Lessons
Learned
NaturalGas
EPA POLLUTION PREVENTER
SBl
N
\
CD
From Natural Gas STAR Partners
CONVERT GAS PNEUMATIC CONTROLS TO INSTRUMENT AIR
Executive Summary
Pneumatic instrument systems powered by high-pressure natural gas are often used across the natural gas and
petroleum industries for process control. Typical process control applications include pressure, temperature, liquid
level, and flow rate regulation. The constant bleed of natural gas from these controllers is collectively one of the
largest sources of methane emissions in the natural gas industry, estimated at approximately 24 billion cubic feet
(Bcf) per year in the production sector, 1Q Bcf from processing and 14 Bcf per year in the transmission sector.
Companies can achieve significant cost savings and methane emission reductions by converting natural gas-
powered pneumatic control systems to compressed instrument air systems. Instrument air systems substitute
compressed air for the pressurized natural gas, eliminating methane emissions and providing additional safety
benefits. Cost effective applications, however, are limited to those field sites with available electrical power, either
from a utility or self-generated.
Natural Gas STAR partners have reported savings of up to 70,000 thousand cubic feet (Mcf) per year per facility
by replacing natural gas-powered pneumatic systems with instrument air systems, representing annual savings of
up to $210,000 per facility. Partners have found that most investments to convert pneumatic systems pay for
themselves in just over one year. Individual savings will vary depending on the design, condition and specific
operating conditions of the controllers.
Method for
Reducing Gas Loss1
Replace Gas with Air
in Pneumatic Systems
(per facility)
Average Volume
of Gas Saved
(Mcf/Year)
20,000
Average Value of
Gas Saved
($/Year)1
60,000
Average Cost of
Implementation
($/Year)2
50,000
Average Payback
(years)
<1
'Assumed value of gas is $3.00/Mcf.
2Cost of installing compressor, dryer and other accessories, and annual electricity requirements.
This is one of a series of Lessons Learned Summaries developed by EPA in cooperation with the natural gas industry on superior
applications of Natural Gas STAR Program Best Management Practices (BMPs) and Partner Reported Opportunities (PROs).
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Technology
Background
The natural gas industry uses a variety of process control devices to operate
valves that regulate pressure, flow, temperature, and liquid levels. Most
instrumentation and control equipment falls into one of three categories: (1)
pneumatic; (2) electrical; or (3) mechanical. In the vast majority of applica-
tions, the natural gas industry uses pneumatic devices, which make use of
readily available high-pressure natural gas to provide the required energy and
control signals. Pneumatic instrument systems powered by high-pressure
natural gas are used throughout the natural gas industry. In the production
sector, an estimated 250,000 pneumatic devices control and monitor gas
and liquid flows and levels in dehydrators and separators, temperature in
dehydrator regenerators, and pressure in flash tanks. Most processing
plants already use instrument air, but some use gas pneumatics, and includ-
ing the gathering/booster stations that feed these processing plants, there
are about 13,000 gas pneumatic devices in this sector. In the transmission
sector, an estimated 90,000 to 130,000 pneumatic devices actuate isolation
valves and regulate gas flow and pressure at compressor stations, pipelines,
and storage facilities. Pneumatic devices also are found on meter runs at
distribution company gate stations and distribution grids where they regulate
flow and pressure.
Exhibit 1 depicts a pneumatic control system powered by natural gas. The
pneumatic control system consists of the process control instruments and
valves that are operated by natural gas regulated at approximately 20-30
pounds per square inch (psi), and a network of distribution tubing to supply
all of the control instruments. Natural gas is also used for a few "utility servic-
es," such as small pneumatic pumps, compressor motor starters, and isola-
tion shutoff valves. Exhibit 2 shows a simplified diagram of a pneumatic con-
trol loop. A process condition, such as liquid level in a separator vessel, is
monitored by a float that is mechanically linked to the liquid level controller
Exhibit 1: Natural Gas Pneumatic Control System
Legend:
PC - Pressure Controller
LLC - Liquid Level Controller
Instrumentation
and Control
Systems Piping
Network
Natural Gas
From Plant
Source: ICF Consulting
Pressure
Regulator
Utility
Services
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outside the vessel. A rise or fall in liquid level moves the float upward or
downward, which is translated to small needle valves inside the controller.
Pneumatic supply gas is either directed to the valve actuator by the
needle valve pinching off an orifice, or gas pressure is bled off the valve
actuator. Increasing gas pressure on the valve actuator pushes down a
diaphragm connected by a rod to the valve plug, causing the plug to
open and increasing the flow of liquid draining out of the separator vessel.
Gas pressure relieved from the valve actuator allows a spring to push the
valve plug closed.
Exhibit 2: Signal and Actuation Schematics
Upward
Movement
Level
1 r
Downward
Movement
Separator
Vessel Wall
Source: ICF Consulting
Fulcr
X'
Instru
Pneun
Suppl]
Liquid Level
um Control Instrument
4n
Tl
DownjET"
" M3____;
ment ^
latic Gas f
' ^**
Valve Actuator
..
Liquid Flow 11^
from Separator~| f
r
* Bleed
t To/from Valve
^Actuator
r*-^
Diaphragm
: |
; *Closei|
at
Open
Valve
As part of normal operation, natural gas powered pneumatic devices
release or bleed gas to the atmosphere and, consequently, are a major
source of methane emissions from the natural gas industry. Pneumatic
control systems emit methane from tube joints, controls, and any number
of points within the distribution tubing network. The actual bleed rate or
emissions level largely depends on the design of the device. In general,
controllers of similar design have similar steady-state bleed rates regard-
less of brand name. The methane emission rate will also vary with the
pneumatic gas supply pressure, actuation frequency, and age or condi-
tion of the equipment.
Many partners have found that it is economic to substitute compressed
air for natural gas in pneumatic systems. The use of instrument air elimi-
nates methane emissions and leads to increased gas sales. In addition,
by eliminating the use of a flammable substance, operational safety is sig-
nificantly increased. The primary costs associated with conversion to
instrument air systems are initial capital expenditures for installing com-
pressors and related equipment and operating costs for electrical energy
to power the compressor motor. Existing pneumatic gas supply piping,
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control instruments, and valve actuators of the gas pneumatic system
can be reused in an instrument air system.
A compressed instrument air system is shown in Exhibit 3. In these sys-
tems, atmospheric air is compressed, stored in a volume tank, filtered
and dried for instrument use. Air used for utility services (e.g. small pneu-
matic pumps, gas compressor motor starters, pneumatic tools, sand
blasting) does not need to be dried. All other parts of a gas pneumatic
system will work the same way with air as they do with gas.
Exhibit 3: Compressed Instrument Air System
pcr
f TLlfc-frGas I
Out
•••IjElfe Liquid
t i Out >
Separator f 20-30 PSI
Vessel J. Hetwork
t Air
J Dryer
Air from
Atmosphere /!*» ^
~rHJ J
Compressor Volume
Tank
Source: ICF Consulting
Legend:
PC - Pressure Controller
LLC - Liquid Level Controller
•\
+
J
~\
^
Instrumentation
•' and Control
Systems Piping
Network
Utility Air
Services
The major components of an instrument air conversion project include the
compressor, power source, dehydrator, and volume tank. The following are
descriptions of each of these components along with important installation
considerations.
* Compressor. Compressors used for instrument air delivery are avail-
able in various types and sizes, from rotary screw (centrifugal) com-
pressors to positive displacement (reciprocating piston) types. The size
of the compressor depends on the size of the facility, the number of
control devices operated by the system, and the typical bleed rates of
these devices. The compressor is usually driven by an electric motor
that turns on and off, depending on the pressure in the volume tank.
For reliability, a full spare compressor is normally installed.
* Power Source. A critical component of the instrument air control sys-
tem is the power source required to operate the compressor. Because
high-pressure natural gas is abundant and readily available, gas pneu-
matic systems can run uninterrupted on a 24-hour, 7-day per week
schedule. The reliability of an instrument air system, however, depends
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Economic and
Environmental
Benefits
on the reliability of the compressor and electric power supply. Most
large natural gas plants have either an existing electric power supply or
have their own power generation system. For smaller facilities and
remote locations, however, a reliable source of electric power can be
difficult to assure. In some instances, solar-powered battery-operated
air compressors can be cost effective for remote locations, which
reduces both methane emissions and energy consumption. Small nat-
ural gas powered fuel cells are also being developed.
* Dehydrators. Dehydrators, or air dryers, are an integral part of the
instrument air compressor system. Water vapor present in atmospher-
ic air condenses when the air is pressurized and cooled, and can
cause a number of problems to these systems, including corrosion of
the instrument parts and blockage of instrument air piping and con-
troller orifices. For smaller systems, membrane dryers have become
economic. These are molecular filters that allow oxygen and nitrogen
molecules to pass through the membrane, and hold back water mole-
cules. They are very reliable, with no moving parts, and the filter ele-
ment can be easily replaced. For larger applications, desiccant (alumi-
na) dryers are more cost effective.
* Volume Tank. The volume tank holds enough air to allow the pneu-
matic control system to have an uninterrupted supply of high pressure
air without having to run the air compressor continuously. The volume
tank allows a large withdrawal of compressed air for a short time, such
as for a motor starter, pneumatic pump, or pneumatic tools, without
affecting the process control functions.
Reducing methane emissions from pneumatic devices by converting to
instrument air control and instrumentation systems can yield significant eco-
nomic and environmental benefits for natural gas companies including:
* Financial Return From Reducing Gas Emission Losses. Assuming
a natural gas price of $3.00 per Mcf, savings from reduced emissions
can be estimated at $360 per year per device or $210,000 or more
per year per facility. In many cases, the cost of converting to instru-
ment air can be recovered in less than a year.
* Increased Life of Control Devices and Improved Operational
Efficiency. Natural gas used in pneumatic control devices and instru-
ments often contains corrosive gases (such as carbon dioxide and
hydrogen sulfide) that can reduce the effective operating life of these
devices. In addition, natural gas often produces by-products of iron
oxidation, which can plug small orifices in the equipment resulting in
operational inefficiencies or hazards. When instrument air is used, and
properly filtered and dried, system degradation is reduced and operat-
ing life is extended.
* Avoided Use Of Flammable Natural Gas. Using compressed air as
an alternative to natural gas eliminates the use of a flammable sub-
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Decision
Process
stance, significantly increasing the safety of natural gas processing
plants and transmission and distribution systems. This can be particu-
larly important at offshore installations, where risks associated with
hazardous and flammable materials are greater.
Lower Methane Emissions. Reductions in methane emissions have
been reported as high as 70,000 Mcf per facility annually, depending
on the device(s) and the type of control application.
The conversion of natural gas
cable to all natural gas facili-
ties and plants. To deter-
mine the most cost-effective
applications, however,
requires a technical and
economic feasibility study.
The six steps outlined
below, and the practical
example with cost tables,
equations, and factors, can
help companies to evaluate
their opportunities.
pneumatics to instrument air system is appli-
Decision Process for Converting Gas
Pneumatic Devices to Instrument Air:
1. Identify possible locations for system installations.
2. Determine optimal system capacity.
3. Estimate the project costs.
4. Estimate gas savings.
5. Evaluate the economics.
6. Develop an implementation plan.
Step 1: Identify Possible Locations For Instrument Air System
Installations. Most natural gas-operated pneumatic control systems can be
replaced with instrument air. Instrument air systems will require new invest-
ments for the compressor, dehydrator, and other related equipment, as well
as a supply of electricity. As a result, a first step in a successful instrument
air conversion project is screening existing facilities to identify locations that
are most suitable for cost effective projects. In general, three main factors
should be considered during this process.
* Facility Layout. The layout of a natural gas facility can significantly
affect equipment and installation costs for an instrument air system.
For example, conversion to instrument air might not be cost effective
at decentralized facilities where tank batteries are remote or widely
scattered. Instrument air is most appropriate when used at offshore
platforms and onshore facilities where pneumatics are consolidated
within a relatively small area.
* Number Of Pneumatics. The more pneumatic controllers converted
to instrument air, the greater the potential for reduced emissions and
increased company savings. Conversion to instrument air is most prof-
itable when a company is planning a facility-wide change.
* Available Power Supply. Since most instrument air systems rely on
electric power for operating the compressor, a cost-effective, uninter-
rupted electrical energy source is essential. While major facilities often
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have an existing power supply or their own power generation system,
many smaller and remote facilities do not. For these facilities, the cost
of power generation generally makes the use of instrument air unprof-
itable. In addition, facilities with dedicated generators need to assess
whether the generators have enough available capacity to support an
air compression system, as the cost of a generator upgrade can be
prohibitive. Remote facilities should examine alternatives for power
generation, which range from microturbines to solar power.
Step 2: Determine Optimal System Capacity. Once project sites have
been identified, it is important to determine the appropriate capacity of the
new instrument air system. The capacity needed is a direct function of the
amount of compressed air needed to both operate the pneumatic instru-
mentation and meet any utility air requirements.
* Instrument Air Requirements. The com-
pressed air needs for the pneumatic sys-
tern are equivalent to the volume of gas 1 rfm ajr/contro| |oop
being used to run the existing instrumen-
tation—adjusted for air losses during the
drying process. The current volume of gas usage can be determined
by a direct meter reading (if a meter has been installed). In non-
metered systems, a conservative rule of thumb for sizing air systems is
one cubic foot per minute (cfm) of instrument air for each control loop
(consisting of a pneumatic controller and a control valve).
The initial estimate of instrument air needs
should then be adjusted to account for air loss-
es during the drying process. Typically, the 17 percent of air input
membrane filters in the air dryer consume is consumed by the
about 17 percent of the air input. As a result, membrane dryer
the estimated volume of instrument air usage is
83 percent of the total compressed air supply: i.e. divide estimated air usage
by 83 percent. Desiccant dryers do not consume air and therefore require
no adjustment.
* Utility Air Requirements. It is common to use compressed air for utility
purposes, such as engine starters, pneumatic driven pumps, pneumatic
tools (e.g., impact wrenches), and sand
blasting. Unlike instrument air, utility air
does not have to be dried. The frequency
and volumes of such utility air uses are Pneumatic air uses: 1/3
additive. Companies will need to evaluate for instrument air; 2/3 for
utility air
these other compressed air services on a
site-specific basis, allowing for the possibili-
ty of expansion at the site. A general rule of thumb is to assume that the
maximum rate of compressed air needed periodically for utility purposes
will be double the steady rate used for instrument air.
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Exhibit 4 illustrates how the instrument air compressor size can be estimated.
Using the rule of thumb of 1 cfm/control loop, the current gas usage would
translate to approximately 35 cfm of dry instrument air. Adjusting for the
dryer's air consumption (17 percent of air input), the total instrument air supply
requirement will be 42 cfm. Factoring in utility air needs of about 70 cfm, the
project would require a total of 112 cfm of compressed air.
Exhibit 4: Calculate Compressor Size for Converting Gas Pneumatics
to Instrument Air
Given:
A
lAu
lAs
UAs
L
A
lAu
lAs
UAs
A
For an average size production site with pneumatics, glycol dehydration, com-
pression, 35 control loops, and an average of 10 cfm utility gas usage for pneu-
matic pumps and compressor engine starting.
= Total Compressed Air
= Instrument air use
= Instrument air supply
= Utility air supply
= Control Loops
Rule-of-thumb: 1 cfm per control loop for estimating instrument air systems.
Rule-of-thumb: 17% of air is bypassed in membrane dryers.
Rule-of-thumb: 1/3 of total air used for instruments, 2/3 of total air used for
utility services.
Calculate: A = Air compressor capacity required.
= lAs + UAs
= L*(1 cfm/loop)
= IAu/(100% -% air bypassed in dryer)
= IAu*(fraction of utility air use) / (fraction of instrument air use)
= (35*1) / (100%-17%) + (35*1) * (2/3) / (1/3) = 112 cfm
Step 3: Estimate the Project Costs. The major costs associated with
installing and operating an instrument air system are the installation costs for
compressors, dryers, and volume tanks, and energy costs. The actual instal-
lation costs will be a function of the size, location, and other location specific
factors. A typical conversion of a natural gas pneumatic control system to
compressed instrument air costs approximately $35,000 to $60,000.
To estimate the cost for a given project, all expenses associated with the
compressor, dryer, volume tank, and power supply must be calculated. Most
vendors are willing to provide estimates of the equipment costs and installa-
tion requirements (including compressor size, motor horsepower, electrical
power requirements, and storage capacity). Alternatively, operators can use
the following information on the major system components to estimate the
total installed cost of the instrument air system.
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* Compressor Costs. It is common to install two compressors at a facili-
ty (one operating and one stand-by spare) to ensure reliability and allow
for maintenance and overhauls without service interruptions. The capaci-
ty for each of the compressors must be sufficient to handle the total
expected compressed air volume for the project (i.e., both instrument
and utility air). Exhibit 5 presents cost estimates for purchasing and serv-
icing small, medium, and large compressors. For screw-type compres-
sors, operators should expect to overhaul the unit every 5 to 6 years.
This normally involves exchanging the compressor core for a rebuilt
compressor at a cost of approximately $3,000, with an additional $500
in labor expense and a $500 core exchange credit.
Exhibit !
Service
Size
Small
Medium
Large
5: Air Co
Air
Volume
(cfm)
30
125
350
mpressor Costs
Compressor
Type
Reciprocating
Screw
Screw
Horsepower
10
30
75
Equipment
Costs
($)
2,5001
12,500
22,000
Annual
Service
($/yr)
300
600
600
Service
Life
(yrs)
1
5-62
5-62
1 Cost included package compressor with a volume tank.
2 Rebuilt compressor costs $3,000 plus $500 labor minus $500 core exchange credit.
* Volume Tank. Compressed air supply systems include a volume tank,
which maintains a steady pressure with the on-off operation of the air
compressor. The rule-of-thumb in determining the size of the volume
tank is 1 -gallon capacity for each cfm of
compressed air. Exhibit 6 presents equip-
ment costs for small, medium, and large
volume tanks. Volume tanks have essential- 1 gallon tank capacity/1 cfm air
ly no operating and maintenance costs.
Rule-of-Thumb:
Exhibit 6: Volume Tank Costs
Service Size
Small1
Medium
Large
Air Volume (gallons)
80
400
1,000
Equipment Cost ($)
500
1,500
3,000
1 Small reciprocating air compressors, 10 horsepower and less, are commonly supplied with a surge
tank.
* Air Dryer Costs. Because instrument air must be very dry to avoid
plugging and corrosion, the compressed air is commonly put through a
dryer. The most common dryer used in small to medium applications is
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a permeable membrane dryer. Larger air systems can use multiple mem-
brane dryers, or, more cost effectively, alumina bed desiccant dryers.
Membrane dryers filter out oil mist and particulate solids and have no
moving parts. As a result, annual operating costs are kept low. Exhibit 7
presents equipment and service cost data for different size dryers. The
appropriate sized dryer would need to accommodate the expected vol-
ume of gas needed for the instrument air system.
Exhibit 7: Air Dryer Costs
Service
Size
Small
Medium
Large
Air
Volume
(cfm)
30
601
350
Dryer
Type
membrane
membrane
alumina
Equipment
Cost
($)
1,500
4,500
10,000
Annual Service
($/yr)
500
2,000
3,000
1 Largest membrane size; use multiple units larger volumes.
Using the equipment information described above, the total installed cost for
a project can be calculated. Exhibit 8 illustrates this using the earlier example
of a medium-sized production facility with an instrument air requirement of
42 cfm and a maximum utility air requirement of 70 cfm (for a total of 112
cfm of compressed air). To estimate the installed cost of equipment, it is a
common practice in industry to assume that installation labor is equivalent to
equipment purchase cost (i.e. double equipment purchase cost to estimate
the installed cost). This would be suitable for large, desiccant dried instru-
ment air systems, but for small, skid-mounted instrument air systems a fac-
tor of 1.5 is used to estimate the total installed cost (installation labor is half
the cost of equipment).
Exhibit 8: Calculate Total Installation Costs
Given:
Compressors (2)
Volume Tanks (2-small)
Membrane Dryer
Installed Cost Factor
= $25,000 (exhibit 5)
= $1,000 (exhibit 6)
= $4,500 (exhibit 7)
= 1.5
Calculate Total Installed Cost:
Equipment Cost
Total Cost
= Compressor Cost + Tank Cost
= $25,000 + $1,000 + $4,500
= $30,500
= Equipment Cost * Installation
= $30,500*1.5
= $45,750
+ Dryer Cost
Cost Factor
10
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In addition to the facility costs, it is also necessary to estimate the energy
costs associated with operating the system. The most significant operating
cost of an air compressor is electricity, unless the site has excess self-gener-
ation capacity. To continue the example from above, assuming that electricity
is purchased at 7.5 cents per kilowatt-hour (kWh) and that one compressor
is in standby while the other compressor runs at full capacity half the time (a
50 percent operating factor), the electrical power cost amounts to $13,140
per year. This calculation is shown in Exhibit 9.
Exhibit 9: Calculate Electricity Cost
Given:
Engine Power
Operating Factor (OF)
Electricity Cost
= 30 HP
= 50 percent
= $0.075/kwh
Calculate Required Power:
Electrical Power
= Engine Power * OF * Electricity Cost
= [30 HP * 8,760hrs/yr * 0.5 * $0.075/kwh] 70.75 HP/kw
= $13,140/yr
Step 4: Estimate Gas Savings. To estimate the gas savings that result from
the installation of an instrument air system, it is important to determine the
normal bleed rates (continuous leak from piping networks, control devices,
etc.), as well as the peak bleed rates (associated with movements in the
control devices). One approach is to list all the control devices, assess their
normal and peak bleed rates, frequency of actuation, and estimates of leak-
age from the piping networks. Manufacturers of the control devices usually
publish the emission rates for each type of device, and for each type of
operation. Rates should be increased by 25 percent for devices that have
been in service without overhaul for five to 10 years, and by about 50 per-
cent for devices that have not been overhauled for more than 10 years to
account for increased leakage associated with wear and tear. Alternatively,
installing a meter can be more accurate, provided monitoring occurs over a
long enough period of time to take account of all the utility uses of gas (i.e.,
pumps, motor starters, activation of isolation valves).
EPA's Lessons Learned: Options for Reducing Methane Emissions from
Pneumatic Devices in the Natural Gas Industry, provides brand name,
model, and gas consumption information for a wide variety of currently used
pneumatic devices. Manufacturer information and actual field measurement
data, wherever available, are provided as well (see Appendix of that report).
To simplify the calculation of gas savings for the purpose of this lesson
learned analysis, we can use the earlier rules-of-thumb to estimate the gas
savings. The gas savings for the medium-sized production facility example in
Exhibit 4 include the conservatively estimated 35 cfm used in the 35 gas
11
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pneumatic controllers plus the gas used occasionally for compressor motor
starters and small pneumatic chemical and transfer pumps. (Note that
replacing these gas usages will result in direct savings of gas emissions.)
Natural gas is not used for pneumatic tools or sand blasting, so additional
compressed air provided for these services does not reduce methane emis-
sions. Assuming an annual average of 10 cfm gas use for natural gas pow-
ered non-instrument services, the gas savings would be 45 cfm. As shown
in Exhibit 10, this is equivalent to 23,652 Mcf per year and annual savings of
$71,000.
Exhibit 10: Calculate Gas Savings
Given:
Pneumatic instrument gas usage
Other non-instrument gas usage
= 35 cfm
= 10 cfm
Calculate Value of Gas Saved:
Volume of Natural Gas Saved
Annual Volume of Gas Saved
Annual Value of Gas Saved
= Instrument Usage + Other Usage
= 35 cfm + 10 cfm
= 45 cfm
= 45 cfm * 525,600 min/yr/ 1000
= 23,652 Mcf/yr
= volume * $3.00/Mcf
= 23,652 Mcf/yr * $3.00/Mcf
= $71 ,000/year
Step 5: Evaluate the Economics. The cost effectiveness of replacing the
natural gas pneumatic control systems with instrument air systems can be
evaluated using straightforward cost-benefit economic analyses.
Exhibit 11 illustrates a cost-benefit analysis for the medium-sized production
facility example. The cash flow over a five-year period is analyzed by show-
ing the magnitude and timing of costs from Exhibits 8 and 9 (shown in
parentheses) and benefits from Exhibit 10. The annual maintenance costs
associated with the compressors and air dryer, from Exhibits 5 and 7, are
accounted for, as well as a five-year major overhaul of a compressor per
Exhibit 5. The net present value (NPV) is equal to the benefits minus the
costs accrued over five years and discounted by 10 percent each year. The
Internal Rate of Return (IRR) reflects the discount rate at which the NPV gen-
erated by the investment equals zero.
12
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Exhibit 11: Economic Analysis of Instrument Air System Conversion
Installation Cost ($)
O&M Cost ($)
Overhaul Cost ($)
Total Cost ($)
Gas Savings ($)
Annual Cash
Flow ($)
Cumulative
Cash Flow($)
YearO
(45,750)
0
0
(45,750)
0
(45,750)
(45,750)
Yearl
(13,140)1
(3,200)2
0
(16,340)
71,0004
54,660
8,910
Year 2
(13,140)
(3,200)
0
(16,340)
71,000
54,660
63,570
Years
(13,140)
(3,200)
0
(16,340)
71,000
54,660
118,230
Year 4
(13,140)
(3,200)
0
(16,340)
71,000
54,660
172,890
Payback Period (months)
IRR
NPV5
Years
(13,140)
(3,200)
(4,800)3
(21,140)
71,000
49,860
222,750
10
177%
$158,454
1 Electrical power at 7.5 cents per kilowatt-hour
2 Maintenance costs include $1 ,200 compressor service and $2,000 air dryer membrane replacement
3 Compressor overhaul cost of $3,000, inflated at 10% per year.
4 Value of gas = $3.00/Mcf
5 Net Present Value (NPV) based on 10% discount rate for 5 years.
Step 6: Develop an Implementation Plan. After determining the feasibility
and economics of converting to an instrument air system, develop a system-
atic plan for implementing the required changes. This can include installing a
gas measuring meter in the gas supply line, making an estimate of the num-
ber of control loops, ensuring an uninterrupted supply of electric energy for
operating the compressors, and replacing old, obsolete and high-bleed con-
trollers. It is recommended that all necessary changes be made at one time
to minimize labor costs and disruption of operations. This might include a
parallel strategy to install low-bleed devices in conjunction with the switch to
instrument air systems. There are similar economic savings for conserving
instrument air use as for conserving methane emissions with low bleed
pneumatic devices. Whenever specific pneumatic devices are being
replaced, such as in the case of alternative mechanical and/or electronic
systems, the existing pneumatic devices should be replaced on a similar
economic basis as discussed in the companion document Lessons
Learned: Options for Reducing Methane Emissions from Pneumatic Devices
in the Natural Gas Industry.
13
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Partner
Experiences
Several EPA Natural Gas Star partners have reported the conversion of natu-
ral gas pneumatic control systems to compressed instrument air systems as
the single most significant source of methane emission reduction and a
source of substantial cost savings. Exhibit 12 below highlights the accom-
plishments that several Natural Gas STAR partners have reported.
Exhibit 12: Partner Reported Experience
Gas STAR Description Project Annual Annual Payback
Partner of Project Cost($) Emissions Savings (Months)2
Reductions ($/Year)1
(Met/Year)
Unocal
Texaco3
Chevron3
Exxon/
Mobil4
Shell
Marathon
Installed an air
compression system
in its Fresh Water
Bayou facility in
southern Vermillion
Parish, Louisiana
Installed compressed
air system to drive
pneumatic devices in
10 South Louisiana
facilities
Converted to pneu-
matic controllers to
compressed air,
including new
installations
Installed instrument
air systems at 3 pro-
duction satellites and
1 central tank battery
at Postle C02 unit
Used instrument air
operated devices on
over 4,300 valves at
off-shore platforms
Installed 15 instru-
ment air systems in
New Mexico facilities
60,000
40,000
173,000
over 2
years
55,000
Not
available
Not
available
69,350
23,000
31,700
19,163
532,800
120-38,000
per facility
208,050
69,000
95,100
57,489
1,598,400
360-
114,000
<4
7
11
12
Not
available
Not
available
1 Value of gas = $3.00/Mcf.
2 Calculated based on partner-reported costs and gas savings.
3 Data for this report were collected prior to the Chevron-Texaco merger in 2001.
4 Data for this report were collected prior to the Exxon/Mobil merger in 1999.
14
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Other
Technologies
The majority of partners' experiences in substituting natural gas-powered
pneumatic devices and control instrumentation with alternative controllers
have involved the installation of compressed instrument air systems. Some
additional alternatives to gas pneumatics implemented by partners are
described below:
* Liquid Nitrogen. In a system using liquid nitrogen, the volume tank,
air compressor, and dryer are replaced with a cylinder containing cryo-
genic liquid nitrogen. A pressure regulator allows expansion of the
nitrogen gas into the instrument and control-piping network at the
desired pressure. Liquid nitrogen bottles are replaced periodically.
Liquid nitrogen-operated devices require handling of cryogenic liquids,
which can be expensive as well as a potential safety hazard. Large
volume demands on a liquid nitrogen system require a vaporizer.
* Mechanical Controls and Instrumentation System. Mechanical
instrument and control devices have a long history of use in the natu-
ral gas and petroleum industry. They are usually distinguished by the
absence of pneumatic and electric components, are simple in design,
and require no power source. Such equipment operates using
springs, levers, baffles, flow channels, and hand wheels. They have
several disadvantages, such as limited application, the need for con-
tinuous calibration, lack of sensitivity, inability to handle large varia-
tions, and potential for sticking parts.
* Electric and Electro-Pneumatic Devices. As a result of advanced
technology and increasing sophistication, the use of electronic instru-
ment and control devices is increasing. The advantage of these
devices is that they require no compression devices to supply energy
to operate the equipment; a simple 120-volt electric supply is used for
power. Another advantage is that the use of electronic instrument and
control devices is far less dangerous than using combustible natural
gas or cryogenic liquid nitrogen cylinders. The disadvantage of these
devices is their reliance on an uninterrupted source of electric supply,
and significantly higher costs.
Although these options have advantages, systems using air instead of natu-
ral gas are the most widely employed alternative in replacing natural gas-
operated pneumatic control devices. It is important to note that maintaining
a constant, reliable supply of dry, compressed air in a plant environment is a
significant cost, albeit more economic than natural gas. Therefore, a parallel
strategy to install low-bleed devices in conjunction with the switch to instru-
ment air systems (refer to Lessons Learned: Options for Reducing Methane
Emissions from Pneumatic Devices in the Natural Gas Industry), and to
design a maintenance schedule to keep the instruments and control devices
in tune, is often economic. Such actions can significantly reduce the con-
sumption of instrument air in the overall system and, therefore, minimize
both the size of the compression system and the electricity consumption
over the life of the plant.
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Lessons
Learned
References
The lessons learned from Natural Gas STAR partners are:
* Installing instrument air systems has the potential to increase revenues
and substantially reduce methane emissions.
* Instrument air systems can extend the life cycle of system equipment,
which can accumulate trace amounts of sulfur and various acid gases
when controlled by natural gas, thus adding to the potential savings and
increasing operational efficiencies.
* Remote locations and facilities without a reliable source of electric supply
often need to evaluate alternate power generation sources. When feasi-
ble, solar-powered air compressors provide an economical and ecologi-
cally beneficial alternative to expensive electricity in remote production
areas. On site generation using microturbines running on natural gas is
another alternative.
* A parallel strategy of installing low-bleed devices in conjunction with the
switch to instrument air systems is often economic.
* Existing infrastructure can be used; therefore, no pipe replacement is
needed. However, existing piping and tubing should be flushed clear of
accumulated debris.
* Rotary air compressors are normally lubricated with oil, which must be
filtered to maintain the life and proper performance of membrane dryers.
* Use of instrument air will eliminate safety hazards associated with flam-
mable natural gas usage in pneumatic devices.
* Nitrogen-drive systems may be an alternative to instrument air in special
cases, but tends to be expensive and handling of cryogenic gas is a
safety concern.
* Report reductions in methane emissions from converting gas pneumatic
controls to instrument air in your Natural Gas STAR Annual Report.
Adams, Mark. Pneumatic Instrument Bleed Reduction Strategy and Practical
Application, Fisher Controls International, Inc. 1995.
Beitler, C.M., Reif, D.L., Reuter, C.O. and James M. Evans. Control Devices
Monitoring for Glycol Dehydrator Condensers: Testing and Modeling
Approaches, Radian International LLC, Gas Research Institute, SPE 37879,
1997.
Cober, Bill. C&B Sales and Services, Inc. Personal contact.
Fisher, Kevin S., Reuter, Curtis, Lyon, Mel and Jorge Gamez. Glycol
Dehydrator Emission Control Improved, Radian Corp., Public Service Co. of
Colorado Denver, Gas Research Institute.
16
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Frederick, James. Spirit Energy 76. Personal contact.
Games, J.R, Reuter, C.O. and C.M. Beitler, Field Testing Results for the
R-BTEX Process for Controlling Glycol Dehydrator Emissions, Gas Research
Institute, Radian Corporation, SPE 29742, 1995.
Gunning, Paul M. U.S. EPA Natural Gas STAR Program. Personal contact.
Gupta, Arun, Ansari, R. Rai and A.K. Sah. Reduction of Glycol Loss From
Gas Dehydration Unit At Offshore Platform in Bombay Offshore—A Case
Study, N.A.K.R. IOGPT, ONGC, India, SPPE 36225, 1996.
Reid, Laurance, S. Predicting the Capabilities of Glycol Dehydrators,
SPE-AIME, Laurance Reid Associates.
Scalfana, David B., Case History Reducing Methane Emissions From High
Bleed Pneumatic Controllers Offshore, Chevron U.S.A. Production Co. SPE
37927, 1997.
Schievelbein, V.H., Hydrocarbon Recovery from Glycol Reboiler Vapor With
Glycol-Cooled Condenser, Texaco, Inc. SPE 25949. 1993.
Schievelbein, Vernon H. Reducing Methane Emissions from Glycol
Dehydrators, Texaco EPTD, SPE 37929, 1997.
Soules, J.R. and RV Tran. Solar-Powered Air Compressor: An Economical
and Ecological Power Source for Remote Locations, Otis Engineering Corp.
SPE 25550, 1993.
17
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&EPA
United States
Environmental Protection Agency
Air and Radiation (6202J)
1200 Pennsylvania Ave., NW
Washington, DC 20460
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