Natural Gas Industry Methane Emission Factor Improvement Study 

 Final Report 

Cooperative Agreement No. XA-83376101 

Prepared by: 

Matthew R. Harrison 

Katherine E. Galloway 

Al Hendler 

Theresa M. Shires 

URS Corporation 

9400 Amberglen Boulevard 

Austin, TX 78729 

Dr. David Allen 

Dr. Michelle Foss 

James Thomas 

Jarett Spinhirne 

The University of Texas at Austin 

10100 Burnet Road 

Austin, Texas 78758 

Prepared for: 

Lisa Hanle 

United States Environmental Protection Agency 

1200 Pennsylvania Avenue 

Washington D.C. 20460 

 

December 2011 
DISCLAIMER 

The scope of services performed during the Natural Gas Industry Methane Emission Factor 

Improvement Study may not be appropriate to satisfy the needs of other users, and any use or reuse of this document or of the findings, conclusions, or recommendations presented herein is at the 

sole risk of said user. 

This publication was developed under Assistance Agreement No. XA-83376101 awarded by the 

U.S. Environmental Protection Agency. It has not been formally reviewed by EPA, and EPA does 

not endorse any products or commercial services mentioned in this publication. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report ES 

Executive Summary 

This report presents the work performed by the University of Texas at Austin, and URS 

Corporation, with the goal of updating default methane (CH4) emission factors for selected 

processes and equipment used in the natural gas industry. The initial impetus of this study was to 

establish new emission factors that were both statistically superior to the 1996 Gas Research 

Institute (GRI) and U.S. EPA emissions inventory project (GRI and EPA, 1996) emission factors, 

and more relevant than the GRI/EPA factors (by including more recent samples). The default 

emission factors for various sources were compiled and synthesized for a variety of source 

categories, and new emission rate measurements were conducted for selected sources where 

existing data had large uncertainties or were thought to possibly be insufficiently representative of 

current practices and equipment. 

The study focus was high emission rate leaks (fugitive leaks) from transmission, 

gathering/boosting, and gas processing reciprocating and centrifugal compressor components, 

including emissions from compressor vents (i.e., blowdown lines and compressor seals). Samples 

were collected from 66 reciprocating compressors and 18 centrifugal compressors, with a total of 

48 reciprocating compressors at transmission compressor stations. Emissions from other fugitive 

sources such as valves, flanges, and other components were also measured in a few locations. 

As found in other similar studies, the largest single emission sources at a compressor 

station are the compressor blowdown (BD) vent lines and the compressor seal vents. 

The new measurements made for this project on fugitive components (i.e., valves, flanges, 

etc.) produced lower emission factors than the previous GRI/EPA study. This may be due to 

improved LDAR practices for accessible fugitive components that have been implemented by 

companies in the past two decades. For centrifugal transmission compressors this project found 

the average blowdown line emission factors were significantly lower than the GRI/EPA study, but 

found wet seal degassing vent emissions were much higher. For reciprocating transmission 

compressors, this project found average blowdown line emission factors that were significantly 

higher than the GRI/EPA study, and rod packing vent emissions that were also much higher. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report i 

Table of Contents 

1.0 Introduction and Background ................................................................................................... 1

1.1 Selection of Scope .................................................................................................... 4

1.2 Technical Background .............................................................................................. 5

1.2.1 Centrifugal Compressors .......................................................................................... 7

1.2.2 Reciprocating Compressors ...................................................................................... 8

1.2.3 Blowdown Open-Ended Lines ................................................................................ 10

1.3 Review of Existing Emission Factors .................................................................... 11

1.3.1 Transmission– Fugitive Emissions from Reciprocating and Centrifugal 

Compressors ........................................................................................................... 11

1.3.2 Processing– Fugitive Emissions from Reciprocating and Centrifugal 

Compressors ........................................................................................................... 13

1.3.3 Compressor Seals .................................................................................................... 14

2.0 Measurement Campaigns ....................................................................................... 18

2.1 Selection of Test Sites ............................................................................................ 18

2.2 Site Descriptions ..................................................................................................... 20

2.3 Field Measurement Methods .................................................................................. 20

2.3.1 Equipment Component Counts and Component Leaks .......................................... 26

2.3.2 Open-Ended Lines .................................................................................................. 27

2.3.3 VPAC Measurements.............................................................................................. 29

2.3.4 Compressor Seals .................................................................................................... 30

2.4 Quality Control ....................................................................................................... 35

3.0 Measurement Results and Analyses ....................................................................... 36

3.1 Measurement Results ............................................................................................. 36

3.3 Comparison to Existing Data ................................................................................. 38

3.4 Comparison of VPAC to Direct Measurement Results .......................................... 39

4.0 Conclusions and Recommendations ....................................................................... 41

Appendix A: Summary of Document Review 

Appendix B: Data Summary 

Appendix C: References 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

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List of Tables 

Table 1-1. GRI/EPA Emission Factors Summary ......................................................................... 3 

Table 1-2. Operating Modes of Compressors in Gas Transmission ............................................ 13 

Table 1-3. Operating Modes of Compressors in Gas Processing Service ................................... 14 

Table 1-4. Comparison of Compressor Seal Emission Factors ................................................... 16

Table 2-1. Description of Sites Surveyed .................................................................................... 20 

Table 2-2. Site-Specific Sources Sampled ................................................................................... 22 

Table 3-1. Component Count Summary ...................................................................................... 36 

Table 3-2. Fugitive Emissions from Valves and Flanges using Hi-Flow Sampler Compared 

to Previous Results .................................................................................................... 36 

Table 3-3. Sampling Results for Centrifugal Compressors Vents ............................................... 37 

Table 3-4. Sampling Results for Reciprocating Compressors (Transmission Compressors) ...... 37 

Table 3-5. Sampling Results for Reciprocating Compressors (Gathering/Boosting 

Compressors) ............................................................................................................. 38 

List of Figures 

Figure 1-1. Centrifugal Compressor Wet Seal Degassing Vent .....................................................6 

Figure 1-2. Compressor Blowdown Vents .....................................................................................6 

Figure 1-3. Centrifugal Compressor Schematic (Rolls Royce, 2010) ............................................7 

Figure 1-4. Centrifugal Compressors (foreground) with Trubine Driver (background) ................7 

Figure 1-5. Reciprocating Compressor Schematic (EPA, 2006) ....................................................8 

Figure 1-6. Reciprocating Compressors .........................................................................................9 

Figure 1-7. Reciprocating Compressor Rod Packing Vent (Gathering Service) ..........................10 

Figure 1-8. Typical Compressor Blowdown Arrangement ...........................................................11 

Figure 2-1. Concentrations of Natural Gas Processing Plants, 2004 ............................................18 

Figure 2-2. Natural Gas Pipeline Compressor Stations, 2008 ......................................................19 

Figure 2-3. FLIR Camera and Hi-Flow Sampler ..........................................................................26 

Figure 2-4. Measurement of Compressor Blowdown Vent Line Leakage with Encased Rotary 

Van Anemometer .......................................................................................................29 

Figure 2-5. Sampling a Blowdown Vent with Hot Wire Anemometer ........................................29 

Figure 2-6. Through Valve Acoustic Leak Detection (VPAC) ....................................................30 

Figure 2-7. Sampling a Rod Packing Vent Stack with Enclosed Rotary Vane Anemometer ......31 

Figure 2-8. Sampling a Rod Packing Vent with Hand-held Hot Wire Anemometer ...................31 

Figure 2-9. Centrifugal Compressor Seal Oil Gas Measurement using Calibrated Bag 

 Technique ...................................................................................................................32 

Figure 2-10. Centrifugal Compressor Lube Oil System Process Flow Diagram (Picard, 2011) ..34 

Figure 3-1. VPAC vs. Direct Flow Measurement Method, complete data set .............................40 

Figure 3-2. Modified VPAC vs. Direct Flow Measurement Method, modified data set for lower 

flow .............................................................................................................................41 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

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1.0 Introduction and Background 

This report describes work conducted under a cooperative agreement sponsored by the U.S. 

Environmental Protection Agency (EPA). The work was performed by the University of Texas at 

Austin, and URS Corporation, with the goal of updating default methane (CH4) emission factors 

for selected processes and equipment used in the natural gas industry. The default emission factors 

for various sources were compiled and synthesized for a variety of source categories, and new 

emission rate measurements were conducted for selected sources where existing data have large 

uncertainties or are insufficiently representative of current practices and equipment. 

Methane is the primary component of natural gas and is also a potent greenhouse gas 

(GHG). About 32% of the U.S. anthropogenic CH4 emissions have been attributed to natural gas 

systems, the highest emission source category (EPA, 2011). Published estimates of CH4 emissions 

from the U.S. natural gas industry are based mostly on default average emission factors that were 

developed in the early 1990s and published in 1996 by the Gas Research Institute (GRI) and EPA 

(GRI and EPA, 1996).1 In the intervening years, economic incentives and greater awareness of 

CH4 as a GHG have inspired new emission reduction approaches that are not reflected in the 1996 

GRI/EPA emission factors. Large uncertainties in some of the GRI/EPA emission factors resulted 

in some cases from small sample populations, variations in equipment design, and variations in 

equipment operating practices. Consequentially, these uncertainties may also affect the quality of 

emissions estimates derived from some of the existing default factors. 

EPA, in collaboration with the American Petroleum Institute (API), the American Gas 

Association (AGA), and the Interstate Natural Gas Association of America (INGAA), identified a 

subset of the emission factors from the 1996 GRI/EPA reports believed to contribute the greatest 

uncertainty in the U.S. natural gas industry CH4 emissions inventory. In 2008, EPA awarded a 

cooperative agreement to the University of Texas to update emission factors for the source 

categories listed below: 

 

1

 The GRI/EPA study is documented in a 15 volume set. This report refers to the collective GRI/EPA study as well 

as specific volumes as necessary. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

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 Production: Well clean-ups, completion flaring, well workovers, and pipelines leaks; 

 Processing: Fugitive emissions from reciprocating and centrifugal compressors; 

 Transmission and Storage: Fugitive emissions from reciprocating and centrifugal 

compressors, pneumatic devices, and M&R stations; and 

 Distribution: Residential customer meters, plastic mains, and services. 

The objective for the project was initially to develop new emission factors for these sources 

that could be used to replace the existing emission factors, most of which were developed in the 

mid-1990’s under the GRI/EPA program. An extensive search of publicly available GHG data for 

the emission sources of interest to this project was conducted; a summary of this review is included 

in Appendix A. The detailed literature review is available as an addendum to this report and on the 

project web site: (www.utexas.edu/research/ceer/GHG ). 

The GRI/EPA study gathered data in the early 1990’s with the intent of characterizing CH4

emissions from the U.S. natural gas industry. The goal of the GRI/EPA report was a single, 

accurate national estimate for total CH4 emissions. Tight confidence bounds on the national total 

was important, but confidence bounds on any one of the myriad of individual sources could be 

wider (poorer). As a result, while the GRI/EPA study focused on understanding typical emission 

characteristics and extrapolating data gathered, for some strata of emission sources, data were 

gathered for a relatively few number of samples. The GRI/EPA study gathered measurements 

across broad ranges of technologies and operating practices for a discreet number of emissions 

sources as they existed in the early 1990s. This produced national average emissions factors which 

were applied to national level activity data. Table 1-1 summarizes the emission factors for the 

sources listed above and their contribution to sector and national emissions for the U.S. natural gas 

industry at the time of the GRI/EPA study. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

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Table 1-1. GRI/EPA Emission Factors Summary 

Source Emission Factor 

Contribution to 

Sector Emissions, 

% 

Contribution to 

Total National 

Emissions, % 

Well Workovers 

(i.e., cleanups) 

0.047 ± 459% tonne CH4/well 

workover-yr 

0.01% 

(± 1,298%) 

Gas Well Unloading 0.95 ± 344% tonne CH4/unloading 

gas well-yr 

6.7% 1.8% (± 380%) 

Well Completions 0.014 ± 200% tonne 

CH4/completion well-yr 

0.027% 0.007% (± 201%) 

Production Gathering Pipelines: 

Protected Steel 0.327± 85% tonne CH4/leak-yr 7.8% 2.1% (± 108%) 

Production Gathering Pipelines: 

Unprotected Steel 0.836 ± 93% tonne CH4/leak-yr 

Production Gathering Pipelines: 

Plastic 1.61±166% tonne CH4/leak-yr 

Production Gathering Pipelines: 

Cast Iron 3.85 ± 64% tonne CH4/leak-yr 

Processing: Reciprocating 

compressors 

78± 74% tonne CH4/ reciprocating 

compressor-yr 

61% 5.32% (± 95.1%) 

Processing: Centrifugal 

compressors 

148± 39% tonne CH4/centrifugal 

compressor-yr 

1.79% (± 91.4%) 

Transmission: Reciprocating 

compressors 

106± 68% tonne CH4/ 

reciprocating compressor-yr 

39% 12.1% (± 68.1%) 

Transmission: Centrifugal 

compressors 

212 ± 44% tonne CH4/centrifugal 

compressor-yr 

2.4% (± 43.7%) 

Transmission: Continuous bleed 

pneumatics 

8.915 ± 29% tonnes CH4/device-yr 12% 4.5% (±60%) 

Transmission: 

Pneumatic/hydraulic devices 

0.1008 ± 112% tonne CH4/deviceyr 

Transmission: Turbine pneumatic 

devices 

1.211 ± 276 tonne CH4/ 

device-yr 

Transmission M&R Stations: 

transmission-to-transmission 

7.62± 80% tonne CH4/ 

transmission-to-transmission 

station-yr 

3.9% 1.43% 

(± 1,000%) 

Transmission M&R Stations: 

Farm taps 

0.000597± 80% tonne CH4/farm 

tap-yr 

Distribution: Residential meters 0.00265 ± 17% tonne CH4/meteryr 7.2% 1.76% (± 19.8%) 

Distribution: Plastic mains 1.91± 166% tonne CH4/leak-yr 6.4% 1.56% (± 282%) 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

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Although the GRI/EPA project was not intended to be the basis for estimating CH4

emissions from any particular company source, the data collected during this study provided (and 

continues to provide) the most comprehensive quantification of CH4 emissions data for a variety of 

natural gas industry sources. While the GRI/EPA study serves as the basis for most natural gas 

industry CH4 emission estimates worldwide, there have been some changes in the natural gas 

industry since the study was performed. New and improved technologies (such as horizontal 

drilling of unconventional gas sources), improved operating practices, government regulation of 

emissions, significant participation in voluntary programs like Natural Gas STAR, and economic 

conservation prompted by the rising price of natural gas are driving forces for reviewing and 

potentially updating specific emission factors. 

1.1 Selection of Scope

Stakeholders for this study, which included the U.S. EPA, as well as some industry 

participants, determined that this study should focus primarily on the natural gas compressor 

station fugitive emissions from reciprocating and centrifugal compressors. Those were among a 

few sources in the GRI/EPA study with high overall emission rate and relatively wide confidence 

bounds. For all compressors in natural gas service, the highest source of fugitive emissions from 

compressor stations occurs in transmission stations, in particular the compressor vents (continuous 

leaks from blowdown lines vent stacks and from compressor seals). Therefore this study initially 

focused on transmission stations. 

While this study focuses on transmission, some nearly identical compressors in gas processing 

were sampled as opportunistic surrogates, and in one region, some additional opportunistic data 

was gathered for upstream production gathering/boosting station compressor emissions. 

The study focus was high emission rate leaks (fugitive leaks) from compressor components, 

including emissions from compressor vents (i.e., blowdown lines and compressor seals). 

Emissions from other fugitive sources such as valves, flanges, and other components were also 

measured in a few locations. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

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1.2 Technical Background 

Compressors are used at various points along the natural gas collection, transportation, and 

delivery system to move gas by means of a pressure differential. Compressors at natural gas 

gathering/boosting stations are used to move gas from production to either central treatment 

facilities, to gas processing plants, or to transmission pipelines. Within a gas processing plant, 

compressors are used to draw gas to the plant and/or to move gas through various portions of the 

plant. In transmission, compressors are used to overcome the pressure drop that occurs in the long 

distances of the transmission pipeline. 

Fugitive emissions include leaks from normally sealed components on the pressurized 

piping and equipment systems. Those include flanges, screwed fittings (connectors), valve stem 

packing (valves), pressure relief valves (PRV’s), valve seats where one side of the closed valve is 

open to the atmosphere (open-ended lines [OELs]), and seals, such as compressor rod packing or 

rotating seals. The most significant fugitive emissions are associated with three sources: 

1) centrifugal compressor seal oil gas, 2) reciprocating compressor piston rod packing systems, and 

3) compressor blowdown line OELs. 

For this study, fugitive emissions from centrifugal compressors and reciprocating 

compressors were measured at gathering/boosting facilities, gas processing plants, and 

transmission compressor stations. A photo of a centrifugal compressor wet seal degassing vent is 

shown in Figure 1-1 and a photo of compressor blown vents are shown in Figure 1-2. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

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Figure 1-1. Centrifugal Compressor Wet Seal Degassing Vent 

Figure 1-2. Compressor Blowdown Vents 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

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1.2.1 Centrifugal Compressors 

Centrifugal compressors are widely used to handle large volumes of gas at high pressure. 

A centrifugal compressor, in its simplest form, consists of an impeller rotating within a casing, 

using centrifugal force to propel the gas to the outside of the casing at an increased pressure. A 

diagram of a centrifugal compressor is shown in Figure 1-3, and a photo is shown in Figure 1-4. 

Figure 1-3. Centrifugal Compressor Schematic (Rolls Royce, 2010) 

Figure 1-4. Centrifugal Compressor (foreground) with Turbine Driver 

(background)
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

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Centrifugal compressors require seals around the rotating shaft to prevent gases from 

escaping where the shaft exits the compressor casing. These seals use oil, which is circulated 

under high pressure between three rings around the compressor shaft, forming a barrier against the 

compressed gas leakage. The center ring is attached to the rotating shaft, while the two rings on 

each side are stationary in the seal housing, pressed against a thin film of oil flowing between the 

rings to both lubricate and act as a leak barrier. “O-ring” rubber seals prevent leakage around the 

stationary rings. Very little gas escapes through the oil barrier; considerably more gas is absorbed 

by the oil under the high pressures at the “inboard” (compressor side) seal oil/gas interface, thus 

contaminating the seal oil. Seal oil is purged of the absorbed gas (using heaters, flash tanks, and 

degassing techniques) and recirculated. (EPA, 2006) 

1.2.2 Reciprocating Compressors 

A reciprocating compressor is a positive displacement device which employs pistons driven 

by a crank shaft to move gas. The pistons increase the pressure of the gas by reducing its volume. 

Areas of high leak frequency from reciprocating compressors include flanges, valves, and 

fittings located on compressors, due to the high vibration. The highest volume of gas loss, 

however, is associated with two sources: 1) piston rod packing systems, and 2) compressor 

blowdown line OELs. 

Packing systems are used to maintain a tight seal around the piston rod, preventing the gas 

compressed to high pressure in the compressor cylinder from leaking, while allowing the rod to 

move freely. A schematic of a reciprocating compressor is shown in Figure 1-5. 

Figure 1-5. Reciprocating Compressor Schematic (EPA, 2006)
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Compressor rod packing consists of a series of flexible rings that fit around the shaft to 

create a seal against leakage. The packing rings are lubricated with circulating oil to reduce wear, 

help seal the unit, and draw off heat. Packing rings are held in place by a set of packing cups, 

normally one for each pair of rings, and kept tight against the shaft by a surrounding spring. A 

“nose gasket” on the end of the packing case prevents leaks around the packing cups. Higher leak 

rates are a consequence of fit, alignment of the packing parts, and wear. Leakage typically occurs 

from four areas: 

 Around the packing case through the nose gasket; 

 Between the packing cups, which are typically mounted metal-to metal against each 

other; 

 Around the rings from slight movement in the cup groove as the rod moves back and 

forth; and 

 Between the rings and shaft. (EPA, 2006) 

Leaking gases are vented to the atmosphere either directly from the end of the packing seal, 

or through the compressor distance piece vent. Photos are shown in Figures 1-6 and 1-7. 

Sometimes, on larger compressors, these distance piece vents are piped together and joined into 

one larger vent line. 

Figure 1-6. Reciprocating Compressors 

(note the 2” rod packing line with orange elbows)
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 10 

Figure 1-7. Reciprocating Compressor Rod Packing Vent (Gathering Service) 

(1/2” tubing)

1.2.3 Blowdown Open-Ended Lines 

The second significant source from compressors is large OELs. In particular, those OELs 

that are connected to the compressor blowdown vent line. The details of this source are described 

below. 

Compressor blowdown OELs allow a compressor to be depressurized when idle, and 

typically leak when the compressor is operating or idle. 

There are two primary modes of operation leading to different emission rates for 

compressor blowdown OELs. For this report, compressor blowdown emissions refer to the 

venting of natural gas contained inside the compressor. The first operating mode is when the 

blowdown valve is closed and the compressor is pressurized, either during normal operation or 

when idle. The second operating mode is when the blowdown valve is open. This occurs when 

the compressor is idle, isolated from the compressor suction and discharge manifolds, and the 

blowdown valve is opened to depressure the compressor. (Note: Fugitive losses do not include the 

vented emissions from depressuring the compressor.) A diagram of a typical compressor 

blowdown valve arrangement is presented in Figure 1-8. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 11 

Figure 1-8. Typical Compressor Blowdown Arrangement 

(adapted from Hummel et. al, 1996) 

1.3 Review of Existing Emission Factors 

1.3.1 Transmission – Fugitive Emissions from Reciprocating and 

Centrifugal Compressors 

Most fugitive emission measurements are conducted for a specific period of time 

(relatively short in comparison to annual operations), and assumed to leak continuously, at the 

same rate. However, compressor operations are different, in that they are often cycled for 

maintenance or due to changes in load requirements. Emission factors need to account for the 

portion of time that compressors are (1) not pressurized, (2) pressurized and running, and (3) 

pressurized and idle. When compressors are depressurized, most components are assumed not to 

have fugitive emissions. The exception is compressor blowdown lines, which can emit at higher 

rates when depressurized (note: fugitive losses do not include the vented emissions from 

depressuring the compressor). 

The emission factors developed for the GRI/EPA study combined data from U.S. and 

Canadian measurements. The U.S. measurements for the depressurized blowdown fugitive 

emission factors were much higher than the pressurized blowdown factor, but this difference was 

not observed in the Canadian measurements. The GRI/EPA study found much higher emission 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 12 

rates for depressurized reciprocating compressors than for pressurized reciprocating compressors, 

as will be shown in Section 3.0 on Measurement Results and Analyses. The difference was 

attributed to compressor age, where compressors installed in the 1950’s were found to have a 

statistically higher leak rate than other installation years. The emission factors in the GRI/EPA 

study are considered the primary industry reference for transmission compressor blowdown 

emissions 

Emissions from compressor-related components were estimated separately due to 

differences in leakage characteristics for components subject to vibrational conditions. For 

example, compressor seal emission rates were determined for the following modes: (1) operating 

and pressurized, (2) idle and fully pressurized, (3) idle and partially pressurized, and (4) idle and 

depressured. 

Transmission compressor emission factors from the GRI/EPA study are 

5.55 ± 68% MMscf CH4/reciprocating compressor-yr 

= 106 tonne CH4/ reciprocating compressor-yr 

11.1 ± 44% MMscf CH4/centrifugal compressor-yr 

= 212 tonne CH4/centrifugal compressor-yr 

These emission factors are a summary per average compressor, and include individual 

sources such as seal leakage, starter line leakage, compressor PRV leakage, certain fugitive 

components on the compressors (such as cylinder valve covers and fuel valves) and blowdown 

open-ended-line leakage. The overall average component emission factors are based on the 

fraction of time associated with each operating mode for reciprocating and centrifugal compressors 

in gas transmission. The operating modes are shown in Table 1-2. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

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Table 1-2. Operating Modes of Compressors in Gas Transmission 

(GRI and EPA, 1996, Table 4-20) 

Operating Mode 

Percent of Time Associated with Operating Mode 

Reciprocating Compressor Centrifugal Compressor 

Pressurized: In Operation 45.2% 24.2% 

Pressurized: Idle 33.9% 5.8% 

Depressurized: Idle 20.9% 70.0% 

Of the total CH4 emissions from natural gas operations determined from the GRI/EPA 

study, transmission compressor fugitive emissions contributed 12.1% (± 68.1%) for reciprocating 

compressors and 2.4% (± 43.7%) for centrifugal compressors (39% of CH4 emissions from 

transmission activities). 

1.3.2 Processing– Fugitive Emissions from Reciprocating and Centrifugal 

Compressors 

Component emission factors for the GRI/EPA study for compressors in gas processing 

plants were based on the screening data noted above for transmission compressors. Adjustments 

were made for the fraction of time reciprocating and centrifugal compressors are pressurized in gas 

processing (89.7% and 43.6% for reciprocating and centrifugal compressors, respectively). In 

addition, it was found that approximately 11% of compressors in gas processing have blowdown 

valves and PRVs routed to a flare rather than vented to the atmosphere. 

Component counts for gas processing plants, including the compressors, were based on 

data from 21 sites compiled through the GRI/EPA study and three separate studies. 

Gas Processing compressor emission factors from the GRI/EPA study are: 

4.09± 74% MMscf CH4/reciprocating compressor-yr 

= 78 tonne CH4/ reciprocating compressor-yr 

7.75 ± 39% MMscf CH4/centrifugal compressor-yr 

= 148 tonne CH4/centrifugal compressor-yr 

These emission factors are a summary per average compressor, and include individual 

sources such as seal leakage, starter line leakage, compressor PRV leakage, certain fugitive 

components on the compressors (such as cylinder valve covers and fuel valves) and blowdown 

open-ended-line leakage. The overall average component emission factors are based on the 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 14 

fraction of time associated with each operating mode for reciprocating and centrifugal 

compressors in gas processing service. The operating modes are shown in Table 1-3. 

Table 1-3. Operating Modes of Compressors in Gas Processing Service 

(GRI and EPA, 1996) 

Operating Mode 

Percent of Time Associated with Operating Mode 

Reciprocating Compressor Centrifugal Compressor 

Pressurized 89.7% 43.6% 

Depressurized 10.3% 56.4% 

Of the total CH4 emissions from natural gas operations determined from the GRI/EPA 

study, gas processing compressor fugitive emissions contributed 5.32% (± 95.1%) for 

reciprocating compressors and 1.79% (± 91.4%) for centrifugal compressors (61% of CH4

emissions from gas processing activities). 

1.3.3 Compressor Seals 

This section addresses more detailed emission source information from EPA’s Natural Gas 

STAR program and from further studies conducted on compressor seals since the GRI/EPA study. 

A 2006 California Energy Commission (CEC, 2006) document reviewed the API Compendium

(American Petroleum Institute, 2004) emission factors and commented that centrifugal and 

reciprocating seals have different emission characteristics and should not be combined in one 

emission factor. Further, the CEC document noted that centrifugal compressor seals can be 

classified as either wet or dry technologies. The wet seals use oil to form a barrier to prevent 

leakage from the compressor seal. The circulating oil is stripped of gas that it absorbs at the highpressure seal face and vents the gas to the atmosphere. Therefore, wet seals have emissions both 

from fugitive leakage at the seal face as well as the vented emissions from the circulating oil. The 

CEC paper indicated that the wet seals were more commonly used when the 1996 GRI/EPA study 

was developed, which is the basis of the Compendium emission factors. 

Dry seals have much lower emissions than wet seals since the barrier fluid is high pressure 

gas, which does not involve venting from stripping oil, and the seal design is different, resulting in 

much lower seal leakage. An EPA Natural Gas STAR Lessons Learned paper on replacing wet 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 15 

seals with dry seals states that dry gas seals substantially reduce methane emissions (EPA, 2006). 

The main difference in the emissions from wet and dry seals is the vented emissions from the 

recirculation oil associated with wet seals. 

Based on guidance in the CEC paper, the original emission factors from the fugitives report 

of the 1996 GRI/EPA natural gas CH4 emissions study were reviewed. The individual seal 

emission factors from this study were never presented in the GRI/EPA study, as compressor 

equipment level emission factors were presented instead. Additionally, the Gas STAR reports 

were reviewed for compressor seal emissions information. The CEC document notes that the 

authors had discussions with individuals associated with the original GRI/EPA measurements, who 

indicated that measurements were made of the seal gas leakage for the fugitives estimate, while the 

seal oil degassing emissions were captured in the station venting measurements. The station 

venting raw data from the 1996 GRI/EPA study were reviewed to see if they include specific seal 

oil vented emissions, but unfortunately the available data were not disaggregated to that level of 

detail. 

The CEC document recommended reviewing the 1996 GRI/EPA data for separate 

reciprocating and centrifugal emission factors, as well as the Gas STAR reports. The CEC 

document also recommended reviewing a study by the Wuppertal Institute for measurements on 

the Russian Gazprom transmission system for wet seal measurement data. Unfortunately, this 

report has not been located. However, compressor seal leak data from the 1996 GRI/EPA fugitive 

report and the Natural Gas STAR reports have been reviewed and summarized in Table 1-4. 

As shown in Table 1-4, there are several potential compressor seal emission factors. The 

only emission factor found that is believed to include both the wet seal vented and fugitive 

emissions is the last entry taken from Natural Gas STAR (the other entries are probably fugitive 

emissions from the seal face). The emission factors shown in Table 1-4 were taken from a variety 

of sources and provided in different units (e.g., metric tonnes, as total gas, as CH4); but are shown 

in Table 1-4 on the same basis (scf gas/seal-hour) for comparison purposes. As observed in 

Table 1-4, there is a wide range of variability in the reported compressor seal emission factors for 

both hourly and annual emissions. 
Natural Gas Industry Methane Emission Factor Improvement Study 

Draft Final Report 16 

Table 1-4. Comparison of Compressor Seal Emission Factors 

Seal Type 

Emission Factor Default CH4

content 

(scf CH4/seal-yr) (scf gas/seal-yr) (scf gas/seal-hr) (mole %) Source 

ProductionUnspecified 2,370 3,008 0.343 78.8% 

Vol. 8, Table 4-8, 1996 GRI/EPA CH4 study 

Based on whourly basi

Unspecified* 42.2 Not given 1992 Picard study, in 

Table B-17 of the Feb. 2004 API Compendium

Converted gas density the Feb. 20

ProcessingReciprocating 450,000 517,241 65.8 87.0% Vol. 8, Table 4-12, 1996 

GRI/EPA CH4 study 

Converted of the year 

Centrifugal 228,000 262,069 68.6 87.0% 

Converted of the year 

TransmissionReciprocating 396,000 423,983 61.2 93.4% Vol. 8, Table 4-15, 1996 

GRI/EPA CH4 study 

Converted of the year 

Centrifugal 165,000 176,660 83.3 93.4% 

Converted of the year 

Recip. - rod packing - running 865,000 98.7 Not given 

Indaco study, provided in Gas STAR Lessons Learned, directed inspection at comp. stns. 

Converted 

Recip. - rod packing - idle 1,266,000 144.5 Not given Centrifugal - Dry 62,700 7.16 Not given Indaco study, provided in Gas 

STAR Lessons Learned, directed inspection at comp. stns. 

Converted 

Centrifugal - Wet 278,000 31.7 Not given Reciprocating* 34.7 Not given 1992 Picard study, in 

Table B-16 of the Feb. 2004 API Compendium

Original EFcomponent natural gas 

Centrifugal* 42.7 Not given Table 3-5 oStorageReciprocating 300,000 321,199 54.3 93.4% Vol. 8, Table 4-22, 1996 

GRI/EPA CH4 study 

Converted of the year 

Centrifugal 126,000 134,904 68.7 93.4% 

Converted of the year 
Natural Gas Industry Methane Emission Factor Improvement Study 

Draft Final Report 17 

Table 1-4. Comparison of Compressor Seal Emission Factors, continued 

Seal Type 

Emission Factor Default CH4

content 

(scf CH4/seal-yr) (scf gas/seal-yr) (scf gas/seal-hr) (mole %) Source 

Unspecified Gas IndustryCentrifugal - Dry** 

up to 3,154,000 up to 

3,376,000 

up to 385 

Not given; 

assume 

93.4% 

EPA Gas STAR Lessons Learned, replacing wet seals with dry seals for centrifugal comp. 

Converted continuous terms of CH

emissions a

Centrifugal - Dry** 

262,800 - 

1,576,800 

281,370 - 

1,688,200 

32.1 - 192.7 

Centrifugal - Wet (includes vented)

21,024,000 - 

105,120,000 

22,510,000 - 

112,548,000 

2570 - 12,850 

*Emission factors already presented in the API Compendium. 

** The EPA Gas STAR paper presents two sets of emissions data for dry centrifugal seals within the same paper.
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 18 

2.0 Measurement Campaigns 

 This section provides details on the test sites and measurements collected during this 

study. 

2.1 Selection of Test Sites 

Figures 2-1 and 2-2 show the locations of natural gas processing plants and interstate 

pipeline compressor stations operating in the lower 48 states, respectively. About one-half the 

total number of gas plants are in Texas, Louisiana, and Oklahoma, while a large fraction of the 

rest are in Wyoming, Colorado, and New Mexico. Interstate pipeline compressor stations are 

more dispersed. 

Figure 2-1. Concentrations of Natural Gas Processing Plants, 2004 

Voluntary partnerships with gas plant and pipeline operating companies were relied on 

for site access. Access to natural gas processing plants and transmission compressor stations was 

requested via emails sent on behalf of the project team by the AGA, API, GPA, and INGAA to 

their respective members. Companies choosing to participate in the study were then asked to 

provide a list of candidate test sites that were representative of their U.S. operations. Test sites 

were selected from the pool of candidates to represent a mix of comparatively older and newer 

facilities and different geographic regions and pipelines. Logistical and budgetary issues were 

considered by selecting sites with relatively large numbers of reciprocating and/or centrifugal 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 19 

compressors and comparatively easy access to emissions sources. Other practical issues such as 

distances between sites were also considered (to control travel expenses). Incentives to 

participate in this study include anonymity, and site-specific emission factors. The pool of test 

sites ideally included: 

 Reciprocating and centrifugal compressors; 

 Construction dates before and after 1996 (the year the GRI/EPA study was 

published); 

 Wet and dry centrifugal compressor seals; 

 Compressors in operating, standby, and idle modes; 

 Accessible open ended blowdown and starter lines; 

 Accessible compressor seal vents; 

 A mix of compressor manufacturers, models, sizes, and ages; 

 Different approaches to leak detection and repair; and 

 Natural Gas STAR Partners and non-partners. 

Figure 2-2. Natural Gas Pipeline Compressor Stations, 2008 

(Source: Energy Information Administration, Office of Oil & Gas, Natural Gas Division, 

Natural Gas Transportation Information System.) 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 20 

A total of three companies granted access to their compressor sampling sites and signed 

access agreements with the University of Texas. Measurements were conducted during three 

separate campaigns during 2009 – 2011. Measurements at the compressor stations included the 

following: 

 Station fugitive screening by FLIR camera (non-quantitative); 

 Fugitive measurement on identified leakers by High Volume Sampler device; and 

 Reciprocating and centrifugal compressor vent measurements by alternate methods 

(pitot tube anemometer or calibrated bag). 

Sampling at the first group of sites in East Texas occurred for a week in November, 2009. 

A second week of sampling occurred in February, 2010, at a group of sites in West Texas. A 

third week of sampling occurred in May, 2011, at a group of sites in New Mexico. The strategy 

in conducting the sampling was to collect as much data as possible at the sites, using multiple 

instruments.. Measurements included compressor related fugitive components (flanges, valves, 

OELs, PRVs), as well as blowdown vent lines and compressor seal and rod packing emissions. 

2.2 Site Descriptions 

Measurements were made at a total of eleven sites which were constructed between the 

1950s and 2000s. For comparison, the 1996 GRI/EPA compressor emission factors were based 

on measurements at 15 compressor stations. Samples were collected from 66 reciprocating 

compressors and 18 centrifugal compressors, with a total of 48 reciprocating compressors at 

transmission compressor stations. A list of the sites surveyed is provided in Table 2-1. 

Table 2-1. Description of Sites Surveyed 

Site Survey Date Description Measured Equipment Year Built 

Site 1 11/2009, 5/2011 Transmission Compressor Station 6 Reciprocating Compressors 1965 

Site 2 11/2009, 5/2011 Transmission Compressor Station 3 Centrifugal Compressors 1982 

Site 3 11/2009, 5/2011 Transmission Compressor Station 5 Reciprocating Compressors 1992-2009 

Site 4 2/2010 Transmission Compressor Station 15 Reciprocating Compressors 1948 

Site 5 2/2010 Transmission Compressor Station 5 Reciprocating Compressors 1948 

Site 6 5/2011 Gathering/Boosting compressors 13 Reciprocating Compressors 1992 

Site 7 5/2011 Gathering/Boosting compressors 6 Reciprocating Compressors 1993 

Site 8 5/2011 Gathering/Boosting compressors 6 Reciprocating Compressors 1992 

Site 9 5/2011 Gas Processing Plant 2 Centrifugal Compressors 1993 

Site 10 5/2011 Gathering/Boosting compressors 4 Centrifugal Compressors 1971 

Site 11 5/2011 Gas Processing Plant 6 Centrifugal Compressors 1950 

Each of the volunteer companies that provided sites in Table 2-1 is a Participant in the 

EPA’s Natural Gas STAR program, however, that was not a requirement for site selection. It is 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 21 

unknown whether the visited sites made any changes due to Natural Gas Star recommended best 

practices. 

2.3 Field Measurement Methods 

Measurements of the compressor-related “vent lines” was the primary focus of the field 

measurement visits, since these sources were the largest single emitters in the previous studies. 

The “vent lines” were often elevated stacks, open to the atmosphere. The compressor-related 

“vent lines” that were measured were 1) the combined rod packing vent lines on reciprocating 

compressors, 2) the wet seal vent line on centrifugal compressors with wet seals, and 3) the 

compressor open-ended line (OEL) leakage through the compressor blowdown vent line. The 

reciprocating compressor rod packing vent lines were most often combined into a single elevated 

vent for each compressor (though some smaller gathering compressors did have individual, nonelevated vents for each cylinder). The compressor Blowdown (BD) vent line was usually a 

direct line from each compressor, but at many sites the BD vent lines were joined into a common 

elevated station vent stack that also had other sources routed to the same stack. The actual 

blowdown events were not measured; instead the amount of continuous leakage through the 

blowdown line was measured. 

In addition to the compressor vents, fugitive screening and fugitive measurements of 

ordinary leaking components such as flanges, valves, OELs, PRVs were conducted throughout 

the site at five of the eleven sites. Not all eleven sites were measured for ordinary fugitives since 

the compressor vents were the main focus, and because the total site fugitive screening is an 

expensive measurement approach. 

Details of the sampling conducted at each host site are provided in Table 2-2. Columns 

in the table show which vents and components were measured. A summary of the sampling 

methods are provided in the remainder of this section. Details of the sampling procedures are 

available in the project’s Quality Assurance Project Plans, provided in as an addendum to this 

report..
Natural Gas Industry Methane Emission Factor Improvement Study 

Draft Final Report 22 

Table 2-2. Site-Specific Sources Sampled 

Site Equipment Type HP Status 

Rod Packing 

Wet Seal 

Degas 

Compressor 

OEL leak 

through BD 

Line 

Lube Oil 

Tank 

Rod 

Packing 

Combined 

Vents 

Individual Rod 

Packing Seal 

Leaks 

Site 1 

Recip 1 2,650 Idle but pressurized M 

N/A 

N/A 

M 

N/A 

Recip 2 2,650 Idle but pressurized M M Recip 3 2,650 Idle but pressurized EI M Recip 4 4,200 Idle but pressurized EI M Recip 5 8,200 

Idle, Depressurized, 

open EI M 

Debrine piping N/A N/A N/A N/A M 

Site 2 

Recip 1 2,800 Idle but pressurized 

N/A 

M,NE 

N/A 

M,NE 

N/A 

Recip 2 2,800 Idle but pressurized M,NE M,NE Recip 3 2,800 Idle but pressurized M M,NE Recip 4 2,800 Idle but pressurized M M,NE Recip 5 2,800 Idle but pressurized M M,NE Recip 6 2,800 Idle but pressurized EI M,NE 

Station BD Vent #1 - Yard Piping N/A -- 

N/A N/A 

M 

Station BD Vent #2 - Idle Line piping N/A -- NM 

Station BD Vent #3 - Yard Piping N/A -- M 

Site 3 

Centrif 1 4,800 Idle, depressurized 

N/A N/A 

M,NE M 

N/A 

Centrif 2 4,800 Running M M Centrif 3 4,680 Running M M 

M = Measured NM = Not Measured N/A = Not Applicable or Not Present EI = Inaccessible M,NE = Measured, No Emissions 
Natural Gas Industry Methane Emission Factor Improvement Study 

Draft Final Report 23 

Table 2-2. Site-Specific Sources Sampled, continued

M = Measured NM = Not Measured N/A = Not Applicable or Not Present EI = Inaccessible M,NE = Measured, No Emissions 

Site Equipment Type HP Status 

Rod Packing 

Wet Seal 

Degas 

Compressor 

OEL leak 

through BD Line 

Lube Oil 

Tank 

Rod 

Packing 

Combined 

Vents 

Individual 

Rod Packing 

Seal Leaks 

Di

Site 4 

Recip 1 1,235 Idle, depressured NM 

N/A N/A 

M 

N/A 

Recip 2 1,235 Running M M Recip 3 1,235 Idle, depressured NM M Recip 4 1,235 Running M M Recip 5 1,235 Idle, depressured NM M Recip 6 1,235 Running M M Recip 7 1,235 Idle, depressured NM M Recip 8 1,235 Running M M Recip 9 1,235 Idle, depressured NM M 

Recip 10 1,235 Idle, depressured NM M Recip 11 1,800 Idle, depressured NM M Recip 12 1,235 Idle, depressured NM M Recip 13 3,500 Idle, depressured NM M Recip 14 3,500 Idle, depressured NM M Recip 15 1,800 Idle, depressured NM M 

Site 5 

Recip 1 1,002 Running NM 

N/A N/A 

NE 

N/A 

Recip 2 1,002 Idle, depressured NM M Recip 3 1,002 Idle, depressured NM M Recip 4 1,002 Running NM NE Recip 5 1,002 Idle, depressured NM M Recip 6 2,278 Idle, depressured M NM Recip 7 2,278 Idle, depressured M NM 
Natural Gas Industry Methane Emission Factor Improvement Study 

Draft Final Report 24 

Table 2-2. Site-Specific Sources Sampled, continued

Site Equipment Type HP Status 

Rod Packing 

Wet Seal 

Degas 

Compressor OEL 

leak through BD 

Line 

Lube Oil 

Tank 

Rod Packing 

Combined 

Vents 

Individual Rod 

Packing Seal 

Leaks 

Disch

Site 6 

Recip 1 Running 

N/A 

M 

N/A 

M,NE M M

Recip 2 Running M,NE M,NE M MRecip 3 Running M M,NE M Recip 4 Running M M,NE M MRecip 5 Running M M,NE M Recip 6 Running NM M NM MRecip 7 Idle, Depressured NM M,NE NM MRecip 8 Idle, Depressured NM M,NE NM Recip 9 Running M M,NE M 

Recip 10 Idle, Depressured NM M,NE NM Recip 11 Idle, Depressured NM M,NE NM MRecip 12 Running M M,NE M M

Recip 13 Idle, Depressured NM M,NE NM 

Site 7 

Recip 1 Running 

N/A 

M 

N/A 

M,NE M,NE 

Recip 2 Running M M,NE M,NE MRecip 3 Running M M,NE M,NE MRecip 4 Idle, Depressured NM M,NE M,NE MRecip 5 Idle, Depressured NM M,NE M,NE MRecip 6 Running M M,NE M,NE M

Site 8 

Recip 1 Idle, depressured NM 

N/A N/A 

M,NE NM M

Recip 2 Running M M,NE M,NE Recip 3 Running M M,NE M,NE MRecip 4 Running M M,NE M,NE MRecip 5 Idle, Depressured NM NM NM Recip 6 Idle, Disassembled NM M,NE NM 

Recip 7 Running M M,NE M,NE M

M = Measured NM = Not Measured N/A = Not Applicable or Not Present EI = Inaccessible M,NE = Measured, No Emissions
Natural Gas Industry Methane Emission Factor Improvement Study 

Draft Final Report 25 

Table 2-2. Site-Specific Sources Sampled, continued

Site Equipment Type HP Status 

Rod Packing 

Wet Seal 

Degas 

Compressor 

OEL leak 

through BD Line 

Lube Oil 

Tank 

Rod 

Packing 

Combined 

Vents 

Individual 

Rod Packing 

Seal Leaks 

Di

Site 9 Centrif 1 Running N/A N/A M M,NE N/A 

Centrif 2 Running M M 

Site 10 

Centrif 1 Running 

N/A N/A 

M M 

N/A 

Centrif 2 Idle, Depressured NM M Centrif 3 Running M M Centrif 4 Running M M 

Site 11 

Centrif 1 Running 

N/A N/A 

M M 

N/A 

Centrif 2 Idle, Depressured NM M Centrif 3 Running M M Centrif 4 Running M M Centrif 5 Running M M Centrif 6 Running M M 

M = Measured NM = Not Measured N/A = Not Applicable or Not Present EI = Inaccessible M,NE = Measured, No Emissions 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 26 

2.3.1 Equipment Component Counts and Component Leaks 

At five stations fugitive leak sources associated with each compressor were classified 

(i.e., connector, valve, etc.) and counted. First, a FLIR infrared camera was used to locate (but 

not quantify) any potential leaking components. A trained team from the company that owned 

the FLIR was used to make those measurements. Once the FLIR camera team located a leaking 

component (usually only a few sources per station), then a quantification was performed using 

the high volume (Hi-Flow) sampler. The high volume sampler operates by pulling in a relatively 

large volume of air (up to 8 cfm), and determining the CH4 concentration in the air. Where 

possible, packing seal vents were selected randomly, disconnected from the common vent, and 

measured with the Hi-Flow. A photograph of the FLIR camera and Hi-Flow Sampler in use is 

shown in Figure 2-3. 

 Figure 2-3. FLIR Camera (right) and Hi-Flow Sampler (left)

While this approach was used early in the first few stations, it became clear that, as in the 

past GRI/EPA study, only the few very large leaks on the compressor seals and on the compressor 

blowdown OELs were important contributors to the total emissions. 

2.3.1.1 High Volume Sampler 

The high volume (Hi-Flow) sampler is portable, intrinsically safe, battery-powered 

instrument designed to determine the rate of gas leakage around various pipe fittings, valve 

packings, and compressor seals found in natural gas transmission, storage, and processing 

facilities. This instrument was used to measure leak rates for fugitive components that were 

identified as leaking by an initial IR camera evaluation of the site. A component’s leak rate is 

measured by sampling at a high flow rate so as to capture all the gas leaking from the component 

along with a certain amount of surrounding air. By accurately measuring the flow rate of the 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 27 

sampling stream and the natural gas concentration within that stream, the gas leak rate can be 

calculated using Equation 1. The instrument automatically compensates for the different specific 

gravity values of air and natural gas, thus assuring accurate flow rate calculations. 

Leak = Flow x (Gas sample – Gas background) x 10–2 Equation 1 

Where, 

Leak = rate of gas leakage from source (cfm); 

Flow = sample flow rate (cfm); 

Gas sample = concentration of gas from leak source (%); and 

Gas background = background gas concentration (%). 

2.3.2 Open-Ended Lines 

The emission rates from OELs and vents were determined by measuring the velocity 

profile across the vent line and the flow area at that point. Measurements on these lines were made 

using an enclosed rotary vane anemometer, or a hot-wire anemometer probe. For measurements 

made at a vent tip, flow velocities were measured with an encased rotary vane anemometer 

(Omega, model number HHF710). Rod packing tubing vents were measured using a low-flow 

slim probe hot wire anemometer (Omega, HHF42). The anemometer was enclosed in a fixed 

plastic pipe of approximately 3” diameter. The process vent line was sealed to the anemometer 

pipe, using a variety of simple seals such as gasket material and duct tape. The plastic pipe of a 

fixed size meant that only the anemometer readout (velocity) was needed, rather than the exact size 

of the process vent pipe. With the velocity and diameter, the volume released per unit time was 

calculated. 

In ideal cases, there was one blowdown line per vent. In a few cases, multiple compressors 

were routed to a single co-mingled vent line, with no access to individual compressor vent lines 

upstream. These latter cases made it impossible to produce average compressor measurements for 

the site, especially where compressors in different modes (idle & depressurized and operating) 

were connected to one combined vent line. Photos of sampling a blowdown vent using a hot wire 

anemometer are shown in Figure 2-4 and 2-5. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 28 

Also, in one case the top of the combined vent was inaccessible, so a hot wire anemometer 

was inserted through a port (an open 1” diameter bleed valve) to make the vent velocity 

measurement. 

When measuring flows from vents, a distinction is made between continuous fugitive leak 

emissions and the short duration discharge of de-pressured equipment, which is often called a 

blowdown. Blowdown quantities from the equipment is not the topic of this report, so those 

intermittent discharges were ignored. However, continuous leakage emissions that flow through 

the blowdown vent line when it is idle were included as fugitive emissions. 

Blowdown emissions from continuous vent systems and intermittent vent systems during 

venting events are defined as “venting” emissions. For example, blowdown valves are opened to 

depressurize the system when a compressor is being shut down. The gas that is intentionally 

discharged during the blowdown is considered vented – not fugitive – emissions, and was not 

measured as part of this study. When a blowdown valve is closed, any gas leakage through the 

valve and out the vent stack was measured as fugitive emissions. 

Additionally, some locations had lube oil tanks with separate elevated vents. If this 

configuration was present at a facility, the anemometer was used to measure emissions from the 

lube oil tank vents. Lube oil vents may include fuel/exhaust from the engine driver side of the 

compressor, unless there are separate lubrication systems for the engine side and the compression 

side of a compressor. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 29 

 

Figure 2-4. Measurement of Compressor Blowdown Vent Line Leakage 

with Encased Rotary Vane Anemometer 

Figure 2-5. Sampling a Blowdown Vent with Hot Wire Anemometer 

2.3.3 VPAC Measurements 

In addition to the vent rate measurement equipment listed above, a “through-valve acoustic 

leak detection device” was utilized at six sites (four gathering/boosting compressor stations and 

two gas processing plants). Originally designed as a valve loss control system for flare and steam 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 30 

valves, the VPAC device uses a leak quantification algorithm to quantify a leak rate of valves. A 

photo of the VPAC in use is shown in Figure 2-6. A representative of the acoustic device 

manufacturer attended the site visits and conducted the VPAC tests. Dual measurements were 

taken where possible with both the acoustic device and the direct flow measurements to compare 

readings from the two devices. The VPAC measurements were reported to this project in a singleblind fashion. That is, the VPAC results were reported without knowledge of the results of the 

other direct measurements. 

Figure 2-6. Through-Valve Acoustic Leak Detection (VPAC) 

2.3.4 Compressor Seals 

Measurements on compressor seals depended on the type of compressor. For reciprocating 

compressors, the following techniques were employed: 

 Where rod packing vent lines were piped together (multiple cylinders joined into a 

single vent line per compressor), the enclosed rotary vane anemometer was used to 

make the measurements at the top of the rod packing vent line. In those cases, one 

single measurement per compressor was made. An example is shown in 

Figure 2-7. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 31 

 Where rod packing vent lines were individually vented to the atmosphere at the 

distance piece, each distance piece vent was measured with a hand-held hot wire 

anemometer. An example of this sampling is shown in Figure 2-8. 

Figure 2-7. Sampling a Rod Packing Vent Stack with Enclosed Rotary Vane 

Anemometer 

Figure 2-8. Sampling a Rod Packing Vent with Hand-held Hot Wire 

Anemometer 

For centrifugal compressors with wet seals, measurements were made of wet seal 

degassing at the fill port to the seal oil sump. Each sump had a dedicated vent line, but in every 

case on this project, the tip of that vent line was placed above the turbine driver exhaust stack, 

preventing any safe access. The dedicated vent line had no valves and therefore could not be 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 32 

blocked/closed. For these measurements, the seal oil fill cap into the sump was opened, and it was 

assumed that this opening was now the path of least resistance, since the dedicated vent lines were 

20-50 feet in length from the sump. Calibrated plastic bags of known internal volume were used to 

measure the vent rate, by timing how long each sump took to fill the bag. A set of three repeat bag 

measurements were made for each sump. In one case, there was no access to the fill port, and 

measurements were made using the enclosed anemometer directly off the top of the sump knockout pot. This measurement technique is shown in Figure 2-9. 

Figure 2-9. Centrifugal Compressor Seal Oil Gas Measurement 

Using Calibrated Bag Technique 

On the wet seal data for centrifugal compressors, this study has found, after the fact, that 

there may be several technical issues surrounding a proper measurement from a wet seal system. 

Seal oil systems for centrifugal compressors can be complex, and may require the use of a detailed 

P&ID (piping and instrumentation diagram) to determine where all the flash locations in a system 

are. For example, as can be seen in Figure 2-10, a seal oil system may contain a flash drum/pot, 

where a separate vent line discharges any absorbed gas from the oil circulating to the seal. This is 

not the same line as the main vent line from the sump, where many companies (and this project) 

took samples. In addition, some seal oil systems may also have blowers, as shown in Figure 2-10, 

which would affect calibrated bag measurements from the sump. Many operators are not fully 

aware of the detailed workings of a particular compressor’s seal oil system because it is a selfcontained, skid mounted package that is not altered day-to-day by operations personnel. Therefore, 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 33 

this project’s measurements of wet seal degassing should be used as a benchmark, but would 

require further analysis before the measurements could be used to develop new emission factors. 
Natural Gas Industry Methane Emission Factor Improvement Study 

Draft Final Report 34 

Figure 2-10. Centrifugal Compressor Lube Oil System Process Flow Diagram (Pic
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 35 

2.4 Quality Control 

Where total site fugitives were measured, CH4 background concentrations were measured 

simultaneous with every leak test and subtracted from the main sample flow CH4 concentration. 

This is necessary to prevent background concentrations, which may be elevated by other nearby 

leaks, from influencing the leak rate determinations for individual components. The Hi-Flow 

Sampler uses two detectors simultaneously to determine the background and the main sample flow 

concentration. One detector draws air flow from the main sample hose and the second detector 

draws air from a separate background probe. The background probe was held near the leak being 

measured while the sample hose is held at the leak. 

To check if the instrument is capturing all the gas that is escaping from the leak source, two 

measurements were performed at two different flow rates. The first measurement was taken at the 

highest possible flow rate, followed by a second measurement at a flow rate that is approximately 

70–80% of the first. If the two calculated leak rates are within 10% of each other, then it was 

assumed that all gas has been captured during the test. 

Calibration checks of both the background and leak-gas detectors were performed at the 

beginning and end of every day using a certified 2.5% CH4 gas standard. The Hi-Flow Sampler 

has a built in feature to perform these checks. 

The high flow sampler was calibrated upon arrival at each test site or whenever a single 

point check of the calibration is outside the ±10% acceptability range. The calibration check was 

performed using the vendor-supplied calibration kit with zero air and gas standards of 2.5% and 

100% CH4. The leak-gas and background gas detectors were calibrated at these concentrations 

using the menu-driven programmed procedure given in the Hi-Flow Sampler operation and 

maintenance manual. 

For the other instruments, such as the anemometers, the instruments were calibrated before 

and after the tests to assure their accuracy. Readings made with these instruments were corrected 

for the actual gas composition because in most cases the instruments were calibrated on air and had 

to be corrected for measuring natural gas. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 36 

Results of this measurement project were also cross checked by independent technical 

review of a scientist and engineer not associated with this project. 

3.0 Measurement Results and Analyses 

 This section presents the results from the field measurement campaigns of this study. 

Raw measurement data is presented in Appendix B. 

3.1 Measurement Results 

Component counts were collected for five transmission compressor stations, and are 

shown as an average per station in Table 3-1. 

Table 3-1. Component Count Summary 

Component 

Average Count 

per Facility 

Standard 

Deviation 

Valves 327 112 

Flanges 415 163 

Compressor Seals 22 14 

PRVs 13 5 

OELs 16 22 

Connectors 376 209 

The overall fugitive emissions from valves, flanges, and other sources are reported as an 

average over all five sites in Table 3-2 below. As the table shows, the new factors show a lower 

leak rate from an average component when compared to the GRI/EPA data from the 1990’s. 

While this is from a relatively small sample set compared to the GRI/EPA study, these lower 

emission factors may be the result of better Leak Detection and Repair (LDAR) practices that 

have developed since the 1990s’ for ordinary fugitives in natural gas systems. 

Table 3-2. Fugitive Emissions from Valves and Flanges using Hi-Flow Sampler 

Compared to Previous Results 

Facility 

Component 

Total Number of 

Components 

Screened 

Component Emission 

Factor 

(Mscfy) 

1996 GRI/EPA 

Emission Factor 

(Mscfy) 

Valves 1,634 0.093 0.184 

Flanges 1,244 0.051 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 37 

As was found in the previous GRI/EPA study, the largest single emission sources at a 

compressor station are the compressor blowdown (BD) vent lines and the compressor seal vents. 

These remain the largest sources in the sampling for this project. Sampling results for 

compressor vents are provided in Tables 3-3, 3-4, and 3-5. It is assumed that transmission 

compression, gathering/boosting, and gas processing centrifugal compressors emission factors 

can be grouped together; whereas, transmission compression reciprocating compressors are 

grouped separately from other reciprocating compressors. 

Table 3-3. Sampling Results for Centrifugal Compressors Vents 

(Includes transmission compression, gathering/boosting, and gas processing plant 

measurements) 

Scenario Sample Size 

Average CH4

Emission Factor 

(Mscfy) 

1996 GRI/EPA 

Emission Factor1

(Mscfy) 

Average BD vent for Run 3 83 9,352 

Average BD vent for Idle + Run* 11 1,584 

Wet Seal (Run) 9 8,137 165 

*For some sites, the running and idle compressors were routed to vent with blowdown lines and PRVs, therefore individual 

compressor operating mode emission factors could not be calculated. 1

Hummel, K.E., Campbell, L.M., and M.R. Harrison (Vol. 8), Table 4-15, 1996 GRI/EPA CH4 study, adjusted for 24.2% time 

the compressor is pressurized. 

Table 3-4. Sampling Results for Reciprocating Compressors 

(Transmission Compressors) 

Scenario Sample Size 

Average 

(Mscfy) 

1996 GRI/EPA 

Emission Factor1

(Mscfy) 

Average BD vent for Idle + pressurized 10 1,957 3,683 Average BD + PRV for Run* 6 8,512 

Average BD + PRV vent for Idle + depressurized* 15 15,792 

Average Rod Packing for Idle + pressurized 5 12,236 396 

Average Rod Packing for Run 2 29,603 

*Includes PRVs. 1Vol. 8, Table 4-15, 1996 GRI/EPA CH4 study. 1

Hummel, K.E., Campbell, L.M., and M.R. Harrison (Vol. 8), Table 4-15, 1996 GRI/EPA CH4 study, adjusted for 79.1% time the 

compressor is pressurized. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 38 

Table 3-5. Sampling Results for Reciprocating Compressors 

(Gathering/Boosting Compressors) 

Scenario Sample Size 

Average 

(Mscfy) 

1996 GRI/EPA 

Emission Factor 

(Mscfy) 

Average BD vent for Run 16 28 0.2151

Average BD vent for Idle + depressurized 8 0 

Average PRV for Run 12 1,097 1.3322

Average PRV for Idle + depressurized 6 0 N/A 

Average Rod Packing for Run 15 241 9.483

1

Hummel, K.E., Campbell, L.M., and M.R. Harrison (Vol. 8), Appendix B-4, 1996 GRI/EPA CH4 study. (Gathering Compressors OEL). 

GRI/EPA treated all OELs in production identically, and did not separate out the compressor BD line OELs. 2

Hummel, K.E., Campbell, L.M., and M.R. Harrison (Vol. 8), Appendix B-4, 1996 GRI/EPA CH4 study (Gathering Compressors PRV). 3

Hummel, K.E., Campbell, L.M., and M.R. Harrison (Vol. 8), Appendix B-4, 1996 GRI/EPA CH4 study (Gathering Compressors, 

Compressor Seal). Assumes four seals per compressor. 

3.3 Comparison to Existing Data 

The new measurements made for this project on fugitive components produced lower 

emission factors than the previous GRI/EPA study, as shown in Table 3-1. This may be due to 

improved LDAR practices for accessible fugitive components that have been implemented by 

companies in the past two decades. Some of the sites visited were for companies that have 

participated in Natural Gas STAR. Programs like EPA’s Natural Gas STAR and general 

awareness of GHG emissions may have led to an actual reduction. However, it is premature to 

make a definitive finding given the limited sample set for this project. 

For compressor vents (rod packing vent and blowdown line vents), the results are mixed. 

As shown in Table 3-3, for centrifugal transmission compressors this project found the average 

blowdown line emission factors were significantly lower than the GRI/EPA study, but found wet 

seal degassing vent emissions that were much higher. Both of the new measurements are from a 

very limited sample set, smaller than the previous sample set of the GRI/EPA study. It should 

also be noted that it is difficult to compare the simple emission factors from this study with the 

GRI/EPA emission factors for an average compressor, which were already adjusted for operating 

time of the compressors. 

For the wet seal degassing vents, it is possible that this project over-measured, by 

capturing some exhaust gas blowthrough from the engine side of the shared seal oil system, or 

from systems with blowers if the blowers were not identified. Since no gas analysis was 

performed on the measured volumes, we are unable to rule out that possibility. It is also possible 

that the volumes were under-measured since the main seal oil vent line could not be blocked 

during the measurement. Additional tests and investigations are needed to make a definitive 

determination; however, the experiences from this study can help inform the design of sampling 

procedures under the GHG reporting rule. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 39 

For reciprocating transmission compressors, this project found average blowdown line 

emission factors that were significantly higher than the GRI/EPA study, and rod packing vent 

emissions that were also much higher as shown in Table 3-4. Both of the new measurements are 

from a very limited sample set, smaller than the previous sample set of the GRI/EPA study. 

While there is not enough data to draw a definitive conclusion, it is clear that the new data set 

does not indicate that reciprocating compressor vent emission factors have been reduced since 

the 1990’s. This may be valid given the age of many of the reciprocating compressors, which 

have only aged further since the tests in the 1990’s. Technology for seal packing and isolation 

valve seats has not changed significantly since the GRI/EPA report. Given the data from EPA’s 

Natural Gas STAR and the GRI/EPA study, a company could monitor rod packing emissions 

more carefully if they so elected, and thus replace rod packing more frequently. The 

measurements from this project do not seem to show any reductions from those practices, if they 

exist. 

3.4 Comparison of VPAC to Direct Measurement Results 

A comparison was made between the direct measurement readings and the VPAC method 

measurements. A plot of the data along with a linear regression of the data are shown in 

Figures 3-1 and 3-2. Ideally, it would be expected that a one-to-one relationship would be 

demonstrated between two comparable measurement methods. Driven by two high values 

(greater than 7,000 scf/day) measured by the direct measurement approach, a slope of 0.0038 

was observed for the complete set of VPAC measurements as a function of direct measurements. 

If the two direct measurement high flow data points are removed from the data set, a negative 

correlation (slope) of the two measurement methods is observed, at a slope of -0.0024. 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 40 

Figure 3-1. VPAC vs. Direct Flow Measurement Method, complete data set 

(please note the axes scales) 

y = 0.0038x + 18.563

R2 = 0.6937

0

50

100

150

200

250

0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 50,000

VPAC Values (SCFD)

Direct Flow Measurement Values (SCFD)

VPAC (SCFD) vs Direct Flow Measurement (SCFD)
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 41 

Figure 3-2. VPAC vs. Direct Flow Measurement Method, modified data set for 

lower flow 

(please note the axes scales) 

There appears to be no statistically significant correlation between the VPAC and the 

direct flow measurement methods. Therefore, it does not appear that the VPAC method should 

be considered as an accurate alternative to direct measurement for the few sources tested in this 

study at gathering/boosting compressors and gas processing plants. 

4.0 Conclusions and Recommendations 

The initial impetus of this study was to establish new emission factors that were both 

statistically superior to the GRI/EPA emission factors, and more relevant than the GRI/EPA 

factors (by including more recent samples). Some stakeholders in the process expected that the 

factors might be lower, given improvements in practices. 

The data from this report is not as robust as the GRI/EPA data, since fewer sites were 

sampled. For centrifugal transmission compressors, the measurements of fugitive components 

and blowdown line vents produced lower emission factors than the previous GRI/EPA study; 

however, centrifugal wet seal degassing vent emissions were much higher. On the wet seal data 

y = ‐0.0024x + 20.377

R2 = 0.0015

0

10

20

30

40

50

60

70

80

90

0 200 400 600 800 1,000 1,200 1,400

VPAC Values (SCFD)

Direct Flow Measurement Values (SCFD)

VPAC (SCFD) vs Direct Flow Measurement (SCFD)
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 42 

for centrifugal compressors, this study has found after the fact, that there may be several 

technical issues surrounding a proper measurement from a wet seal system; therefore, the 

emission factors from this report for wet seal degassing should be used only as an evaluative 

measurement. A useful conclusion from this study may be that future measurement campaigns 

need to understand the detailed design of each unique sampled compressor seal oil system, 

understand all the possible degassing locations, and only then determine where to sample the 

system. Since wet seal gas systems require a more detailed evaluation than was previously 

anticipated, installation of measurement ports may require a careful review of the P&ID 

drawings for the seal oil system, finding flash points such as the flash drum pot for measurement. 

If blowers are used, a sample from the seal oil sump vent may not be representative of the total 

generation of all flash gas. 

For reciprocating transmission compressors, this project found average blowdown line 

and rod packing vent emission factors that were significantly higher than the GRI/EPA study. 

While there is not enough data to draw a definitive conclusion, it is clear that the new data set 

does not indicate that reciprocating compressor vent emission factors have been reduced since 

the 1990’s. These higher results may be due to continued aging of the compressors since the 

1996 GRI/EPA study. 

There appears to be no statistically significant correlation between the VPAC 

measurements and the direct flow measurement methods. Therefore, it does not appear that the 

VPAC method should be considered an accurate alternative to direct measurement for the few 

sources tested in this study at gathering/boosting compressors and gas processing plants. 

Some of the insight from this report might be useful for future measurement campaigns 

or compressor operators. For example: 

 More explicit specifications for location of the wet seal degassing vent measurement 

(including possible protocols for that measurement); 

 Use of calibrated bag measurements for wet seal degassing since the oil mist may 

adversely affect other measurement instruments; 

 Reconsideration of alternate methods to the acoustic through-valve leak device for 

this service, since the acoustic through-valve device appears non-correlated to direct 

measurements; 

 More explicit specifications for location of ports for individual compressor BD line 

leakage measurements; and 

 Protocols for certain measurement techniques, such as use of inserted anemometers 

and encased anemometers.

 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 43 

Appendix A – Summary of Document Review 

Doc. # Document or Reference Reviewed Review Comments 

1 American Gas Association. Gas Facts, 2007, A Statistical Record of the 

Gas Industry, 2006 Data. 01 March 2008. 

Provides activity data: miles of 

plastic pipeline mains 

2 American Gas Association. Greenhouse Gas Emission Estimation 

Methodologies, Procedures, and Guidelines for the Natural Gas 

Distribution Sector, Draft, Prepared by innovative environmental 

solutions, inc., April 4, 2008. 

All emission factors are derived 

from the GRI/EPA study 

3 American Petroleum Institute. Fugitive Hydrocarbon Emissions for Oil 

and Gas Production Operations, API Publication Number 4589, 

Prepared by Star Environmental. December 1993. 

Provides emission factors, but 

not for the sources of interest 

4 Australian Government Department of Climate Change. Australian 

Methodology for the Estimation of Greenhouse Gas Emissions and 

Sinks 2006 Energy (Fugitive Fuel Emissions), National Greenhouse Gas 

Inventory Committee, Australia, December 2007. 

http://www.greenhouse.gov.au/inventory/methodology/index.html

References E&P forum for a 

fugitive emission factor based on 

volume flared. 

5 Australian Government Department of Climate Change. Australian 

Methodology for the Estimation of Greenhouse Gas Emissions and 

Sinks 2006 Energy (Stationary Sources), National Greenhouse Gas 

Inventory Committee, December 2007. 

http://www.greenhouse.gov.au/inventory/methodology/index.html

Does not address emission 

sources of interest 

6 Australian Government Department of Climate Change. National 

Greenhouse Accounts (NGA) Factors, Canberra ACT, January 2008. 

Does not address emission 

sources of interest 

7 California Air Resources Board. Documentation of California’s 

Greenhouse Gas Inventory (website), April 10, 2008, 

http://www.arb.ca.gov/cc/inventory/doc/doc_index.php

New data sources reference Gas 

STAR 

8 California Climate Action Registry. Natural Gas Transmission and 

Distribution Greenhouse Gas Reporting Protocol, Draft, February 2008. 

Does not address emission 

sources of interest 

9 California Energy Commission, Evaluation of Oil and Gas Sector 

Greenhouse Gas Emissions Estimation and Reporting, Final Draft 

Report FR-05-100, April 2006. 

Provides plastic pipeline 

emission factor comparison 

derived from GRI/EPA study 

data 

10 California Energy Commission, Research Roadmap for Greenhouse 

Gas Inventory Methods, Prepared by University of California, Berkeley, 

Lawrence Berkeley National Laboratory, California Energy 

Commission, Consultant Report CEC-500-2005-097, July 2005. 

http://www.energy.ca.gov/pier/final_project_reports/CEC-500-2005-

097.html 

Provides emission factors for 

production wells, compressor 

station, plastic pipeline, and 

distribution services. Cite EIIP 

and ICF, which are believed to 

be derived from GRI/EPA study 

11 Canadian Association of Petroleum Producers. A National Inventory of 

Greenhouse Gas (GHG), Criteria Air Contaminant (CAC) and 

Hydrogen Sulphide (H2S) Emissions by the Upstream Oil and Gas 

Industry, Vols. 1-5, Prepared by Clearstone Engineering Ltd., Calgary, 

Alberta, Canada, September 2004. 

http://www.capp.ca/default.asp?V_DOC_ID=763&SubjectID=414802

Does not address emission 

sources of interest 

12 Delucchi, Mark A. A Lifecycle Emissions Model (LEM): Lifecycle 

Emissions from Transportation Fuels, Motor Vehicles, Transportation 

Modes, Electricity Use, Heating and Cooking Fuels, and Materials, 

Appendix E: Methane Emissions from Natural Gas Production, Oil 

Production, Coal Mining and Other Sources, Institute of Transportation 

Studies, University of California Davis, UCD-ITS-RR-03-17E, 

December 2003. http://pubs.its.ucdavis.edu/author_detail.php?id=5

Does not address emission 

sources of interest 

13 Environment Canada Greenhouse Gas Division. Inventory Report: 

Greenhouse Gas Sources and Sinks in Canada 1990-2005, April 2007. 

Does not address emission 

sources of interest 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 44 

Appendix A –Summary of Document Review, continued 

Doc. # Document or Reference Reviewed Review Comments 

14 European Environment Agency. Annual European Community 

Greenhouse Gas Inventory 1990-2005 and Inventory Report 2007, 

Technical Report No 7/2007, 27 May 2007. 

http://reports.eea.europa.eu/technical_report_2007_7/en 

Does not address emission 

sources of interest 

15 Fernandez, Roger, Robin Petrusak, Donald Robinson, and Duane 

Zavadil. Cost Effective Methane Emission Reductions for Small and 

Midsize Natural Gas Producers, Journal of Petroleum Technology, June 

2005. http://www.icfi.com/Markets/Environment/doc_files/methaneemissions.pdf

Does not provide emission 

factors or specific data of 

interest to the study 

16 Intergovernmental Panel on Climate Change. 2006 IPCC Guidelines 

for National Greenhouse Gas Inventories, Volume 2, Chapter 4: 

Fugitive Emissions, Prepared by the National Greenhouse Gas 

Inventories Programme, Eggleston H.S., Buendia L., Miwa K., Ngara T. 

and Tanabe K. (eds), IGES, Japan, 2006. 

References the API 

Compendium and provides high 

level national emission factors. 

Provide combined venting and 

flaring emission factors 

associated with well testing and 

well servicing activities in 

developed and developing 

countries.

17 Intergovernmental Panel on Climate Change, National Greenhouse Gas 

Inventories Programme. IPCC Emission Factor Database (EFDB) 

(website), April 10, 2008. http://www.ipccnggip.iges.or.jp/EFDB/find_ef_main.php

Does not address emission 

sources of interest 

18 International Association of Oil and Gas Producers. Environmental 

Performance in the E&P Industry 2006 Data, Report No. 399, October 

2007. http://www.ogp.org.uk/pubs/399.pdf 

Does not provide emission 

factors or specific data of 

interest to the study 

19 International Association of Oil and Gas Producers. Flaring & Venting 

in the Oil & Gas Exploration & Production Industry: An Overview of 

Purpose, Quantities, Issues, Practices and Trends, Report No. 2.79/288, 

January 2000. http://www.ogp.org.uk/pubs/288.pdf 

Does not provide emission 

factors or specific data of 

interest to the study 

20 International Petroleum Industry Environmental Conservation 

Association, International Association of Oil and Gas Producers and 

American Petroleum Institute. Petroleum Industry Guidelines for 

Reporting Greenhouse Gas Emissions, Prepared by Battelle, December 

2003. 

Does not provide emission 

factors or specific data of 

interest to the study 

21 Interstate Natural Gas Association of America. Activities to Reduce 

Greenhouse Gas Emissions from Natural Gas, Prepared by Energy 

Resources International Inc., Publication F-2000-05, Washington, D.C. 

20036, 2000. http://www.ingaa.org/cms/31/3243/3247/3229/277.aspx 

Does not address emission 

sources of interest 

22 Interstate Natural Gas Association of America. Greenhouse Gas 

Emission Estimation Guidelines for Natural Gas Transmission and 

Storage, Volume 1: GHG Emission Estimation Methodologies and 

Procedures, Prepared by Innovative Environmental Solutions, Inc., 

Washington, D.C. 20002, Revision 2, September 28, 2005. 

Provides emission factors for 

specific types of pneumatic 

devices: actuators or controllers, 

continuous bleed pneumatics, 

pneumatic/ hydraulic valve 

operator, and turbine valve 

operator. Data derived from 

GRI/EPA study 

23 Interstate Natural Gas Association of America, American Petroleum 

Institute, and American Gas Association. INGAA/API/AGA Natural 

Gas Systems GHG Emission Factor Comparison & Improvement 

Collaborative Project – Status Report: Task 2 - Emission Factor 

Comparison, Draft Technical Report, 27 November 2005. 

References data derived from the 

GRI/EPA study. Identifies 

emission estimate approaches 

that can most benefit from 

improved methodology and data 

gaps for natural gas systems 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 45 

Appendix A –Summary of Document Review, continued 

Doc. # Document or Reference Reviewed Review Comments 

24 Methane to Markets. Methane to Markets Documents, Tools and 

Resources, April 10, 2008. http://www.methanetomarkets.org/index.htm

Does not provide emission 

factors or specific data of 

interest to the study 

25 Robinson, D.R., R. Fernandez, and R.K. Kantamaneni. Methane 

Emissions Mitigation Options in the Global Oil and Natural Gas 

Industries. http://www.epa.gov/gasstar/pdf/ng020.pdf 

Does not provide emission 

factors or specific data of 

interest to the study 

26 United Nations Economic Commission for Europe Convention on 

Long-range Transboundary Air Pollution Co-operative Programme for 

Monitoring and Evaluation of the Long-range transmission of Air 

Pollutants in Europe. EMEP/CORINAIR Emission Inventory Guidebook 

- 2007, Technical Report No 16/2007, December 2007. 

http://reports.eea.europa.eu/EMEPCORINAIR5/en/page002.html 

Provides fugitive component 

emission factors for oil and gas 

facilities based on a 1992 

Canadian study 

27 U.S. Department of Energy, Energy Information Administration Office 

of Integrated Analysis and Forecasting. Documentation for Emissions of 

Greenhouse Gases in the United States 2005, DOE/EIA-0638 (2005), 

Washington, D.C. 20585, October 2007. 

Does not provide emission 

factors or specific data of 

interest to the study 

28 U.S. Department of Energy, Energy Information Administration. 

Monthly Energy Review, Washington, D.C., various years. 

http://www.eia.doe.gov/overview_hd.html

Does not provide emission 

factors. May provide 

information of interest to activity 

data 

29 U.S. Department of Energy, Energy Information Administration. 

Natural Gas Annual, Washington, D.C., various years. 

http://tonto.eia.doe.gov/dnav/ng/ng_pub_publist.asp

Does not provide emission 

factors. May provide 

information of interest to activity 

data 

30 U.S. Department of Energy, Energy Information Administration. 

Natural Gas Monthly, Washington, D.C., various years. 

http://tonto.eia.doe.gov/dnav/ng/ng_pub_publist.asp

Does not provide emission 

factors. May provide 

information of interest to activity 

data 

31 U.S. Department of Energy, Energy Information Administration Office 

of Policy and International Affairs. Technical Guidelines Voluntary 

Reporting of Greenhouse Gases (1605(b)) Program, Office of Policy 

and Internal Affairs, United States Department of Energy, January 

2007. 

References emission factors 

derived from the GRI/EPA study 

32 U.S. Department of the Interior, Minerals Management Service Gulf of 

Mexico OCS Region. Year 2005 Gulfwide Emission Inventory Study, 

OCS Study MMS 2007-067, Prepared by Eastern Research Group, Inc., 

New Orleans, December 2007. 

http://www.gomr.mms.gov/PI/PDFImages/ESPIS/4/4276.pdf

Report only provides summary 

data. Access to the original 

database could be valuable 

33 U.S. Department of Transportation, Office of Pipeline Safety. Pipeline 

Statistics, April 15, 2008. http://ops.dot.gov/stats/stats.htm 

Does not provide emission 

factors. May provide 

information of interest to activity 

data 

34 U.S. Environmental Protection Agency. Inventory of U.S. Greenhouse 

Gas Emissions and Sinks: 1990 - 2005, Washington, D.C. 20460, 

April 15, 2007. 

http://www.epa.gov/climatechange/emissions/downloads06/07CR.pdf

Does not provide emission 

factors or specific data of 

interest to the study 

35 U.S. Environmental Protection Agency. Inventory of U.S. Greenhouse 

Gas Emissions and Sinks: 1990 - 2006, Public Review Draft, 

Washington, D.C. 20460, February 22, 2008. 

http://www.epa.gov/climatechange/emissions/usinventoryreport.html 

Does not provide emission 

factors or specific data of 

interest to the study 

 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 46 

Appendix A –Summary of Document Review, continued 

Doc. # Document or Reference Reviewed Review Comments 

36 U.S. Environmental Protection Agency. Natural Gas STAR 

Documents, Tools and Resources. http://www.epa.gov/gasstar/

Some specific information is 

presented below 

37 CAPP. CH4 and VOC Emission from the Canadian Upstream Oil and 

Gas Industry, Volume 1. Organic and Common-Pollutant Emissions by 

the Canadian Upstream Oil and Gas Industry. 

Provides Canadian emission 

factors for well servicing 

operations (by 

Province/Territory), counts of 

gas wells (high pressure sour, 

high pressure sweet, and low 

pressure sour), fugitive 

component counts for production 

wells (high pressure and shallow 

gas wells), and fugitive 

component counts and emissions 

for reciprocating versus 

centrifugal compressors. 

Reports are from the mid-90s. 

Information is used or cited in 

the API Compendium.

38 CAPP. A Detailed Inventory of CH4 and VOC Emission from the 

Canadian Upstream Oil and Gas Industry, Volume 2. Development of 

the Inventory. 

Canadian Petroleum Association (CPA). A Detailed Inventory of 

CH4 and VOC Emissions from Upstream Oil and Gas Operations 

in Alberta, Volumes I-III, March 1992. 

39 CPA. A Detailed Inventory of CH4 and VOC Emissions From 

Upstream Oil and Gas Operations in Alberta. Volume 1. Overview of 

the Emission Data. 

Canadian Petroleum Association (CPA). A Detailed Inventory of 

CH4 and VOC Emissions from Upstream Oil and Gas Operations 

in Alberta, Volumes I-III, March 1992. 

In addition to information noted 

above for CAPP study, these 

reports provide fugitive 

compressor seal emission 

factors. Reports are from the 

mid-90s. Information is used or 

cited in the API Compendium. 

40 CPA. A Detailed Inventory of CH4 and VOC Emissions From 

Upstream Oil and Gas Operations in Alberta. Volume 2. Development 

of the Inventory. 

Canadian Petroleum Association (CPA). A Detailed Inventory of 

CH4 and VOC Emissions from Upstream Oil and Gas Operations 

in Alberta, Volumes I-III, March 1992. 

41 CPA. A Detailed Inventory of CH4 and VOC Emissions From 

Upstream Oil and Gas Operations in Alberta. Volume 3. Results of the 

Field Validation Program. 

Canadian Petroleum Association (CPA). A Detailed Inventory of 

CH4 and VOC Emissions from Upstream Oil and Gas Operations 

in Alberta, Volumes I-III, March 1992. 

 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 47 

Appendix B – Data Summary 

(A full compilation of the data collected are available in a spreadsheet addendum to this report) 

Table B-1. Fugitive Emissions from Valves and Flanges using Hi-Flow Sampler 

Hundreds of components were screened using the FLIR camera, most components showed no leaks. The few 

components that did indicate a leak with the FLIR were measured using the Hi-Flow device and are presented in this 

table. 

Site Leaking 

Component Mscf/yr 

Site 1 

Valve 1 83.52 

Flange 1 77.95 

Flange 2 11.14 

Flange 3 53.82 

Flange 4 7.42 

Flange 5 22.27 

Site 2 

Valve 1 53.82 

Valve 2 40.83 

Flange 1 111.35 

Flange 2 118.78 

Flange 3 87.23 

Flange 4 87.23 

Site 3 

Valve 1 1,033.73 

Valve 2 11.14 

Valve 3 139.19 

Site 4 

Valve 1 14.72 

Valve 2 331.65 

Valve 3 208.66 

Flange 1 161.88 

Flange 2 38.37 

Flange 3 157.15 

Flange 4 45.73 

Flange 5 19.97 

Flange 6 7.36 

Site 5 

Valve 1 62.02 

Valve 2 176.60 

Valve 3 259.65 

Flange 1 29.43 

Flange 2 129.82 

 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 48 

Table B-2. Centrifugal Compressor Vents 

(Transmission, Gathering/Boosting, and Gas Processing Plant) 

Site Equipment 

Name Run Mode 

Wet Seal Degassing Blowdown Line Leaks 

SCFD 

VPAC 

Comparison 

SCFD 

SCFD 

VPAC 

Comparison 

SCFD 

Site 3 

Centrif 1 Idle, depressurized 0.00 - 129,756.40 - 

Centrif 2 Running 37,028.57 - 0.00 - 

Centrif 3 Running 51,840.00 - 0.00 - 

Site 9 Centrif 1 Running 3,021.06 - 0.00 50.28 

Centrif 2 Running 3,756.20 - 195.49 22.24 

Site 10 

Centrif 1 Running 18,103.71 - - - 

Centrif 2 Idle, depressured 0.00 - - - 

Centrif 3 Running 60,413.51 - - - 

Centrif 4 Running 0.00 - - - 

Blow Down #1 - 

Combined (4 

compressors + 2 

stations + 2 

PRVs) 

n/a - - 44,745.54 193.54 

Blow Down #2 - 

Emergency 

Shutoff Valve n/a 

- - 7,890.81 17.57 

Site 11 

Centrif 1 Running 12,713.28 - 

747.88 

5.78 

Centrif 2 Idle, depressured 0.00 - 84.28 

Centrif 3 Running 14,645.70 - 6.04 

Centrif 4 Running 35,698.89 - 6.72 

Centrif 5 Running 22,782.20 - 339.95 

5.36 

Centrif 6 Running 29,494.81 - 5.52 

 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 49 

Table B-3. Reciprocating Compressor Vents 

(Transmission Compressors) 

Site Equipment Name Run Mode 

Blowdown Line 

Leaks Rod Packing Vent 

SCFD SCFD 

Site 1 

Recip 1 Idle, pressurized 13,404.59 47,934.80 

Recip 2 Idle, pressurized 13,404.59 57,157.16 

Recip 3 Idle, pressurized 13,404.59 - 

Recip 4 Idle, pressurized 13,404.59 - 

Recip 5 Idle, depressurized, open 4,825.65 - 

Site 2 

Recip 1 Idle, pressurized 0.00 0.00 

Recip 2 Idle, pressurized 0.00 0.00 

Recip 3 Idle, pressurized 0.00 18,766.42 

Recip 4 Idle, pressurized 0.00 35,709.82 

Recip 5 Idle, pressurized 0.00 8,042.75 

Recip 6 Idle, pressurized 0.00 - 

Station BD Vent #1 - 

Yard Piping n/a 10,187.49 - 

Station BD Vent #2 - 

Idle Line piping n/a - - 

Station BD Vent #3 - 

Yard Piping n/a 9,115.12 - 

Site 4 

Recip 1 Idle, depressured 29,021.41 - 

Recip 2 Running 27,984.93 - 

Recip 3 Idle, depressured 35,758.52 11,401.27 

Recip 4 Running 26,948.45 - 

Recip 5 Idle, depressured 67,371.13 10,364.79 

Recip 6 Running 32,130.84 - 

Recip 7 Idle, depressured 77,735.91 8,291.83 

Recip 8 Running 52,860.42 - 

Recip 9 Idle, depressured 150,289.43 26,948.45 

Recip 10 Idle, depressured 15,547.18 

Recip 11 Idle, depressured 33,167.32 - 

Recip 12 Idle, depressured 18,656.62 - 

Recip 13 Idle, depressured 34,203.80 - 

Recip 14 Idle, depressured 25,911.97 - 

Recip 15 Idle, depressured 21,766.06 - 

 
Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

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Table B-3. Reciprocating Compressor Vents 

(Transmission Compressors), continued 

Site Equipment 

Name Run Mode

Blowdown Line 

Leaks

Rod Packing 

Vent

SCFD SCFD

Site 5 

Recip 1 Running 0.00 NM 

Recip 2 Idle, depressured 17,620.14 NM 

Recip 3 Idle, depressured 9,328.31 NM 

Recip 4 Running 0.00 NM 

Recip 5 Idle, depressured 107,793.80 NM 

Recip 6 Idle, depressured NM 55,969.86 

Recip 7 Idle, depressured NM 106,239.08 

NM = Not Measured
Natural Gas Industry Methane Emission Factor Improvement Study 

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Table B-4. Reciprocating Compressor Vents (Gathering/Boosting Compressors) 

Site Equipment Name Run Mode 

Rod Packing 

Seal Leaks Blowdown Line Leaks Discha

SCFD SCFD 

VPAC 

Measurement 

SCFD S

Site 6 

Recip 1 Running 387.03 0.00 7.28 

Recip 2 Running 0.00 0.00 7.46 0Recip 3 Running 91.15 0.00 7.29 0Recip 4 Running 154.25 0.00 7.02 0Recip 5 Running 3,187.36 0.00 7.84 17,Recip 6 Running 0.00 1,216.83 7.61 0Recip 7 Idle, depressured 0.00 0.00 40.17 0Recip 8 Idle, depressured 0.00 0.00 40.17 0Recip 9 Running 882.03 0.00 40.17 14,

Recip 10 Idle, depressured 0.00 0.00 40.17 Recip 11 Idle, depressured 0.00 0.00 40.17 0Recip 12 Running 112.18 0.00 7.33 0Recip 13 Idle, depressured 0.00 0.00 40.17 

Site 7 

Recip 1 Running 15.44 0.00 9.02 3,9

Recip 2 Running 262.40 0.00 9.10 0Recip 3 Running 370.45 0.00 9.27 0Recip 4 Idle, depressured 0.00 0.00 - 0Recip 5 Idle, depressured 0.00 0.00 - 0Recip 6 Running 308.71 0.00 9.03 0

Site 8 

Recip 1 Idle, depressured 0.00 0.00 38.02 0

Recip 2 Running 78.33 0.00 5.29 Recip 3 Running 1,345.65 0.00 8.13 0Recip 4 Running 170.65 0.00 8.18 0Recip 5 Running 2,544.43 0.00 7.99 0
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Natural Gas Industry Methane Emission Factor Improvement Study December 2011 

Draft Final Report 53 

Appendix C – References 

American Petroleum Institute (API). Compendium of Greenhouse Gas Emissions Estimation 

Methodologies for the Oil and Gas Industry, API, Washington, DC, 2004. Errata, February, 

2005. http://www.api.org/ehs/climate/new/upload/2004_COMPENDIUM.pdf 

http://ghg.api.org/documents/CompendiumErrata205.pdf 

California Energy Commission (CEC). Evaluation of Oil and Gas Sector Greenhouse Gas 

Emissions Estimation and Reporting, California Energy Commission, Consultant Report, Final 

Draft, April 14, 2006. 

GRI and EPA. Harrison, M.R., Shires, T.M., Wessels, J.K., Cowgill, R. M. Methane Emissions 

from the Natural Gas Industry, Volumes 1 – 15, Final Report, GRI-94/0257 and EPA-600/R-96-

080, Gas Research Institute and US Environmental Protection Agency, June 1996. 

Hummel, K.E., Campbell, L.M., and M.R. Harrison. Methane Emissions from the Natural Gas 

Industry, Volume 8: Equipment Leaks, Final Report, GRI-94/0257.25 and EPA-600/R-96-080h, 

Gas Research Institute and US Environmental Protection Agency, June 1996. (Part of the 

GRI/EPA methane emissions inventory project.) www.gastechnology.org

Picard, Dave. Centrifugal Wet Seal Schematic. Clearstone Engineering, LTD. E-mail to the 

authors. 15 August 2011. 

Rolls Royce. Centrifugal compressors: Pipeline and barrel compressors for the energy industry. 

2010. http://www.rolls-royce.com/Images/EC201(rev)_tcm92-22934.pdf 

U.S. Environmental Protection Agency (EPA). Lessons Learned from Natural Gas STAR 

Partners. Replacing Wet Seals with Dry Seals in Centrifugal Compressors, October 2006. 

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

U.S. Environmental Protection Agency (EPA). Lessons Learned from Natural Gas STAR 

Partners. Reducing Methane Emissions from Compressor Rod Packing Systems, October 2006.

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

U.S. Environmental Protection Agency (EPA). Inventory of U.S. Greenhouse Gas Emissions 

and Sinks: 1990-2009. April, 2011.