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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

OFFICE OF 

 AIR AND RADIATION 

Petroleum and Natural Gas Systems (Subpart W) of the Greenhouse Gas Reporting Rule 

Docket: EPA-HQ-OAR-2009-0923 

Title: Compressor Modes and Thresholds 

Summary: This memo discusses the compressor monitoring requirements required in 40 CFR 

Part 98 Subpart W – Petroleum and Natural Gas Systems of the Greenhouse Gas Reporting Rule, 

which has been modified from the April 2010 proposed rule in response to comments on the 

burden and potential adverse consequences of implementing the proposed rule. 
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Compressor Modes and Threshold 

EPA determined through experience reported by industry partners in the Natural Gas STAR 

Program1 that different and significant vented emissions occur from large gas compressors in 

three different operational modes: emissions from drive shaft seals while compressors are 

operating, leakage through blowdown valves when closed and compressors are either operating 

or standby under pressure, and unit isolation valve leaks during time when compressors are shut 

down and depressurized. This memo discusses the compressor monitoring requirements required 

in 40 CFR Part 98 Subpart W – Petroleum and Natural Gas Systems of the Greenhouse Gas 

Reporting Rule, which has been modified from the April 2010 proposed rule in response to 

comments on the burden and potential adverse consequences of implementing the proposed rule. 

Background of Three Modes of Compressors and Reporting Requirements

Vented emissions from compressors are different, and from different components during the 

three modes of operation. These three modes are: (1) operating, (2) standby under pressure, and 

(3) standby and de-pressurized. Reciprocating compressors may spend significant time in all 

three modes, while centrifugal compressors are more commonly in either the operating or 

standby and de-pressurized modes. Centrifugal compressor means any equipment that increases 

the pressure of a process natural gas or CO2 by centrifugal action, employing rotating movement 

of the driven shaft. Reciprocating compressor means a piece of equipment that increases the 

pressure of a process natural gas or CO2 by positive displacement, employing linear movement 

of a shaft driving a piston in a cylinder. When a compressor is fully pressurized, natural gas can 

leak from the closed blowdown valve and compressor drive shaft seals (piston rods on 

reciprocating compressors, rotating drive shaft on centrifugal compressors). When compressors 

are shut down but not depressurized, the blowdown valve is closed but can still leak. 

Furthermore, reciprocating compressor rod packing is a dynamic seal, which can leak more when 

the compressor is shutdown under pressure. Centrifugal compressor seals “close” when the shaft 

stops spinning and seal oil (in wet seal compressors) stops circulating. 

When compressors are shut down for long periods of time in stand-by, or for maintenance, they 

are commonly blown down (depressurized by opening a blowdown valve to vent the high 

pressure gas in the compressor case or cylinders, suction and discharge bottles for reciprocating 

compressors and piping between the unit isolation valves). This gas is typically vented out of a 

stack on the roof of a compressor building, or a high vent stack in outdoor compressors for safety 

reasons. . The unit isolation valves are large and seal against a high pressure differential between 

suction or discharge pressure and atmospheric pressure, typically between 300 and 1000 psig. 

During this shutdown-depressurized period leakage through the large unit isolation valves can be 

very large and go unnoticed as the emission is through the blowdown vent stack on a compressor 

building roof or outdoor elevated blowdown vent stack. If the compressor is shutdown for 

internal maintenance, leaking unit isolation valves will be “blinded” (flat, metal plate inserted 

between the flanges of the isolation valve on the compressor side ... a positive seal against gas 

leaking into the compressor during maintenance). Obviously unit isolation valve leakage cannot 

be detected or measured with a blind in place. 

 

1

 http://www.epa.gov/gasstar/documents/savings.pdf 
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Requirement for Emissions Measurement in Each Compressor Mode

Based on comments received on the April 2010 proposed rule, compressors are not shutdown 

and depressurized on a forward schedule every year, but are taken offline for scheduled 

maintenance at least once during a three year period. EPA received comments indicating that 

coordinating the once per year compressor emissions analysis with unscheduled shutdown for 

measuring emissions in the not-operating, de-pressurized mode would be difficult and 

burdensome. They pointed out that their operating commitments could not necessarily be met by 

shutting down each compressor when the emissions measurement personnel were onsite. Further, 

since detecting and measuring the unit valve leakage must be done without blind flanges in 

place, this activity must be pre-planned and mobilized well ahead of a compressor shutdown for 

maintenance. EPA did not intend for reporters to perform unscheduled shutdown of compressors 

simply for monitoring emissions in this made. Therefore, today’s final rule clarifies that reporters 

are allowed to measure vented emissions of each compressor in the mode found during an annual 

measurement, but must measure each compressor in the shutdown-depressurized mode at least 

once during any three consecutive year period. Emissions measurement in other modes must be 

performed each year in the mode that compressors are found during the annual measurement. 

Hence, reporters are not required to shutdown compressors during the reporting period for the 

purposes of monitoring under subpart W. However, reporters will be expected to schedule 

monitoring in conjunction with planned compressor shutdowns for maintenance such that unit 

isolation valves are tested for through-valve leakage before blind flanges are swung. EPA has 

added a more cost effective alternative for this testing by allowing the use of an acoustic leak 

detection instrument that is calibrated for estimating through-valve leakage from the acoustic 

signal, valve type and size, and stream contents and pressure. This instrument can do the leak 

detection and quantification at one time at the unit valve and not involve insertion meters or endof-pipe measurements on the blowdown vent stack. 

Reporter Level Emission Factors for Emissions Measurement 

With the above provision in the final rule, wherein not every compressor vented emissions will 

be measured in every mode of operation each calendar year, EPA is requiring that reporter-level 

emission factors be developed for each mode of operation. All measurements made in a given 

mode of operation during the reporting year and two preceding reporting years will be summed 

and divided by the number of compressors measured. This reporter specific emission factor, in 

cubic feet per hour, will be used to estimate and report the emissions for all compressors not 

measured in that mode in the current reporting year, for the time in that mode. The reporter will 

still report emissions for compressors measured in the reporting year based on the compressor’s 

measured vented emissions and duration in that mode. This way, if reporters make repairs (not 

required by the MRR) to leaking unit isolation valves, they can take advantage of that eliminated 

emission on that compressor which is measured. 

For a hypothetical example, where one reporter has four compressor stations (i.e. four 

transmission compressor facilities), each with five compressors for a total of 20 compressors, he 

conducts monitoring of each compressor in each compressor station the first year in the “asfound” mode. This monitoring finds that 75% of centrifugal compressors were in the operating 

mode, while the remaining 25% of centrifugal compressors were in the not-operating, 

depressurized mode. In this case the reporter does not have complete emissions data to represent 
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both modes of operation for each individual centrifugal compressor in each station. EPA 

requires that the reporter develop an average per-hour emission factor for centrifugal compressor 

wet seal degassing vents in the operating mode and a separate per-hour emission factor for 

centrifugal compressors unit isolation valve leakage in the not-operating depressurized mode. 

Then, the reporter will use these average emission factors for those centrifugal compressors not 

measured in the associated mode by multiplying the factor by the cumulative time that 

compressors in each station were in the mode but not measured in that mode in this reporting 

year. The compressors measured in a mode will use their unique measurement for the time they 

were in that mode in that year. In a subsequent year, a compressor measured in one of the modes 

in the previous year, but not in the present reporting year will use the average emission factor. 

Emission Factor for Onshore Production Compressors

The onshore production sector has many small compressors dispersed over hundreds of miles at 

production well-sites. These smaller compressors are typically reciprocating type and not 

“spared” (i.e. a single compressor that does not have a standby spare). Hence, they have a high 

operating factor and because of their type and size, have smaller process emissions (vented and 

equipment leaks) primarily from rod packing. By contrast the processing, transmission, gas 

storage, LNG storage and LNG import/export terminal sectors generally have larger compressors 

within a single facility boundary or adjacent to that boundary. These large compressors are of 

different types (reciprocating, centrifugal wet seal and centrifugal dry seal). They are almost 

always spared, and therefore have lower operating factors (more time in the standby 

depressurized mode) and higher emissions, particularly from wet seal centrifugals. 

Insufficient publicly available data was available for EPA to reliably quantify the distribution of 

onshore production compressors by sizes and services to facilitate the definition of a threshold 

between “smaller” and “larger” wellhead compressors to make it possible to provide an emission 

factor for “small” compressors and require measurements from “large” compressors. As such, 

today’s final rule requires the use of a process emissions (vented and equipment leaks) factor for 

all onshore petroleum and natural gas production compressors. For reciprocating compressors, 

the Final Rule provides process emission factors which were adopted from the Gas Research 

Institute study2 (Volume 8, 1994; available online at the EPA GasStar site). For wet seal 

degassing emissions from centrifugal compressors, the Final Rule provides process emission 

factors which were adopted from a study reported at the 24th World Gas Conference3 which 

aggregated all available measurement data into three factors: high, medium and low. For 

conservative estimation, EPA is using the low emission factor from this study. 

 

2

 GRI study available at http://epa.gov/gasstar/tools/related.html 3

 “ Methane’s Role in Promoting Sustainable Development in the Oil and Natural Gas Industry”, 

http://www.epa.gov/gasstar/documents/best_paper_award.pdf