of the Venturi to be atomized by the force
of the high velocity gas.
TUB TYPE P-A VKNTX7RX
The Chemico Type P-A Venturi Scrubber is most ef-
fective in the very difficult applications requiring
efficient removal of sub-micron dust fume, and mist
particles.
Chemical Construction Corp.
Figure 6
1-21
-------
Venturi Scrubbers
V PERFORMANCE DATA
Gas flow ---
Gas velocity through throat- - -
Pressure loss
Gas temperature ------
Overall efficiency
High efficiency on dusts with mass median
size greater than »
Humid air influence on efficiency-
Water usage
REFERENCES
1 Stairmand, C. J. The Design and Per-
formance of Modern Gas-Cleaning
Equipment. Paper read before
A.lnst. P. London. November. 1955.
2 Nicklcn, G. T. Some Recent Developments
and Applications of Scrubbers in In-
dustrial Gas Cleaning. Proceedings
APCA. 52nd Annual Meeting APCA.
Los Angeles. June. 1959.
200 to over 145. 000 cfm
200-600 fps
up to 25-30 in. wa'.ergape
"unlimited"
-usually high (97 - 99*%)
0. 5-2n
none
-5-7 gpm of water per
M cfm gas
3 Jones, W. P. Development of the Venturi
Scrubber. Ind. Eng. Chem. Nov. 1949.
4 Basse, B. Gases Cleaned by the Use of
Scrubbers. Blast Furnace and Steel
Plant. Nov. 1956.
5 Chemico Gas Scrubbers for Industry.
Bulletin M-104, Chemical Construction
Corporation. 525 West 43rd Street,
N.Y. 36, N. Y.
6 Venturi Scrubbers for Industry. Bulletin
M-103A, Chemical Construction Cor-
poration, 525 West 43rd St. N.Y. 36, N.Y.
1-22
-------
COLLECTORS WITH SELF-INDUCED SPRAYS
I MECHANISM OF PARTICLE
COLLECTION
A In this equipment, the particle collection
zone is a spray curtain which is induced
by the gas flow itself through a specially
designed orifice. (The spray curtain is
followed by a spray eliminator).
B The Collection of Particles
1 Normal gas velocity of about 50 fps
creates droplets about 320(i.
2 Collection of particles is mostly by
impingement on the droplets during
the free-falling period of the droplets
and also during the period of the accel-
eration of the droplets from rest (when
high relative velocities are available).
II APPLICATION
A Since there is an absence of ledges,
moving parts, and restricted passages.
these units are especially adapted to
materials like:
1 Magnesium and explosive dusts
2 Sticky or linty materials like metallic
buffing exhausts
PERFORMANCE DATA
Water usage - 10-40 gal/1000 cfm gas
cleaned (Much or all of this
urO +*a »« W\f3 ^r lt£> ₯>Af*lV*f*1l1at£»fll
Efficiency - See Figure 3
10-40 gal/1
cleaned (Much or all of this
water may be recirculated).
Sensitivity not particularly sensitive
to cfm change (at least within + 25% of the
design rate)
Concentration- high concentrations (40 grains/
ft3) (There are no fine clear-
ances to cause chokage)
Pressure drop- 2 j - 6 in. w. g.
Maintenance - The whole apparatus is well
irrigated and periodic hos ing-
down of the interior is easily
done. There is an absence of
moving parts. There may be
corrosion difficulties.
CNTRAMMCNT
C.OLUCTING CLEMENTS «*-. iV
Figure 1
TVPICAL WET
ORIMCE TYPE COLLECTOR
PA.C. pm. 70.9. 60
1-23
-------
Collectors With Self-Induced Sprays
»~- < <.;'."::, "7^"*^ *p
' jr^-^'^r;^-0--
^r^^t^.:-^:';^ '
1-24
-------
I
to
en
16 20 24
Particle Size, Microns
Sire-Efficiency Curve for Self-Induced Spray Collector
Figure 3
36
40
n
3
1
U)
/I
-------
Collectors With Self-Induced Sprays
REFERENCES
1 First. M.. et. al., "Performance
Characteristics of Wet Collectors, "
NYO-1587 Waste Disposal. Harvard
University. 1953.
2 Stairmand. C. J. "Mist Collection by
Impingement and Diffusion, " paper
read at the Inaugural Meeting of
Midland Branch of A. Inst. P.,
Birmingham. England. Oct. 14, 1950.
3 Stairmand, C. J. "The Design and
Performance of Modern Gas-Cleaning
Equipment, " paper read before the
A. Inst. P., London. November, 1955.
Kane.J. M. "Operation, Application, and
Effectiveness of Dust Collection Equip-
ment, " Heating and Ventilating. Aug. 1952.
Nicklen, G. T. "Some Recent Develop-
ments and Applications of Scrubbers
in Industrial Gas Cleaning, " Proceed-
ings APCA, 52nd Annual Meeting,
Los Angeles. June, 1959.
Magill, P. L. Air Pollution Handbook,
McGraw-Hill Book Co., Inc. 1956.
1-26
-------
WET DYNAMIC PRECIPITATOR
I OPERATION (Figure 1)
A Wet dynamic precipitators combine the
dynamic forces of a rotating fan to cause
the particles to impinge upon numerous
specially shaped blades.
B A film of water is maintained on the blades
by spray nozzles.
High efficiency on
particles with mass
median greater than.
Efficiency sensitivity
to cfm change ....
Water usage
.1-
.no
.0.5 to 1 gpm/1000
cfm gas
OMT I WATIH.
OIXMAHGCD AT
LAOC TIP1
DWTV AM
IHLtT
Figure 1
REFERENCES
1 First, M.. et al. Performance Character-
istics of Wet Collectors. NYO-1587
Waste Disposal. Harvard University.
1953.
2 Stairmand, C. J. Mist Collection by
pingement and Diffusion. Paper read
at the Inaugural Meeting of Midland
Branch of A. Inst. P., Birmingham,
England. October 14, 1950.
3 Stairmand, C. J. The Design and Per-
formance of Modern Gas-Cleaning
Equipment. Paper read before the A.
Inst. P. London. November, 1955.
II PERFORMANCE DATA
Pressure drop,
.... a function of mechan-
. ical efficiency
Usually less than 1-
in. w. g.
Pressure drop
sensitivity to cfm
change a function of mechan-
ical efficiency.
Particle concentration, .less than 1 grain/ft^.
(For heavy loading.
a pre-cleaner may
be used to lighten the
load on the unit).
Kane, J. M. Operation, Application, and
Effectiveness of Dust Collection
Equipment. Heating and Ventilating.
August, 1952.
Nicklen, G. T. Some Recent Developments
and Applications of Scrubbers in In-
dustrial Gas Cleaning. Proceedings
APCA, 52nd Annual Meeting, Los
Angeles. June, 1959.
Magill, P. L. Air Pollution Handbook.
McGraw-Hill Book Co., Inc. 1956.
PA.C. pm. 71.0. GO
1-27
-------
DISINTEGRATOR SCRt'imiSRS
I MECHANISM OF PARTICLE COLLECTION
A Since for hiph collection efficiency there
must be a small obstacle (Do) and a high
relative velocity between the obstacle and
particle (vp/o), attempt is made to ap-
proach this ideal by:
1 Shooting water drops at the particles
so that a high relative velocity (VD/O)
will be obtained (even if such velocities
are maintained for short periods) and
arranging that this bo done so that a
very large number of impacts will be
achieved.
B Such action is incorporated in the disinte-
grator scrubber (Figure 1).
II OT'ERATION
A A disintegrator scrubber consists of an
outer casing containing alternate rows of
staler and rotor bars, the relative velocity
between adjacent bars being of the order
of 200 - .iOO fps.
B Water is injected axially and is effectively
atomized into fine droplets (say 25n) by
the rapidly rotating vanes.
C The dust-laden gas also enter axially and
passes through the dense spray zone where
the particles are subjected to intense
bombardment by the water droplets.
Water inlets
Statore
Rotors
Dirty gas
inlet
Clean gas
Exit
Effluent
Figure 1
1-28
i* \.c.pm. on.!». <;o
-------
Disintegrator Scrubbers
>>
u
o
456
Particle Size, Microns
Size-Efficiency Curve for Disintegrator Scrubber
Figure 2
14 15
REFER K
1 First. M. . <-tal. Performance Character-
isiies of U'ct Collectors. XYO1587
\Vaste. Disposal, Harvard University. 1953.
2 Stairmand, C..T. Mist Collection by Im-
pingement and Diffusion. Paper read
at the Inaugural Meeting of Midland
Rraneh of A. lust. p. Birmingham,
Kii!.:l:inH. O'tolier 14, HT)0.
'? St-'iirma ml. ('..'. The Design anil Per-
formanee
-------
Disintegrator Scrubbers
III. PERFORMANCE DATA
Efficiency
Pressure drop
Energy usage
Water consumption
Concentration *. .
highly efficient. See Figure 2
for a size-efficiency curve.
less than 1-in. w. g.
high power requirements. Total
power consumption may be
16-20 HP per 1000 cfm gas
cleaned. This power is largely
expended in atomizing and ac-
celerating the water.
extremely high.
usually preceded by convention-
al collectors as cyclones and
scrubbers to insure that low
concentrations of the order of
i to i grains per cu. ft. are
presented to the unit. These
precautions are necessary to
avoid build-up in the disintegra-
tor, which, running at high speed
with fine clearance, is particu-
larly susceptible to trouble if
operated under unsuitable condi-
tions.
1-30
-------
IMPINGEMENT TYPE SCRUBBING TOWER
I TYPES OF SCRUBBING TOWERS
A There are two types of scrubbing towers
commonly used:
1 Those employing impingement target
plates
2 Those employing beds of spherical
obstacles
II TOWER WITH TARGET PLATES (Figure 1)
A Construction and Operation
1 This type of scrubber is a tower con-
sisting of a vertical shell in which are
mounted a large number of equally
spaced, circular, perforated (orifice)
plates.
a At one side of each orifice plate, a
conduit, called a downspout, is pro-
vided to pass the liquid to the plate
below.
b At the opposite side of the orifice
plate, a similar conduit feeds liquid
from the plate above.
2 Over each hole (about 3/16" diameter)
in the orifice plate, a target plate is
positioned.
a The motion of the gas past the edge
of the holes in the orifice plate re-
sults in the formation of spray drop-
lets (about 10(V). These droplets
are initially at rest and provide an
effective relative velocity between
particle and droplet for good
impingement.
b The particle-laden gas passes through
the holes in the plate and the particles
impinge upon the atomized droplets
and on the target plates.
B Particle Concentration
1 An important feature of this design is
freedom from chokage in spite of the
small holes in the orifice plates. This
is due to:
a The very violent circulation induced
below the targets by the air jets,
and
b A preliminary spray zone which
helps to keep the orifice plate free
from deposits.
2 Concentrations of 40 grains/ft^ can
readily be handled.
C Efficiency
1 An example of a size-efficiency curve
is shown in Figure 2.
D Pressure Drop
1 Each plate imposes a pressure drop of
3 in. w. g.
TOWERS WITH BEDS OF SPHERES
A Construction and Operation
(An example of a scrubbing tower with
beds of spheres is shown in Figure 3)
1 Large particles are removed by im-
pingement on wet surfaces and contact
with water spray in an area below the
filter bed.
2 Particle-laden gas then passes upward
through a bed of spheres. In the inter-
stices of the bed, the particles are
subjected to increased velocities which
results in their efficient impingement
upon the surfaces of the spheres.
PA. C. pm. 73. 9. GO
1-31
-------
Impingement Type Scrubbing Tower
(G
TAPGST^V"
PLATE x\ -
\ --
E"
a
>
o
o
UATGR tXWPLETS ATOMIZED
AT KOGGS or ORIFICES.
GAS PLOW
DOWNSPOUT TO-
GA". FLOW
ARRANGEMENT OP "TAKVSJfT PLATES* MECHANISM Of (MPMSEMCNT
IN IMPINGEMENT SCRURBCR. SCRUBBER.
IMnNOtMCNT
MFnt STAOE
ACCtOMOATINO
SIOT STAOI Peabody Engineering Corp.
Figure 1
1-32
-------
I
co
co
4 6 8 10
Particle Size, Microns
Size-efficiency Curve for Wet-Impingement Scrubber
Figure 2
12
14
I
ff
%
(9
3
W
n
i
I
**
TO
-------
Impingement Type Scrubbing Tower
Figure 3
National Dust Collector Corp.
3 The high gas velocity through the
interstices of the packed spheres also
results in pulling water upward with
sufficient force to disintegrate the
water streams into a turbulent mist in
the zone above the filter bed. Here,
ultra- fine particles are trapped by the
mist and constantly flushed downward.
4 Mist carried by the upward flowing
cleaned gas is removed by passage
through a bed packed with porcelain
saddles.
B Particle Concentration
1 Such units have self-cleaning action
and there is freedom from build-up of
solids and ease of cleaning.
2 Concentrations of about 40 grains
arc readily handled.
C Pressure Loss
1 Pressure loss is 4-6 in. w. g.
D Efficiency
1 Efficiency is high on two micron-sized
particles and above.
E Water Consumption
1 Fresh water: l/4gpm per 1000 cfm gas
cleaned
2 Recirculated water: 3 gpm per 1000
cfm gas cleaned (Scrubbing liquid can
have high solids content).
F Capacity
1 Units handle 500 to 40, 000 cfm.
REFERENCES
1 First, M. et al. Performance Character-
istics of Wet Collectors. NYO-1587
Waste Disposal, Harvard University.
1953.
2 Stairmand. C. J. Mist Collection by Im-
pingement and Diffusion. Paper read
at the Inaugural Meeting of Midland
Branch of A. Inst. P. Birmingham,
England. October 14, 1950.
3 Stairmand, C. J. The Design and Per-
formance of Modern Gas-Cleaning
Equipment. Paper read before the A.
Inst. P. London. November,. 1955.
4 Kane. J. M. Operation. Application, and
Effectiveness of Dust Collection Equip-
ment. Heating and Ventilating. August,
1952.
5 Nicklen, G. T. Some Recent Development
and Applications of Scrubbers in In-
dustrial Gas Cleaning. Proceedings
APCA, 52nd Annual Meeting, Los
Angeles. June, 1959.
6 Magill, P. L. Air Pollution Handbook,
McGraw-Hill Book Co., Inc. 1956.
1-34
-------
WET CENTRIFUGAL COLLECTORS
I TYPES OF WET CENTRIFUGAL
COLLECTORS
A Irrigated Types
1 These rely upon the throwing of parti-
cles against wetted collected surfaces,
such as wetted walls or impingement
plates by centrifugal action.
B Spray Chamber Types
1 These depend upon impaction of the
particles upon spray droplets and the
subsequent precipitation of the "dirty"
spray droplets upon the wall of the
unit by centrifugal action.
II IRRIGATED TYPES (Figure 1)
B
ct"t»iruc«l ceiuctg»
A-CW «lr wiM
8 IniyirtU. j»«».
H-W.
1-DHyikkU
l-W.W w4 M* **
Figure 1
American Air Filter Co.
A The efficiency of a centrifugal collector
may be increased by irrigating its walls,
if the attendant disadvantages of a wet
system can be tolerated.
B Water distribution may be from low
pressure nozzles or gravity flow.
C Performance Data
Water rates 3-5 gal/1000 cfm of
gas treated
Draft loss 2} to 6"
Draft loss sensitivity
to cfm change .
High efficiency on
particles of mass
median greater than.
Efficiency sensitivity
to cfm change. . . . ,
Humid air influence
on efficiency
Gas temperature . . .
as (cfm)2
yes
none
"unlimited"
III CYCLONE SPRAY CHAMBERS (Figure 2)
A Operation
1 The dust-laden gas enters tangentially
at the bottom and spirals up through
a spray of high velocity fine water
droplets.
2 The dust particles are collected upon
the fine spray droplets which are then
hurled against the chamber wall by
centrifugal action.
3 An unsprayed section above the nozzles
is provided so that the liquid droplets
containing the collected particles will
have time to reach the walls of the
chamber before the gas stream exits.
PA. C. pm. 72. 9. fiO
1-35
-------
Wet Centrifugal Collectors
PEASE ANTHONY O^UOKIC SPftftT SCRUBgEJL
.SUANCDjUS.
W*Tt<« WATCH
OUTLCT WUT
Figure 2
Chemical Construction Corp.
B Efficiency
1 Efficiency of dust removal is given by:
3r7 rWH
E = 1 -e
2D0Q
where:
E = efficiency of collection
n = individual droplet efficiency
r = radius of the cyclone (the
length of the path of the droplet)
W = volume rate of liquid through
the nozzle
Dj - diameter of the droplets
Q = volume rate of carrier gas
H = height of tower (The drops
should not be made too small
since entrainment may occur,
requiring an increase in the
height of the tower)
2 Operating Conditions
Gas now 500-more than 25. 000
efm
Gas velocity into
cyclone up to 200 fps
Separation factor. . . 50 to 300
Efficiency 97+% on dust above Ip.
High efficiency on
particles of mass
median greater than . 0. 5 - 5n
Efficiency sensitive
to cfm change .... yes
Draft loss 2 - 6" w. g.
Draft loss sensitivity
to cfm change .... as (cfm)^
Water usage 3-10 gal/1000 cuftof
gas cleaned
Humid air influence
on efficiency none
Gas temperature . . .pre-cooling of high
temperature gases
necessary to prevent
rapid evaporation of
fine droplets.
Power requirements . 1 to 3 HP/1000 cfm of
gas
REFERENCES
1 First. M., etal. Performance Char-
acteristics of Wet Collectors. NYO-
1587 Waste Disposal. Harvard
University. 1953.
2 Stairmand, C. J. Mist Collection by
Impingement and Diffusion. Paper
read at the Inaugural Meeting of
Midland Branch of A. Inst. P. Birming-
ham. England. October 14. 1950.
3 Stairmand, C. J. The Design and Per-
formance of Modern Gas-Cleaning
Equipment. Paper read before the A.
Inst. P. London. November. 1955.
1-3&.
-------
Wet Centrifugal Collectors
4 Kane. J. M. Operation, Application, and
Effectiveness of Dust Collection Equip-
ment. Heating and Ventilating.
August, 1952.
5 Nicklen, G. T. Some Recent Developments
and Applications of Scrubbers in
Industrial Gas Cleaning. Proceedings
APCA, 52nd Annual Meeting. Los
Angeles. June, 1959.
6 Magill, P. L. Air Pollution Handbook.
McGraw-Hill Book Co., Inc. 1956.
1-37
-------
VENTURI SCRUBBER
OPERATION AND MAINTENANCE
Prepared by
K. C. Schifftner
Peabody Process Systems
Glenbrook, Connecticut 06906
Prepared for
Environmental Research Information Center
Seminar on
Operation and Maintenance of
Air Pollution Equipment for Particulate Control
April 1979
ENVIRONMENTAL RESEARCH INFORMATION CENTER
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
2-1
-------
E.P.A. SCRUBBER OPERATIONS S MAINTENANCE SEMINAR
Ken Schifftner
A scrubber system is an investment of a peculiar variety. Unlike production
equipment whose pay-back can be gauged and plotted, a scrubber system begins
its life typically unwanted and follows an often ignored course through its
operating life, more a symbol of Government mandate than a willfull, environment
protecting device.
Yet an improperly operated scrubber system can destroy profits in a more
dramatic fashion than most any commercial factor. It can gobble up operating
capital unless controlled, waste energy unless properly maintained, and fail to
reach compliance unless efficiently operated. Many of you know this all too
well.
We will attempt to disclose as many operating and maintenance suggestions as
space will allow so that your "bag of tricks" will be fuller, your confidence
reinforced in your ability to keep air pollution control equipment operating
effectively and at lowest cost.
Types of Scrubbers:
Wet scrubbers can be divided into approximately six (6) basic types:
Venturi -
Using a high velocity zone of various configurations designed to change
fan static pressure into velocity pressure, in doing so creating fine,
dust-removing droplets when scrubbing liquid is introduced. The effici-
ency of these devices vary as the pressure drop across the venturi and
the mechanical efficiency of the throat in the creation of the droplets.
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Impingement -
These use perforated or slotted plates containing target plates opposite
all openings to cause an abrupt change in direction (and acceleration)
of contaminant laden air. When flooded with scrubbing liquid, these de-
signs produce a high surface area froth of scrubbing liquid on its top
surface and turbulence, both factors contributing to particulate removal
and gas absorption.
Packed Tower -
Like the impingement scrubber yet use replaceable packing which extends
the surface area of the scrubbing liquid, providing a tortuous path for
the contaminated gas and enhancing gas absorption.
Spray Tower -
A gas absorption device developing high liquid surface areas through the
use of a spray nozzle(s), hydraulically or pneumatically atomized.
Dynairtic Scrubbers -
Utilize a fan, impeller or other motive device to mechanically produce
small droplets which enhance gas absorption and particulate removal.
These are typically sprayed fans, coupled with droplet removal devices.
Cyclonic Scrubbers -
A spray tower variation in which gas is spun cyclonically in a vessel
with scrubbing liquid sprayed concurrantly into the stream.
General Problems;
Before getting to specifics, lets talk about general wet scrubber problems.
Most scrubber problems, assuming the proper unit was selected for the given
application, involve spray nozzle plugging, liquid circuit restrictions and
entrainment of droplets from the vessel.
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Other less common problems are:
Wet/Dry line build-up: the scrubber design improperly allows dry dust laden
gas to contact the juncture of the scrubbing liquid and the vessel, causing
dust build-up. Good designs prevent this contact by extending ductwork
sections sufficiently into the scrubber and thoroughly wetting all scrubber
surfaces through reliable means (usually gravity flush and sometimes sprays).
Nozzle Plugging: Nozzles plug through improper selection; too small orifices
through which too dense scrubbing liquid must pass; improper header design;
drawing off a sump which also settles (and concentrates) solids; erratic
pump operation; chemical sealing; and mechanical failure.
Flow Imbalance: The headers external to the scrubber are very important.
They must send the manufacturer's required flow to the proper location at
the proper rate. Many problems are solved through the simple adjustment
of flow using existing valves.
Build-up (scaling): Scaling is the plating out of deposits on a scrubber
surface. Usually it is harmless unless the surface in question is a
functional one. It is caused by the chemical composition, solubility,
temperature and pH of the scrubbing liquid. -A difficult problem to
diagnose; a good deal of research has been done on calcium based SO2
scrubber scaling problems. Proper control starts with the scrubber de-
sign and process control.
Localized Corrosion: Corrosion is a major factor in shortening the operating
life of a scrubber whether properly designed or not. Wells or pockets of
liquid should be avoided and points of stress should be adequately flushed.
Internal members attacked from two sides should be thicker than the shell.
Instrumentation fitting blockage: One problem which significantly causes
other problems is instrumentation blockage. Many times a standard fitting
is not adequate in a scrubber, specially designed fittings and connections
must be used.
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Sump Swirling: Especially on cyclonic devices, the swirling of the scrubbing
liquid can cause severe wear and draining problems unless arrested by anti-
swirl plates in the scrubber or rapid continuous draining.
Entrainment: Entrainment occurs when the droplet separator is not functioning
properly. Nearly all scrubbers produce entrainment, only the good ones
remove it prior to discharge.
Re-Entrainment: Re-entrainment occurs beyond the droplet removal device
through improper draining or erratic flow patterns. It can also occur in
stacks of very high velocity or where fittings protrude into the high velocity
air area.
Liquid-Gas Maldistribution: The gas and liquid must be properly distributed
for the given application. Each affects the other aggravated by the influence
of baffles (needed or accidental), build-up, mechanical failure, wear,
scaling in headers or improper design. This is most common in packed and
spray towers.
Thermal Shock: Where hot gas meets' cold scrubber something has to give.
Proper design permits gradual cooling rather than abrupt changes. A
typically simple problem to fix through the use of multiple cooling zones,
thermal shock is sometimes only discovered too late.
Loss of Seal: All scrubbers run in variation with atmospheric or ambient
conditions. The juncture of the liquid circuit with its surroundings is
many times a liquid seal.. This seal may be :SL the top of a quencher or from
an overflow connection. These lines must have seals able to prevent gas
movement to or from the ambient surroundings. Loss of seal can cause en-
trainment or plugging, and instrumentation malfunction.
Wear: Wear can be tolerated unless it is localized. Unfortunately, a
scrubber's functioning parts are also the wear parts. Expect to replace
fan wheels if they are constantly sprayed with water.' (as in dynamic scrubbers)
venturi throats on venturi scrubbers and any other high velocity zone.
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Wear (Cont'd): Remember, gasses take the path of least resistance (not
necessarily the shortest path). When you check for wear, be suspicious
of those parts which inhibit this flow (parts directly along the path
of least resistance).
Vibration: Most common in wet dynamic scrubbers and with the fans on
wet fan venturi or cyclonic systems, it is best controlled by monitoring
and scheduled preventive maintenance.
I
Now let's look at a hypothetical "lemon" system, purchased by some fellow who
bought the system from the company 40% below everyone else's price, using 'j the
water consumption and 1/3 the energy. Let's investigate some of the problems
of this bargain that didn't work.
Please see Figure #1
Here we see a quencher venturi scrubber with a fixed throat, cyclonic separator,
tray absorber, vane type droplet eliminator, and, for sake of further discussion
a chevron droplet eliminator and a mesh pad. The absorber liquid circuit is
separate from the venturi circuit so that a neutralization chemical may be added
to the absorber.
Let's follow some 1800°F gas into the quencher through the venturi and out to
atmosphere.
Quencher;
The manufacturer of this system apparently calculated the exact amount of
water needed to adiabatically saturate this gas flow and introduced it into
a single quencher nozzle. Unfortunately the quencher only cooled to 800°F
as expected. Why?
Possible Reasons;
A. Low Water Flow: Typically I's to 2>j times the theoratical evaporation
demand are used spray quenchers. It is a matter of probility that
dictates the more droplets available to evaporate, the more will.
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Possible Reasons (Cont'd)
B. Poor Distribution: Selecting a cheap hollow cone nozzle, the manufacture
made thorough cooling nearly impossible. Full cone nozzles are usually
used since evaporation is a surface phenomenon. One must generate a
high surface area to adequately evaporate. He should add a full cone
nozzle or a double header to adequately quench and cool. At least 2 dia.
straight duct dun above the quencher should be used.
C. Low Header Pressure: In the heat of bidding this job, the manufacturer
found out that a competition claimed only 10 psig. quencher header
pressure. He then changed his pressure to 5 psig. The 5 psig. pressure
however only gives him a 60° spray cone angle, not 90° as he needs and
the nozzle is producing 1000 micron droplets at 5 psig., not 400 micron
or smaller as he needs. Raise the header pressure to 15-30 psig. after
checking the spray nozzle characteristics for agreement.
D. Short Length: Evaporation is a surface phenomenon, as we said. It is
also time dependent. Evaporation does not occur instantaneously, but
progresses at a rate dictated by the evaporating liquid, the humidity,
and temperature of the surrounding air and physical factors such as
turbulence. Usually 0.15 to 0.25 sec. residence time is used for gases
1000°F and below with 0.2 -0.3 sec. used for hotter gases.
You'll note on figure #1 that the quencher is quite long. The manufacturei
had to double the quencher length.
Seal;
Between the bottom of the quencher and the top of the venturi is a liquid
seal. When first started up, the "lemon" system sucked all the water out
of the seal within seconds. The user was told to fill the seal with sand.
The scrubber then sucked the sand out of the seal. Extra height was then
added to the seal and it now works fine.
Some seal problems and solutions are:
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A. Insufficient height due to suction: Allow at least 4" more free-
board height on all liquid seals over and above the manometer effect
which will occur naturally. On adjustable venturi scrubbers, with a
fan after the scrubber, start-up the venturi with closed so as to
restrict flow. On adjustable Venturis running under pressure, start
{if possible) with the throat open and the fan inlet dampers closed so
as to reduce the pressure at the seal.
B. Low flow: Add enough liquid to continually overflow inside the vessel.
C. Poor distribution: Make certain the seal is level and symetrical. A
seal at an angle will draw more quickly from the side with the lowest
atmosphere to baffle length.
D. Asymetrical liquid flow: Typically one liquid inlet is not enough,
multiply inlets should be used.
Please see figure 1A for the revised liquid inlet.
Now we get to the venturi itself. Figure 2A shows an actual adjustable venturi
used on a successful industrial application. There are many more like it pre-
sently operating well. Problems do occur however.
Let' s go through about a dozen potential problems with venturi scrubbers:
A. Distribution pipes insufficient in size or quantity.
As we mentioned about wet/dry line problems, the venturi inlet must
have the proper liquid distribution in order to function properly. Many
Venturis do not have sufficient liquid inlets to cover the approach
(hopper-shaped section) of the scrubber. An inspection into this zone
usually shows a non-uniform flow pattern or perhaps no flow at all.
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Potential Venturi Scrubber Problems (Cont'd)
Inlet pipes should be no more than about 8'/sec. velocity if tangential
type and no more than about 6'/sec. if weir type. This reduces flow
imbalance due to splashing.
v
B. Distributor pipes plugged: This common occurance, especially on limekiln
scrubbers can be corrected by placing rod-out connections at all plug-
ging prone locations. Headers .should be made in small sub-assemblies to
facilitate handling. Many installations outlaw 90° elbows, using two
45° elbows and a coupling instead.
C. Distribution should cover entire approach surface. Additional headers
may be needed in the "lemon" system to accomplish full coverage.
D. Poor flow, quantity and pressure: Do what the operating manual says to
do. On high solids lines, put the venturi liquid outlet distributor
pipe above the venturi, feeding downward from its bottom surface (rather
than the top) into the venturi inlets. This permits thorough draining
on shut down/and provides more uniform flow under low flow conditions.
E. Solids content too high for the scrubber design: Most scrubbers using
open pipes can't reliably tolerate slurries over 15% by weight. Usually
the instrumentation connection begin to plug near this concentration with
the venturi plugging at 15-20% solids. It is wise to run the unit at
6-8% solids maximum for best operation. Spray nozzle scrubbers sometimes
can't tolerate 2% solids in the liquid circuit. If it must run above
this number, an alternate liquid feed circuit should be used; or designs
used which incorporate larger nozzles.
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F. Wet/Dry Problems: As mentioned under problem A, the liquid distribution
must be proper. Many times the inlet spool (piece of scrubber inlet
extending down into the approach zone is simply not long enough). In
figure 1A you'll note it extends well below the weir. Newer designs
use a straight weir but maintain this extension.
G. Thermal Shock: Most venturi scrubbers can't tolerate an external load
r >
on their inlet flange. More importantly they must not be thermally
loaded. . .
,"*'.
Proper operation of the quencher helps prevent this. A small horizontal
water hold-up ring welded to the interior of the quencher pan help keep
this surface wetted, preventing thermal shock. Always put the quencher-
water circuit on a reliable liquid circuit. You can see from figure 1
that the "lemon" system uses the venturi recirculating pump as the
quencher water pump. What happens if this pump fails? -
The manufacturer does not cover the damage under his warranty.
Put the quencher liquid circuit on a reliable clean water line, not a
recirculation line.
H. Stress Corrosion Cracking: Much like thermal shock, attack by halogenated
compounds can cause stress corrosion cracking. Proper material selection
is important. Don't recirculate if halogens are being scrubbed out. The
venturi circuit will merely serve to concentrate them; Higher alloys are needed.
At 600° F inlet temperature, even 50 ppm HC1 can have devastating effects
on stainless steel.
I. Low L/G: The L/G (liquid to Gas ratio) is important not only for particu-
late removal but also for scrubber operation. Typically only 2 gallons
in 10 of scrubbing liquid really does the scrubbing. The rest is used to
convey, flush, dilute, humidify and cool.
Extremely low L/G ratios invite problems. When the inlet grain loading is
10 and above, most Venturis use L/G ratios of 10-15. When the loading is
5-10 grains, the scrubber operates in the 7->10 L/G range.
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Some new upflow scrubbers use only 4-5 gallons per 1000 ACFM for this same flow.
One manufacturer claimed a few years ago that his design used only 1 gallon per
1000 ACFM. Calculating this out, the scrubber sump would be a muddy unpumpable
glop of 65% solids. He revised his figures to 4-5 gallons/1000 cfm "for this
application". Get specific recommendations from the designer, not from brochures.
Adjustable Venturi Problems;
Adjustable venturi scrubbers include damper blade type (either round such as a
butterfly damper or rectangular such as a louver damper), annular (in which a
center body, usually conical, moves up and down) or a disc design. The basic
function is to vary the area of the venturi in an effort to produce the pressure
drop required for a given efficiency.
Dampers which pivot from a shaft usually vary the throat area by a function of
the sine of the angle of rotation. Center bodies which advance along the center
of the venturi throat may vary as a function of the angle of the center body
cone versus the fixed outer housing. Disc scrubber throats vary the area linearly.
The different types available result from different needs. A draft sensitive
process using a damper ..blade scrubber whose area varies as the sine of the angle
is a potential problem. Changing the angular position of the damper can have a
dramatic effect on process suction. ' An annular venturi or disc venturi would be
less sensitive.
On very low pressure drops however, the annular may not provide adequate mixing and
therefore be at a disadvantage. Typically annular Venturis have a lower mechanical
loss than damper blade designs, a result of better geometry and stream linine.
Adjustable venturi problem salving is best left to the manufacturer. He knows better
the thought processes used in its design and may offer constructive solutions.
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Wear
Before we go on to the separator, lets take a step backward and look at the entire
system for wear locations. Figure 2B shows typical scrubber wear areas.
As mentioned earlier those high velocity points along the path of less resistance
are most likely to wear. Notice it varies whether the unit is forced draft or
induced draft.
In the induced draft mode, the corner of the venturi elbow (if any), will likely
be a wear area. Operating under suction, the air tries to cut this corner, wearing
this area considerably on abrasive applications. A wear plate there is a good idea.
Under forced draft, there is less suction in the separator (technically, the air
is denser) thus the air tries to impact first at the base of the venturi, then
redistribute itself, then proceed to the separator. Forced draft Venturis tend to
be more easily "fed", meaning they distribute better in the approach section.
The separator wear zone if cyclonic like in our example, wears about the same re-
gardless of fan location. Most tangential entry separators have a draft loss of
2-4" w.c. and thus use restrictions at the point of.tangency to provide the higher
velocities used to cyclonically separate droplets from gas.
Not only does wear occur at this point but also through about 100° of rotation be-
yond this point as the air is forced against the wall. Fiberglass scrubbers have
been known to completely wear through at this point. This zone is also a potential
entrainment zone which will be discussed later. The plan view in Figure 2B
shows this area more clearly.
The "lemon" system we are discussing had to have wear plates installed in this
zone to reduce the affects of erosion.
Absorber
Moving on to the absorber, we find a different set of physical constraints at work.
Now we must have good distribution, must move vertically rather than cyclonically
and must maintain good distribution and flow.
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Peabody Process Systems. Inc.
Absorber (Cont'd)
Whether packed tower or tray tower, the absorber must be presented with a uniform
gas flow. Any obstructions, baffles, headers etc., which disrupt this flow
could have an effect on the absorber.
If the flow is too low, channeling can occur in a packed bed and "weeping" can
occur in a tray absorber. "Weeping" exists when the liquid above the tray has a
higher pressure than the gas beneath that particular part of the tray. Scrubbing
liquid will drain through the tray perforations causing a shift in air flow
patterns to a less restrictive point.
Channeling can occur both by liquid distribution and gas flow. A liquid flow
directed!too strongly to the side wall of the unit can cause a deficiency of
liquid in the center, encouraging less resistance and greater gas velocity. If
you install packing, leveling it out one day, only to find it grossly shiffted
the next,; you may have channeling. The hearers, liquid flow or distribution may
have to be altered. Commonly, greater spray is applied to the center of a packed
absorber since the gas does not proceed in plug flow through the tower (though
this is assumed for design purposes).
Here is an absorber checklist.
Packed Tower
Problem .
Poor Gas Distribution
Poor Liquid Distribution
Packing Sized Improperly
Too High Velocity
Too Low Velocity
Possible Solutions
1. Use and injection type support grid.
2. Allow extra vertical height-Inlet to grid.
3. Use less than 8'/sec. vertical velocity
1. Install redistributors every 4-6' of packing.
2. Rearrange headers and liquid entry.
3. Use a reflux distributor grid.
1. Check with packing manufacturer not scrubber
designer. Many companies ingnore recommendations.
1. Put another tower in parallel with existing unit.
2. Cut back in flow rate if possible.
1. Restrict bottom part of grid.
2. Use small packing.
3. Use redistributors.
4. Raise water rate.
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Tray AJbsorber
Weeping
Plugging
Distribution Poor
Problems Possible Solutions
1. Select open area proper to eliminate weeping.
2. Bleed in air if possible
3. Blank off excess area.
1. Use spray wash header. Check operation.
2. Reduce solids content in scrubbing liquid.
3. Clean periodically.
4. Raise flow rate (check factory).
1. Check, inspect trays. Lack of uniformity of
condition indicates poor distribution. Baffle
as required to offset poor gas flow.
2. Use end weirs on the downcomers to retain
extra water on the tray.
3. Clean weir boxes.
1. Size for full flow, maximum condition. If
too small, enlarge or install supplemental
external weirs.
2. Keep them clean.
1. Make certain trays are securely in place.
2. Stiffen warped trays.
3. Tighten all fittings.
4. Reweld baffle strips, caps, keeper plates, etc.,
or replace trays if broken.
Having solved the absorber problem, our hypothetical case now presents us with the
droplet removal devices: The vane eliminator, the chevron, and the mesh pad, and
our previously introduced cyclonic separator.
Weir Sizing
Mechanical
Each have two major problems, extrainment and build-up. Build-up is the easier of
the two problems. It is controlled by judicious cleaning (or flushing) by sprays or
headers as follows:
Vane 1. Center spray (intermittent)
2. Periphery flush (intermittent)
Chevron 1. Spray from below
Mesh 1. Spray from above
2. Spray from both above and beneath
Cyclonic 1. Routine continuous flow
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Entrainment is another matter. Figure 3 shows a vane eliminator in cross section
with some modification to improve droplet separation.
A. Add a cone on the center spool to act as a vortex finder. The swirling air above
the vane needs a surface to pivot from. Making a 20° (from the horizontal)
cone produces a point on which this vortex may pivot. It can produce sometimes
dramatic improvements.
B. Clean or enlarge drain(s).
Internal drains need periodic cleaning. Otherwise they will back up, overflowing
previously removed liquid back into the air stream.
C. Clean or enlarge down pipe. For reasons above.
D. If all else fails, install an external drain.
E. Clean unit on spection. Many 18 gauge vane eliminators can be cleaned with a
rubber mallet. Striking it about a foot out from the center hub sometimes
vibrates build-up off.
F. Re-weld any loose vanes.
G. Make certain center manhole is sealed.
H. Use flush water at the rate of 2 gallons per minute per foot of periphery.
I. Use the center flush spray (if provided).
A vane eliminator too high up in the vessel will typically not function properly
regardless of modification. One low in the vessel can be improved.
Mesh pads are merely filters. Like filters they build up. Spraying from beneath
and above periodically can remove some material.. Spraying from beneath only will
drive the material further into the mesh dictating removal and cleaning or replacement.
Chevrons are typically spray cleaned from below.
If the vertical velocity is above 600 ft./min., most chevrons will begin to entrain.
At about 900 ft./min., mesh pads begin entrainment. Vane eliminators have open
area velocities of 1200-1600 ft
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Cyclonic separators sometimes re-entrain because the scrubbing liquid previously
thrown against the vessel wall streams back down on top of the cyclonic inlet.
Forming a puddle, it will overflow into the air stream creating a spray. This
can be eliminated or reduced by installing a wiper angle as shown in Figure 3 to
direct this flow towards the_center of the scrubber much like a rain gutter.
By reducing the area, (section B-BO of the inlet, higher velocities can be obtained
sometimes resulting in improved droplet removal capability.
Wow, we made it. The gas can now leave up the stack. The stack had better be of
low enough velocity (under 30'/sec) to reduce the chances of re-entrainment though.
What about instrumentation connections? If they don't work, how can you analyze
the problem? Figure 4 shows some typical instrumentation connections including the
pressure tap angled so as to reduce plugging, the vented level control fitting to
reduce turbulence and stabilize readings, and the tank mounted pH or O.K.P. probe
(placed so that you can get at it).
Using these suggestions, your "lemon" should be salvageable. Had this example been a
dynamic scrubber or spray tower, general comments and trends would apply equally
as well. Time does not permit a similar example.
We hope our comments give added insight into solving wet scrubber operational problems.
Specific questions, of course, should be discussed with the particular scrubber
manufacturer prior to start of work.
Thank you.
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Automatic Ventri-Slot
2-18
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OUTLET
QUENCHER
I
M
IO
ssssssssssssssss
MESH PAD OR
CHEVRON OR
VANE ELIMINATOR
ABSORBER
INSTRUMENTATION
o
WATER
TREATMENT
PUMP(S)
INSTRUMENTATION
PUMP(S)
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PFABODf GAS IvJLET QUENCH SKTION
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IMPROVING DROPLET ELIMINATOR PERFORMANCE
VANE
(A) Add Cone to act as vortex finder
(§) Clean or Enlarge Drain
© Clean or Enlarge Down Pipe
(5) Install External Drain
CYCLONIC
,-C
I I T]
A-A '
Install Angle Deflector
Reduce Area
&-B
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TYPICAL WEAR AREAS
INDUCED DRAFT
>/> 4k OUTLET
\ CONE
L .1
WEAR
1
CORNER
SEPARATOR
FORCED DRAFT
> L
L J
y\
SEPARATOR
ELBOW
WEAR
PLAN
2-22
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MISCELLANEOUS
LEVEL-CONTROL FITTINGS
TURBULENCE REDUCER
PH/O.R.P. PROBE CONNECTION
Pressure Taps
Ni 3
GAS FLOW
INSIDE
Tap
Threaded Cap
Elbow
Level
INSIDE
OUTSIDE
Vent
Scrubber
Instrument
OUTS IDE
Valve
INSIDE
Main Junctioi
Secondary Line
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VII-3
Maintaining Venturi-Tray Scrubbers
William J. Kelly
© Copyright 1978 by Chemical Engineering/ A McGraw-Hill
Publication. Reprinted with permission from Chemical
Engineering, December 4, 1978, pp. 133-137.
3-1
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Maintaining venturi-tray scrubbers
Here are ways to design and maintain
these scrubbers so as to prevent most
problems. The author provides
a detailed list of things to look
for and do should problems occur.
William J. Kelly, Swemco Inc.
Q] Venturi-tray-scrubber maintenance starts with equip-
ment design, continues with preventive maintenance, and
ends with troubleshooting. This article gives design tips
that will make maintenance easier, suggests a preventive-
maintenance program, and tells what to do should trouble
develop.
Design considerations
The following will determine service life and mainte-
nance requirements of a combination tray-venturi scrub-
ber:
Corrosion protection.
Temperature protection.
Erosion protection.
Access to equipment.
Solids accumulation.
Corrosion protection
Corrosion considerations are complex and should be
undertaken according to specific application. Corrosion
has been given a great deal of attention in the technical
literature. The protective measures discussed here are
common-sense guidelines for those involved in purchasing
scrubbing equipment. The purchaser should:
Require mill certification for all alloy materials used
in scrubber, fan and duct fabrication.
Deal only with reputable suppliers and check their
past performance.
Request multi-source recommendations on selection
of alloys, resins or refractories from materials manufac-
turers and users.
Use test coupons (whenever possible) on existing
fipment that is being replaced, to determine corrosion
mperature protection
Vhen handling high-temperature inlet gas (above
7(JB°F), it is imperative that proper scrubber-liquor distri-
Kon be maintained. This will ensure continuous
or-film protection on surfaces exposed to the high
perature. Fig. 1 shows a typical venturi scrubber with
Water-sear,
expansion-!
' joint
zjarrf.bf
- -a* :|
wJv&i
' &
Y. Tangential c 1
feedpipe -
Venturi scrubber has wetted walls
Fig.1
"wetted wall" protection. A water-seal expansion joint
compensates for thermal expansion and contraction and
prevents stress from developing in the high-temperature
zones. This seal also floods the upper venturi section to
keep all surfaces wet and at relatively low temperature.
The continuous washing also prevents "wet-dry inter-
faces," where solids build up on the scrubber wall and
where corrosion and stress cracking can occur.
Tangential feed pipes, although used primarily to inject
scrubbing liquor to the throat, also provide a wall-
washing action and assure that surfaces are protected from
high temperature. Proper distribution of liquor on vcry-
high-temperature applications (above 1,200°F) greatly
reduces the possibility of stress cracking, carbide precipita-
tion and sigma-phase contamination of alloy. In addition,
the continuous flush prevents local high-acid concentra-
tion and corrosion. Another zone where corrosion is
related to temperature is the area above the mist elimina-
tor, where dewpoint corrosion is common. Proper materi-
al selection in this zone is critical.
CHEMICAL ENGINEERING DECEMBER 4, 1978
133
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Preventive maintenance
Table I
"..' .::'.' Equipmentcheck : ,,,,,
i ' i
Make internal inspection to remove construction debris.
Test level, temperature and pressure controls and alarms.
For automatic throats, use manual override to check free motion and the
correct position of end-points. Refer to manufacturers' instructions for
lubrication requirements. . .'...-
''!'. ' ' ' ''- ''"V" . ' ' '. t .y gravity* flowi?"~x'".. -.;'-'-. IKA^..^?,^ '^-^ft^^feiV;,:'*. - i^
« ^ ,'". ' '' ' "l',*i ',-**''!* *' "'-. -J '.'''*' *'^fe~''" -'-* t
1 tr watecseat is provided, check vertical and rtorizontal'clearances. Ensure
correct opening* few liquor flow/and proper tolerances for expansio^j
Observe-operation and pattern of quench sprays. If any problem-tv"w
..'.. '. -i , ' . "-' i...^,,' ..... '**: «~&*1
suspected,contact manufacturer.;;- ' . : .% . v-a^.T-u^, "-
.. ,
* Temporarily apply full-'water'f low to the unit and check the I iquor 4
drain foe f ast draindown or pumpdown. If liquor holds up in bottom- <
of unit for several seconds a constriction should- b* suspected. *"
. .fVv:lA--^^^.;v/:;-rA.,r^:^>^,i^^::?.
Initiate ga» flow and take pressure-drop readings across trays, throat and
overall unit.; Readings should be made at full gas flow. Refer to operating
instructions for design pressure-drops. '.;-, s^V
"''.. : ' vK :<".- ';..' - .,-"- :''" ''.'.' .'*-'-
When system reaches steady state, check gas and liquor temperatures in
and out of the scrubber. ''-."._,. ^ .'
Observe stack appearance. '
On units with hot inlet-gas, check outside shell temperature at the gas-
inlet zone-. A high relative temperature difference indicates local hot
. spots and possible stress-crackfng zones.
Analyze recycle liquor for unexpected components. Determine the effect
of these constituents since they may greatly foreshorten service life (for
example; high-chloride liquor in stainless steel service). .
. Note venturi-throat response to changes in pressure drop on units with
automatically variabl* throats. (Typically, changes in gas flow occur at
relatively stow rates so that hunting and lagging are almost never a ; ;v*
problem.J Vr» ':"... .' -. '- V' - '"' '' '.'~'- ''-V-'
.1 ; --. -A;-<-: -' . -. ' ' 'f' ' ' '" ' -'i'."" : ' ,->>"
Check recycle and drain piping and pump for surging and cavitation.
Causes may be scrubber inducedliquor foaming, high scrubber vacuum,
loss of sump.level, etc. :V\" ' ..'&-.": '. '-"'/'.
After an initial period of operation (1-4 weeks) shutdown and make a
thorough internal inpection, specifically looking for:
(a) Plugging or blocking of trays, nozzles or weirs. .
(b) High-velocity-zone wear (throat, sprays, turns, etc.) ;
(c) High-temperature spots in gas-inlet zone
Id) Signs of corrosion ' '
(e) General mechanical condition of internals. ' ::.--
Detecting a progressive type of problem at this stage provides the time
margin necessary to modify the equipment before serious damage can .
occur. If indications of a problem are so slight as to be questionable, V
schedule an. additional shutdown- at a similar interval for an additional
Frequency T ;,: .
Before startup.. " ^
Before startup; 6 to 12-month intervals
thereafter: ' ./ ;.
Before startup; 6 to 12-month intervals
thereafter.
Before startup; 6 to 24-month intervals
thereafter. " ^
n *<
Before startup; 1 to 6-month intervals
thereafter. * ->.tir> ;
Before startup.
Before startup; 3 to-1 2-month, intervals
thereafter. u ^ jg£^
"*~ **"* *« A \ ** '*
Before startup, 3 to 6-month intervals
thereafter.
Before startup, 12 to 24-month interval*
thereafter. v~">-*lT->
Initial, startup; 1-week to 1-month interval* i]
thereafter. «
Initial startup; 1-week to 1-morrth interval*
thereafter.
Initial startup; 1-day to 1-week intervals Y^
thereafter.
Initial startup; 1 to 12-month intervals
thereafter.
Initial startup; 1-day to 1-week intervals , ;,-
thereafter.
V -.- .'".' -'"' ''**
Initial startup; 1-week to 6-month interv»ltt|
thereafter. . '.'..'..'..-.- ,«>'."
Initial startup: 1 to 6-month intervals ,45
^Jrl
thereafter.
1 to 6-month intervals following initial
. shutdown. .. ,
134
3-4
CHEMICAL ENGINEERING DECEMBER 4. 1978
-------
Correcting scrubber problems
Table II
Major cause*
Poor cleaning performance
a. Low scrubbing-liquor. flowrate.
b. Low pressure-drop across venturi.
c. Inlet dust loading or size distribution;-
beyond scrubber design capability.
d. Excessive gas flow.
e Partially Mocked entrapment * --^.- .
;_" separators ** fs "*--?
.... ':" Highexit-gastemperature
, a. Very, low;venturi-liquid flowratBi >.
b. Low tray-water flowrate.
c. Tray(s') (partially) plugged with solids.
d. High cooling-water inlet temperature.
e. High scrubber-inlet temperature or
excess gas-flow.
Exhaust-gas liquor entrapment
a. Moisture eliminator drain plugged
(tangential vane eliminators only).
b. Excessive tray-liquor flow (tower
flooding).
c. Excessive water frothing (possibly due
to foaming agent in liquor).
d. Plugging of chevron-type eliminator.
e. Excessive gas flow.
Fan motor overload
a. Low scrubber-pressure-drop due to
excessive throat opening.
Plugged spray nozzles
a. Nozzle openings too small.
b. Solids concentration too high in spray
liquor.
->..' Action
a. Check pump output. Look for plugged piping and nozzles, incorrectly '-' .*
. opened valves^ overthrottled-pump-discharge valve. . . _ .,
b. Check for low scrubbing-liquor flowrate; low gas flowrate; inoperative . ;
or uncalibrated variable-throat controller; damaged variable-throat . .-. , A
.', blade/disk;. '- '''-* ''v.'-v-L. ./>;.''?. : "- - «'>*. "'$
..' ,-_ *v-. . . . +. -^
« c.. |f operating modifications fail to correct the problem, analyze particle -l.'"". J
size and quantity ' j
* * K ° * "*
d. Check fan-damper setting,.vertturi-throat setting/system fan operation^ -
vs.fan.curve. ^
e. Check washdown sprays if irtitaMed Check composition of spray liquor;.'J;' <
' If scaling occur*, investigate use of low-pH flushing liquor.
^ -** f
a Check pump outputrlook for plugged piping, nozzles, etc , incorrectly-
opened valves, overthrottled pump-discharge valves. * T _% ^»|
b Check pump output, look for plugged piping, nozzles, etc., incorrectly: "
opened valves, overthrottled pump-discharge valves. ,w - .. .-^CTJ
. c. Check condition of tray flushing sprays if installed. If scaling is observed, ' '
use a low-pH wash periodically to dissolve scale. Check percent solids in
recycle liquor.
d. Check heat-exchanger operation and adjust cooling-water flowrate and
temperature. ,
e. Check upstream equipment operation.
a. Shut down and snake out the eliminator drain. If problem recurs, add
flushing water to continuously irrigate drain pipe.
b. Reduce flow; calibrate flow-control mechanism if installed.
c. Sparge a liquor sample. If liquor froth does not disappear quickly,
foaming may be choking downcomers and drains. Analyze liquor for
foaming agents.. .
d. Check flushing-spray conditions and pattern. Use more flushing periods
per hour. Poor gas-cleaning performance will accelerate, buildup. Check
liquor chemistry for scaling agents.
e. Check fan damper position. Check variable venturi-throat opening.
a. Normally, the fan damper wilt provide enough "choke" to prevent
overload, so fan damper must be checked with variable throat opening.
Check throat operator and liquor flow. i
a. Modify strainer/nozzle-opening ratio so that nozzle holes are at least
twice the diameter of strainer openings.
b. Check separation equipment. Check for excessive dust load in gas stream.
Check for purge-line malfunctioning.
CHEMICAL ENGINEERING DECEMBER 4. 1978
135
-------
Correcting scrubber problems
Table II (confd)
Major causes
Plugged spray nozzles (cont'd)
c. Pipe scale or debris entering liquid
stream beyond strainer.
Excessive nozzle wear
a. Solids concentration too high in spray .
liquor:
b. Abrasives in spray liquor.
c. Low pH in combination with abrasives,
causing erosion/corrosion.
Plugged tray*--'' '>;.'//,"= '.* ."
ai Hot gases entering equipment before
. liquid flow-is initiated.
bi Inefficient venturi scrubbing, allowing'
! high solids levels in gas stream-to x-
contact trays.
i . - .
cj High solids in tray liquor, formation
; of insoluble salts in scrubber.
d. No water flow or very low water flow
to trays.
Excessive throat wear
a. High solids recirculation.
b. Corrosion/erosion
c. Excessive gas velocity.
Erratic automatic-throat operation
a. Prime-mover malfunction. '"(. '
b. Sensor signal incorrect.
c. Transmitted signal incorrect.
d. Damaged damper disk-mechanism.
Action
c. Remove spray heads and flush spray piping and nozzles. Replace piping
if corrosion is apparent.
a. Check separation equipment. Check for excessive dust load in gas
..stream and for purge-line malfunctioning.
b. Remove abrasives from liquor stream or install abrasion-resistant linings
' in'wear zones.
c. Check separation equipment. Check for excessive dust load, in gas stream
and for purge-lina malfunctioning. Remove abrasives from liquor stream
- or install abrasion-resistant linings in wear zones. Add alkali for pH
modification. '.' . . . ;'.' -:
a. This condition causes "bakeout" of solids. Shut down. Try "washing"
trays in place by recycling strong detergent. Otherwise, remove trays.
and scrape clean. .-:,. " . . ,
b. Check throat pressure-drop and venturi liquor rate.
c. Check solids-separation equipment. Analyze liquor and determine and
eliminate cause of high solids.
d. Check pump output; look for plugged piping, nozzles, incorrectly
opened valves, overthrottled pump discharge valves.
a. Check solids-separation equipment. Check for excessive dust load in gas
stream and for purge-line malfunctioning.
b. Check separation equipment. Check for excessive dust load in gas
stream and for purge-line malfunctioning. Add alkali for pH
modification. Install abrasion-resistant liners in high-wear zones if liquor
modifications are not practical.
c. Check throat pressure-drop and reduce to design point.
as. Remove from service, repair or replace. Most throats can be held in a
fixed position close to design pressure-drop by mechanical means during
this procedure.
b. Check sensor taps on vessel for solids buildup. Check transmission tubing
for liquid buildup or air leaks. Clean or repair sensor.
c. Clean or repair sensor. Check instrument air-supply pressure and filters.
Check tubing for leaks. Check positioner filter and connections. (Clean
instrument air is critical here.) Thoroughly clean positioner internals and
check freeness of operation.
d. First make external inspection of drive train. If damaged area is not
observed, shut unit down and make internal inspection using a throat-
actuator manual override. Check for packing damage and excessively
tight packing gland.
136
3-6
CHEMICAL ENGINEERING DECEMBER 4, 1978
-------
Erosion protection
In the high-velocity venturi throats, at tangential liquor
and gas zones, and near spray areas, consideration must
be given the possibility of erosion, as well as the combined
effect of corrosion-erosion. This becomes of primary
importance when gas-stream solids are abrasive and the
liquor being recycled is acid. For these operating condi-
tions, design options such as use of silicon carbide and
alumina wear-surfaces must be considered. Fig. 2 shows
Hastelloy alloy C high-velocity spray nozzles that were
used for handling dilute sulfuric acid and abrasive solids.
Service life of these nozzles was approximately two weeks.
The abrasive solids in the liquor continuously eroded the
corrosion-resistant skin of the alloy so that corrosion and
erosion worked together to cause a catastrophic failure.
Nozzles of silicon carbide have replaced alloy nozzles for
this kind of service and, in most instances, have provided
years of troublefree service.
Access to equipment
If critical areas are inaccessible, developing problems
may go undetected during normal inspections. Access
doors, sight ports, or both, should be provided to allow
inspection of wear and corrosion zones, as well as areas
where there is solids buildup potential. When downtime
must be kept at a minimum, fast-opening doors with
reliable latches should be considered.
For many applications, it is reasonable to make spray
headers removable during operation, thus allowing for a
quick check of these critical components. Shutoff valves
and elbow connections directly upstream of each header
will simplify removal.
Solids accumulation
Another consideration in scrubber design is the preven-
tion of plugging and blockage of small-clearance openings
such as spray heads, liquor pipes and tray perforations.
Interference with normal gas and liquor flow patterns can
result in high-temperature and corrosion damage to local
areas of the scrubber and, in some cases, to downstream
equipment. Spray nozzle openings on slurry service
should be above '/2-in. dia. In some cases where the solids
concentrations are high or the solids are coarse and sticky,
minimum openings must be increased to 1-in. dia. or
larger. Low-pressure, open-pipe feeds are often preferred
over sprays for scrubber-liquor injection where high solids
concentrations or high temperature is anticipated.
Tray plugging occurs when solids in the incoming gas
and recycle liquor build up around and eventually bridge
the tray perforations. The basic causes of plugging are
improper design of tray-flushing sprays, plugged flushing
nozzles, low tray-liquor flow, sticky solids in the gas
stream, and chemical scaling (usually in the form of
calcium salts).
Selection of a mist eliminator is usually a compromise
between maximum performance and minimum mainte-
nance. Generally, the smaller the open area that the
eliminator presents to the gas stream, the higher the
mist-removal efficiency and the greater the possibility of
plugging. Mesh-type mist-eliminators must be avoided on
applications where there is anything more than a trace of
solids in the gas or liquor. Parallel-blade chevron designs
3-7
Eroded high-velocity spray nozzles
Fig. 2
are often indicated where sticky materials or high dust-
loads are involved. Chevron units require flush sprays
above or below the blades (or both) on many applications,
and flushing liquor should be clean.
Open-vane separators are a good choice for many
scrubber units up to a maximum diameter of 14 ft. Open
cyclonic separators will not plug up, since they have no
internals, but their efficiencies are below those of impinge-
ment-type entrainment separators.
Preventive maintenance
When a scrubber is properly designed for its specific
application, and unexpected conditions do not turn up in
actual operation, it should provide many years of trouble-
free service. Table I gives a typical preventive-mainte-
nance schedule for a combination venturi-tray scrubber.
The appropriate intervals between inspections differ
substantially from job to job, and the intervals indicated
are approximate minimums and maximums. It is
suggested that preventive-maintenance checks start with
the minimum intervals and be gradually increased toward
the maximum ones. Following a guide of this type will
further assure a troublefree and extended service life.
The author
William J. Kelly is project manager for
Swemco Inc., 470 Park Ave. South, New
York, NY 10016, telephone (212)
725-5450. He previously worked as
product manager of cooling towers for
Blazer Corp., and as a scrubber sales
engineer for Peabody Engineering. After
receiving a B.S.M.E. degree from New
York State University, he spent several
years as U.S. Merchant Marine officer,
and as a Navy nuclear test engineer.
CHEMICAL ENGINEERING DECEMBER 4. 1978
137
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