Lessons

Learned

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 51 billion cubic feet

(Bcf) per year in the production sector, 14 Bcf per year in the transmission sector, and <1 Bcf from processing. 

Companies can achieve significant cost savings and methane emission reductions by converting natural gaspowered 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 $490,000 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. 

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).

Method for Reducing 

Methane 

Emissions

Replace Gas with Air in

Pneumatic Systems

(per facility)

1

Assumed value of gas is $7.00/Mcf.

2

Cost of installing compressor, dryer and other accessories, and annual electricity requirements.

Volume of

Natural Gas

Savings (Mcf/yr)

20,000

Value of Natural

Gas Savings

($/yr)1

$140,000

Cost of

Implementation

($/yr)2

$60,000

Payback

(months)

6
2

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 applications, 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 400,000 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 including 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 85,000 pneumatic devices actuate isolation valves and

regulate gas flow and pressure at compressor stations, pipelines, and storage facilities. Non-bleed pneumatic devices are also 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 services,” such as small pneumatic pumps, compressor motor starters, and isolation shutoff valves. Exhibit 2 shows a simplified diagram of a pneumatic control 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

Technology

Background

Exhibit 1: Natural Gas Pneumatic Control System

Source: ICF International

Instrumentation

and Control

Systems Piping

Network

Utility

Pressure Services

Regulator

Natural Gas

From Plant

20-30 PSI

Network

Liquid

Out

Separator

Vessel

Inlet

Fluids

Gas

Out

PC Legend:

PC - Pressure Controller

LLC - Liquid Level Controller

LLC
3

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. 

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 regardless of brand name. The methane emission rate will also vary with the

pneumatic gas supply pressure, actuation frequency, and age or condition 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 eliminates methane emissions and leads to increased gas sales. In addition,

by eliminating the use of a flammable substance, operational safety is significantly increased. The primary costs associated with conversion to

instrument air systems are initial capital expenditures for installing compressors and related equipment and operating costs for electrical energy

to power the compressor motor. Existing pneumatic gas supply piping,

Exhibit 2: Signal and Actuation Schematics

Source: ICF International

Liquid Level

Control Instrument Upward Fulcrum

Movement

Liquid

Level

Downward

Movement

Separator

Vessel Wall

Instrument

Pneumatic Gas

Supply

Valve Actuator

To/from Valve

Actuator

Diaphragm

Close

Open

Valve

Up

Down

Liquid Flow

from Separator

Bleed
4

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 systems, atmospheric air is compressed, stored in a volume tank, filtered

and dried for instrument use. Air used for utility services (e.g. small pneumatic 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.

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 available in various types and sizes, from rotary screw (centrifugal) compressors 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 system is the power source required to operate the compressor. Because

high-pressure natural gas is abundant and readily available, gas pneumatic systems can run uninterrupted on a 24-hour, 7-day per week

schedule. The reliability of an instrument air system, however, depends

Exhibit 3: Compressed Instrument Air System

Source: ICF International

Instrumentation

and Control

Systems Piping

Network

Utility Air

Services

Compressor

Air from

Atmosphere

20-30 PSI

Network

Liquid

Out

Separator

Vessel

Inlet

Fluids

Gas

Out

PC Legend:

PC - Pressure Controller

LLC - Liquid Level Controller

Volume

Tank

Air

Dryer

LLC
5

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 natural 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 atmospheric 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 controller 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 molecules. They are very reliable, with no moving parts, and the filter element can be easily replaced. For larger applications, desiccant (alumina) dryers are more cost effective.

★ Volume Tank. The volume tank holds enough air to allow the pneumatic 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 economic and environmental benefits for natural gas companies including:

★ Financial Return From Reducing Gas Emission Losses. Assuming

a natural gas price of $7.00 per Mcf, savings from reduced emissions

can be estimated at $840 per year per device or $490,000 or more

per year per facility. In many cases, the cost of converting to instrument 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 instruments 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 operating life is extended.

★ Avoided Use Of Flammable Natural Gas. Using compressed air as

an alternative to natural gas eliminates the use of a flammable subEconomic and

Environmental

Benefits
6

stance, significantly increasing the safety of natural gas processing

plants and transmission and distribution systems. This can be particularly 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 pneumatics to instrument air system is applicable to all natural gas facilities and plants. To determine 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. 

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 investments 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 profitable 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, uninterrupted electrical energy source is essential. While major facilities often

Decision

Process 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.
7

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 unprofitable. 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 instrumentation and meet any utility air requirements. 

★ Instrument Air Requirements. The compressed air needs for the pneumatic system are equivalent to the volume of gas

being used to run the existing instrumentation—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 nonmetered 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 losses during the drying process. Typically, the

membrane filters in the air dryer consume

about 17 percent of the air input. As a result,

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

additive. Companies will need to evaluate

these other compressed air services on a

site-specific basis, allowing for the possibility 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. 

Rule-of-Thumb:

1 cfm air/control loop

Rule-of-Thumb:

17 percent of air input 

is consumed by the 

membrane dryer 

Rule-of-Thumb:

Pneumatic air uses: 1/3 

for instrument air; 2/3 for

utility air
8

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. 

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 installation 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 $45,000 to $75,000. 

Exhibit 4: Calculate Compressor Size for Converting Gas Pneumatics 

to Instrument Air

Given: For an average size production site with pneumatics, glycol dehydration, compression, 35 control loops, and an average of 10 cfm utility gas usage for pneumatic pumps and compressor engine starting. 

A = Total Compressed Air

IAu = Instrument air use

IAs = Instrument air supply

UAs = Utility air supply

L = 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.

A = IAs + UAs

IAu = L*(1 cfm/loop)

IAs = IAu/(100% -% air bypassed in dryer)

UAs = IAu*(fraction of utility air use) / (fraction of instrument air use)

A = (35*1) / (100%-17%) + (35*1) * (2/3) / (1/3) = 112 cfm

Nelson Price Indexes

In order to account for inflation in equipment and operating & maintenance

costs, Nelson-Farrar Quarterly Cost Indexes (available in the first issue of each

quarter in the Oil and Gas Journal) are used to update costs in the Lessons

Learned documents.

The “Refinery Operation Index” is used to revise operating costs while the

“Machinery: Oilfield Itemized Refining Cost Index” is used to update equipment

costs.

To use these indexes in the future, simply look up the most current NelsonFarrar index number, divide that by the February 2006 Nelson-Farrar index number, and, finally, multiply by the appropriate costs in the Lessons Learned.
9

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 installation 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. 

★ Compressor Costs. It is common to install two compressors at a facility (one operating and one stand-by spare) to ensure reliability and allow

for maintenance and overhauls without service interruptions. The capacity 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 

servicing small, medium, and large compressors. For screw-type

compressors, 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 $3,929, with an additional

$720 in labor expense and a $650 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

equipment costs for small, medium, and large volume tanks. 

Volume tanks have essentially no operating and maintenance costs.

Exhibit 5: Air Compressor Costs

Service Air Compressor Horsepower Equipment Annual Service

Size Volume Type Costs Service Life

(cfm) ($) ($/yr) (yrs)

Small 30 Reciprocating 10 3,2751 434 1

Medium 125 Screw 30 16,371 868 5-62

Large 350 Screw 75 28,812 868 5-62

1 Cost included package compressor with a volume tank.

2 Rebuilt compressor costs $3,929 plus $500 labor minus $500 core exchange credit.

Rule-of-Thumb:

1 gallon tank capacity/1 cfm air 

Exhibit 6: Volume Tank Costs

Service Size Air Volume (gallons) Equipment Cost ($)

Small1 80 655

Medium 400 1,964

Large 1,000 3,929

1 Small reciprocating air compressors, 10 horsepower and less, are commonly supplied with a surge tank.
10

★ 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 a

permeable membrane dryer. Larger air systems can use multiple membrane 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 volume of gas needed for the instrument air system. 

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 instrument air systems, but for small, skid-mounted instrument air systems a 

factor of 1.5 is used to estimate the total installed cost (installation labor is

half the cost of equipment).

Exhibit 7: Air Dryer Costs

Service Air Dryer Equipment Annual Service

Size Volume Type Cost ($/yr)

(cfm) ($)

Small 30 membrane 1,964 724

Medium 601 membrane 5,893 2,894

Large 350 alumina 13,096 4,341

1 Largest membrane size; use multiple units larger volumes. 

Exhibit 8: Calculate Total Installation Costs

Given:

Compressors (2) = $32,742 (exhibit 5)

Volume Tanks (2-small) = $1,310 (exhibit 6)

Membrane Dryer = $5,893 (exhibit 7)

Installed Cost Factor = 1.5

Calculate Total Installed Cost:

Equipment Cost = Compressor Cost + Tank Cost + Dryer Cost

= $32,742 + $1,310 + $5,893

= $39,945

Total Cost = Equipment Cost * Installation Cost Factor

= $39,945 * 1.5

= $59,917
11

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-generation 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. 

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 leakage 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 percent 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

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

Exhibit 9: Calculate Electricity Cost

Given:

Engine Power = 30 HP

Operating Factor (OF) = 50 percent

Electricity Cost = $0.075/kwh

Calculate Required Power:

Electrical Power = Engine Power * OF * Electricity Cost

= [30 HP * 8,760hrs/yr * 0.5 * $0.075/kwh] / 0.75 HP/kw

= $13,140/yr

Methane Content of Natural Gas

Non-associated natural gas found in the

production sector contains approximately

78.8% methane and pipeline quality natural gas found in the transmission sector

contains approximately 93% methane.

Methane emission reductions can be

approximated by applying the appropriate

methane content to the natural gas savings calculated in this document.
compressed air provided for these services does not reduce methane emissions. Assuming an annual average of 10 cfm gas use for natural gas powered 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

$165,600. 

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 showing 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 

generated by the investment equals zero.

Step 6: Develop an Implementation Plan. After determining the feasibility

and economics of converting to an instrument air system, develop a systematic plan for implementing the required changes. This can include installing a

gas measuring meter in the gas supply line, making an estimate of the number of control loops, ensuring an uninterrupted supply of electric energy for

operating the compressors, and replacing old, obsolete and high-bleed controllers. 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 

12

Exhibit 10: Calculate Gas Savings

Given:

Pneumatic instrument gas usage = 35 cfm

Other non-instrument gas usage = 10 cfm

Calculate Value of Gas Saved:

Volume of Natural Gas Saved = Instrument Usage + Other Usage

= 35 cfm + 10 cfm

= 45 cfm

Annual Volume of Gas Saved = 45 cfm * 525,600 min/yr / 1000

= 23,652 Mcf/yr

Annual Value of Gas Saved = volume * $7.00/Mcf

= 23,652 Mcf/yr * $7.00/Mcf

= $165,600/year
13

Exhibit 11: Economic Analysis of Instrument Air System Conversion 

Year 0 Year 1 Year 2 Year 3 Year 4 Year 5

Installation Cost ($) (59,917)

O&M Cost ($) 0 (13,140)1 (13,140) (13,140) (13,140) (13,140)

(4,630)2 (4,630) (4,630) (4,630) (4,630)

Overhaul Cost ($) 0 0 0 0 0 (6,286)3

Total Cost ($) (59,917) (17,770) (17,770) (17,770) (17,770) (24,057)

Gas Savings ($) 0 165,6004 165,600 165,600 165,600 165,600

Annual Cash

Flow ($) (59,917) 147,830 147,830 147,830 147,830 141,543

Cumulative

Cash Flow($) (59,917) 87,912 235,742 383,571 531,401 672,944

Payback Period (months) 5

IRR 246%

NPV5 $496,570

1 Electrical power at 7.5 cents per kilowatt-hour

2 Maintenance costs include $1,736 compressor service and $2,894 air dryer membrane replacement

3 Compressor overhaul cost of $3,929, inflated at 10% per year.

4 Value of gas = $7.00/Mcf

5 Net Present Value (NPV) based on 10% discount rate for 5 years.

When assessing options for converting gas pneumatic controls to instrument

air, natural gas price may influence the decision making process. Exhibit 12

shows an economic analysis installing two 30 hp compressors, two medium

sized volume tanks, and a medium sized membrane dryer at different natural

gas prices.

Exhibit 12: Gas Price Impact on Economic Analysis

Internal Rate 

of Return (IRR)

Value of Gas

Saved

Net Present 

Value (i=10%)

Payback Period

(Months)

$3/Mcf

14

$70,971

84%

$137,853

$5/Mcf

$118,286

8

166%

$317,211

$7/Mcf

246%

5

$165,600

$496,570 $586,249

286%

5

$189,257

$8/Mcf $10/Mcf

$236,571

4

365%

$765,607

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. 
14

Partner

Experiences

Exhibit 13: Partner Reported Experience

Gas STAR Description Project Annual Annual Payback

Partner of Project Cost ($) Emissions Savings (Months)2

Reductions ($/yr)1

(Mcf/yr)

Unocal3 Installed an air $79,000 69,350 $485,450 2

(now compression system 

Chevron) in its Fresh Water

Bayou facility in

southern Vermillion

Parish, Louisiana

Texaco3 Installed compressed $52,000 23,000 $161,000 4

(now air system to drive

Chevron) pneumatic devices in

10 South Louisiana

facilities

Chevron3 Converted to pneu- $227,000 31,700 $221,900 7

matic controllers to over 2

compressed air, years

including new

installations

ExxonMobil4 Installed instrument $72,000 19,163 $134,141 7

air systems at 3 production satellites and

1 central tank battery

at Postle CO2 unit

Shell Used instrument air Not 532,800 $3,729,600 Not

operated devices on available available

over 4,300 valves at

off-shore platforms

Marathon Installed 15 instru- Not 120 - 38,000 $840 - Not

ment air systems in available per facility 226,000 available

New Mexico facilities

1 Value of gas = $7.00/Mcf.

2 Calculated based on partner-reported costs and gas savings updated to 2006 costs.

3 Data for this report were collected prior to the Chevron-Texaco and Chevron-Unocal mergers.

4 Data for this report were collected prior to the Exxon/Mobil merger in 1999.

Several EPA Natural Gas Star partners have reported the conversion of natural 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 13 below highlights the accomplishments that several Natural Gas STAR partners have reported.
15

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 cryogenic 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 natural 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 continuous calibration, lack of sensitivity, inability to handle large variations, and potential for sticking parts.

★ Electric and Electro-Pneumatic Devices. As a result of advanced

technology and increasing sophistication, the use of electronic instrument 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 natural gas are the most widely employed alternative in replacing natural gasoperated 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 instrument 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 consumption 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.

Other

Technologies
16

Lessons

Learned

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 feasible, solar-powered air compressors provide an economical and ecologically 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 flammable 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.
17

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.

Frederick, James. Spirit Energy 76. Personal contact.

Games, J.P., 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 P.V. Tran. Solar-Powered Air Compressor: An Economical

and Ecological Power Source for Remote Locations, Otis Engineering Corp.

SPE 25550, 1993.

References
United States

Environmental Protection Agency

Air and Radiation (6202J)

1200 Pennsylvania Ave., NW

Washington, DC 20460

October 2006