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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460 

OFFICE OF 

 AIR AND RADIATION 

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

Docket: EPA-HQ-OAR-2009-0923 

Title: Revisions to Processing Leaker Emission Factors in Rule Table W-2 

Summary: This memo discusses the methodology used to develop the emission factors from 

emission surveys conducted by Clearstone Engineering Ltd. for the U.S. EPA, (Clearstone I and 

Clearstone II) and for the Canadian Energy Partnership for Environmental Innovation (CEPEI)

Based on sample sizes of component leak data EPA has determined that combining the datasets 

provides more robust LEFs in today’s final rule than the April 2010 proposed rule. 

 
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Revisions to Processing Leaker Emission Factors in Rule Table W-2 

In today’s final rule, EPA has provided total hydrocarbon leaker emission factor (LEF) for 

compressor related and non-compressor related components for onshore natural gas processing, 

natural gas transmission compression, and underground natural gas storage. This memo 

discusses the methodology used to develop the emission factors from emission surveys 

conducted by Clearstone Engineering Ltd. for the U.S. EPA, (Clearstone I1 and Clearstone II2) 

and for the Canadian Energy Partnership for Environmental Innovation (CEPEI34) Based on 

sample sizes of component leak data EPA has determined that combining the datasets provides 

more robust LEFs in today’s final rule than the April 2010 proposed rule. These factors suitably 

characterized each applicable industry segment. 

Background 

In considering the option of requiring leak detection and leaker emission factors for quantifying 

equipment leaks in the onshore natural gas processing, onshore natural gas transmission, and 

underground natural gas storage segments, EPA originally investigated the other available data 

sources for leaker emission factors. The first source was the SOCMI leaker factors used in 

conjunction with the VOC Leak Detection and Repair (LDAR) program under the Clean Air Act 

40 CFR Part 60 subpart KKK (see Appendix A) for processing plants. These factors were 

developed from data in petroleum refineries and chemical plants, and are used for state criteria 

pollutant inventories of petroleum refinery and chemical plants as well as the VOC sections of 

processing plants (fractionation section). These SOCMI factors were not developed from data 

sources within the non-VOC, high methane content, sections of processing plants (feed and 

residue gas compression and conditioning) or the gas transmission and gas storage segments of 

the natural gas industry. 

Secondly, EPA considered the emission factors in the API Compendium5

. These factors are 

population factors, and the actual leak detection and measurement data was not publicly 

available for EPA to reconstitute into leaker factors, so it was not suitable for subpart W 

requirement of leak detection and leaker factors as an alternative to leak measurement. 

Clearstone Engineering Ltd. provided all of the field emissions data collected in 1998 and 2004 

from leaking components in the processing sector. It measured hydrocarbon emissions from 

components found in nine processing plants and categorized them by site, process unit, 

component type, and stream type. In 1998, four processing plants were surveyed for the 

 

1

 Clearstone Engineering Ltd. Identification and Evaluation of Opportunities to Reduce Methane Losses at Four Gas 

Plants. Calgary, Alberta: 2002. 2

 Clearstone Engineering LTD. Cost-Effective Directed Inspection and Maintenance Control Opportunities at Five 

Gas Processing Plants and Upstream Gathering Compressor Stations and Well Sites. (Draft): 2006 

3

 Clearstone Engineering Ltd. Identification and Evaluation of Opportunities to Reduce Methane Losses at Four Gas 

Plants. Calgary, Alberta: 2002. 

4

 Clearstone Engineering Ltd. Measurement of Natural Gas Emissions from the Canadian Natural Gas Transmission 

and Distribution Industry, (for CEPEI). 2007. 

5

 API. (2009) Compendium of Greenhouse Gas Emission Methodologies for the Oil and Natural Gas Industry. pg. 

6-5 <http://www.api.org/ehs/climate/new/upload/2009_GHG_COMPENDIUM.pdf> 
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Clearstone I report which was published in 2002. The resulting data is referred to as Clearstone I 

data. In 2004, an additional five processing plants were surveyed, including one processing plant 

surveyed in 1998 was resurveyed for the Clearstone II study, draft report submitted to EPA in 

2005. The data from the five processing facilities surveyed for the Clearstone II study is referred 

to as Clearstone II data. 

The CEPEI data consists of emission surveys conducted by Clearstone Engineering Ltd. in 1998 

and 2007 and leak measurement surveys submitted by individual private sector companies 

between those years. The surveys examined numerous natural gas transmission, storage and 

distribution facilities in Canada. The results of these surveys were used to write two separate 

reports for CEPEI: Handbook for Estimating Methane Emissions from Canadian Natural Gas 

Systems6

 (referred to as CEPEI 1998) and Measurement of Natural Gas Emissions from the 

Canadian Natural Gas Transmission and Distribution Industry7 (referred to as CEPEI 2007). 

This data will henceforth be referred to as CEPEI. 

EPA investigated the raw data from these studies to develop LEFs for 40 CFR 98, subpart W. 

The component definitions in the final reports were not the same as those covered under subpart 

W, so EPA first filtered the raw data to only combine that which is consistent with subpart W’s 

source listing (e.g. compressor related valves, non-compressor related valves, etc). After 

eliminating inapplicable leak data, it investigated the remaining sample sizes and evaluated the 

potential to combine data sets to develop more robust LEFs. 

Methodology for the Developing Emission Factors from Clearstone I and II Data (Onshore 

Natural Gas Processing) 

The LEFs provided by Clearstone were filtered and reconstructed to investigate potential 

configurations of that data for subpart W. First, the Clearstone I and II data was re-categorized 

by converting Clearstone’s component categories to those consistent with the EPA/GRI study 

and the API Compendium, being considered for inclusion in the current draft proposed Subpart 

W. The Clearstone component categories were relabeled to fit: valves, open-ended lines, 

connectors, pressure-relief valves, and meters. 

Next, the Clearstone I and II data was separated into components which occur before and after 

the de-methanizer (or dew point control) unit. This was done using the stream types provided by 

Clearstone. For example, components labeled PG (Process Gas) and SR/PG (Sour Process Gas) 

were considered to occur before the de-methanizer while components labeled FG (Fuel Gas) and 

Residue Gas were considered to occur after the de-methanizer. Stream types such as AIR, C, 

 

6

 Clearstone Engineering Ltd. Handbook for Estimating Methane Emissions from Canadian Natural Gas Systems,

(for CEPEI). 1998 

7

 Clearstone Engineering Ltd. Measurement of Natural Gas Emissions from the Canadian Natural Gas Transmission 

and Distribution Industry, (for CEPEI). 2007. 
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and HC Liq were excluded from the data set because they are not methane related. Also, the 

bottoms streams from the de-methanizer, and all other fractionation unit streams (such as C2, 

C2+, and C3) were excluded because they contain negligible methane content and are regulated 

under the VOC LDAR program, 40 CFR Part 60 subpart KKK (i.e. not representative of 

unregulated methane emissions). 

The Clearstone I and II data were further categorized into compressor and non-compressor 

related components. Several equipment leak studies, including Clearstone studies, demonstrate 

that the mechanical (vibration) and thermal stresses created by compressors put additional stress 

on connected components making them more susceptible to leaking. This was done by filtering 

those components with process unit descriptions referring to compressors. For example, if the 

process unit description contained “compressor” or a comparable label the data point would be 

categorized as compressor, otherwise it would be categorized as non-compressor. 

Compressor components were further separated into reciprocating and centrifugal compressor 

related components. Reciprocating compressors cause higher amplitude, lower frequency 

vibrations than centrifugal compressors. This was done by filtering those data points categorized 

as compressors and searching for “turbine” in the process unit description. If the process 

description contained “turbine”, the component was labeled centrifugal, otherwise it was labeled 

reciprocating. 

Finally, hydrocarbon LEFs for the processing segment were calculated by dividing the sum of 

the leakage rates by the sum of the component counts in each source category. The above 

methodology is displayed as a flowchart in Figure 1. The same methodology described above 

was applied to both the Clearstone I and II data. 

Methodology for Developing Emission Factors from CEPEI Data 

The collective data is described as representative samples of block valve stations, booster 

stations, border meter stations, compressor stations, control valve stations, transmission farm 

taps, receipt meter stations, sales meter stations, storage stations, and storage wells for the 

transmission/storage sector; and commercial meter sets, gate stations, distribution farm taps, 

district regulating stations, industrial meter sets, residential meter sets, and miscellaneous 

pipeline equipment in the distribution sector. EPA isolated the data for compressor stations, 

storage stations, and storage wells to develop LEFs for transmission and storage segments, and 

isolated gate stations and district regulating stations for the distribution segment in subpart W. 

Components in the field data were associated with a site identifier, tag identity, source, 

component count, and leakage rate. The site identifiers designate the facility types and the tag 

identities distinguish the leaking components from the non-leaking components. The components 

were categorized as: blowdown vent, connector, controller, centrifugal compressor seal, 

reciprocating compressor seal, control valve, open-ended line, orifice meter, other flow meter, 
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pressure relief valve, regulator, and valve. The leakage rates provided in the field data indicates 

the total hydrocarbon leaker rates. In cases where there were more than one component 

associated with a leakage rate, it was assumed that the leakage rate was a total of those 

components. 

Using the site identifiers, the components from the compressor stations, gate stations and district 

regulating station were separated out from the raw data. Unlike the Clearstone I and Clearstone II 

studies, it was not possible to separate the compressor station components into centrifugal, 

reciprocating and non-compressor related. It was assumed that any component found at 

compressor stations was compressor related because the count of non-compressor related 

components is relatively small compared with compressor related components. In addition, the 

compressor unit could not be identified as centrifugal or reciprocating given the labeling system 

used in the field data. Hydrocarbon LEFs for distribution were derived from components found 

in gate stations and district regulating stations as specified by Subpart W. 

Components found at compressor stations were grouped together to formulate hydrocarbon LEFs 

for both centrifugal and reciprocating compressors in the transmission sector, where as 

components found at gate stations and district regulating stations were grouped together to 

calculate hydrocarbon LEFs for the distribution sector. The data includes non-leaking and 

leaking components in the raw data requiring the isolation of leaking components using their tag 

identities. The leaking components were then grouped according to source category. Finally, 

hydrocarbon LEFs for the transmission and distribution sectors were calculated by dividing the 

sum of the leakage rates by the sum of the leaking component counts in each source category. To 

convert the hydrocarbon leaker emission factors to methane LEFs, a composition factor of 0.934, 

as stipulated by the GRI8

 report for transmission gas quality, was used. The data analysis process 

is summarized visually in Figure 2. 

Initial Results 

The total hydrocarbon emission factors from the Clearstone I and II data are shown in Table 1. 

The associated component counts are in Table 2. The methane emission factors from the CEPEI 

data analysis for transmission and distribution are shown in Table 3 and 4. 

Several emission source categories, such as open-ended line before the de-methanizer for 

reciprocating compressors, do not have sufficient data points or any data point to make a valid 

emission factor. To eliminate this issue, the segregation of components into reciprocating versus 

centrifugal and before the de-methanizer versus after the de-methanizer was eliminated. The 

resulting emission factors and component counts are shown in Table 5. 

 

8

 EPA/GRI. (1996) Methane Emissions from the Natural Gas Industry

<http://www.epa.gov/gasstar/tools/related.html> 
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Concluding Remarks 

Even after eliminating certain groupings, the emission factor for pressure relief valves for 

compressor related components are supported by only one data point (i.e. only one leak was 

found and quantified). Another emission factor with a small sample set is meter for compressor 

related components. It was not apparent whether these components did not leak, or that there 

were only one of each type component in the facilities studied. The CEPEI data was combined 

with the Clearstone I and II to develop LEF for both transmission and processing. 

Compressors in processing and transmission are comparable in size and discharge pressure. 

Therefore, EPA sought to overcome the problem of small sample sets supporting certain 

emission factors in processing, by combining the CEPEI data with the Clearstone I and II to 

develop LEF for both transmission and processing. EPA also elected to use the same LEFs for 

compressor related components in underground natural gas storage because compressors in this 

sector have a large throughput and inject gas at high pressure into underground reservoirs or 

from reservoir re-production into transmission pipelines. The emission factors for Table 6 shows 

the emission factors and component counts of compressor related components and noncompressor related components for transmission and processing. The final emission factors were 

corrected to 68 ̊F and 14.7 psia per the definition of “standard conditions” set forth in subpart A 

of the 40 CFR 98. 
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Emission Factor Calculation 

Component Grouping 

Source Categorization 

Clearstone II Raw Data 

Methane 

Streams 

Non-Methane 

Streams 

Before 

De-Methanizer 

After 

De-Methanizer 

Compressor NonCompressor 

Reciprocating Centrifugal Reciprocating Centrifugal 

Compressor NonCompressor 

Valves 

Connector 

Open-Ended 

Lines

Pressure Relief 

Valve

Meter 

Component 

Counts 

Hydrocarbon 

LEF 

Component 

Counts 

Hydrocarbon 

LEF 

Component 

Counts 

Hydrocarbon 

LEF 

Component 

Counts 

Hydrocarbon 

LEF 

Component 

Counts 

Hydrocarbon 

LEF 

Component 

Counts 

Hydrocarbon 

LEF 

Figure 1 - Summary of Clearstone data analysis 
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Emission Factor 

Calculations 

Source Categorization Facility Type Leaking 

Components 

Raw Data 

Compressor 

Stations 

Leaking 

Components 

Non-leaking 

Components 

Component 

Counts 

Hydrocarbon 

Leaker Emission 

Factors 

Connector 

Block Valve 

Control Valve 

Compressor 

Blowdown Valve 

Pressure Relief 

Valve 

Orifice Meter 

Regulator 

Open-ended Line 

Methane Leaker 

Emission 

Factors 

TRANSMISSION DISTRIBUTION

Component 

Counts 

Hydrocarbon 

Leaker Emission 

Factors 

Connector 

Block Valve 

Control Valve 

Compressor 

Blowdown Valve 

Pressure Relief 

Valve 

Orifice Meter 

Regulator 

Open-ended Line 

Methane Leaker 

Emission 

Factors 

District 

Regulating 

Stations 

Gate Station 

Leaking 

Components 

Non-leaking 

Components 

Figure 2 – Summary of CEPEI data analysis
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Table 1 – Total hydrocarbon emission factors for processing

Processing

Before De-methanizer

Emission Factor 

(scf/hour/component)

After De-methanizer 

Emission Factor 

(scf/hour/component)

Leaker Emission Factors - Reciprocating Compressor Components, Gas Service

Valve 0.7 2.5 

Connector 2.3 3.1 

Open-ended Line - 16.2 

Pressure Relief Valve - - 

Meter - - 

Leaker Emission Factors - Centrifugal Compressor Components, Gas Service

Valve 15.9 18.1 

Connector 4.3 9.1 

Open-ended Line 17.9 10.3 

Pressure Relief Valve - - 

Meter - 48.3 

Leaker Emission Factors – Non-Compressor Components, Gas Service

Valve 6.4 

Connector 5.7 

Open-ended Line 11.3 

Pressure Relief Valve 2.0 

Meter 2.9 

Table 2 – Component count for each emission factor for processing

Processing

Before De-methanizer

Emission Factor 

(scf/hour/component)

After De-methanizer 

Emission Factor 

(scf/hour/component)

Leaker Emission Factors - Reciprocating Compressor Components, Gas Service

Valve 750 105 

Connector 625 60 

Open-ended Line - 7 

Pressure Relief Valve - - 

Meter - - 

Leaker Emission Factors - Centrifugal Compressor Components, Gas Service

Valve 231 844 

Connector 151 777 

Open-ended Line 26 61 

Pressure Relief Valve - 1 

Meter 1 3 

Leaker Emission Factors - Other Components, Gas Service

Valve 1364 

Connector 1004 

Open-ended Line 192 

Pressure Relief Valve 9 

Meter 18 
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Table 3 – Methane emission factors for transmission 

Transmission Emission Factor 

(scf/hour/component) Component Count 

Leaker Emission Factors - All Components, Gas Service 

Connector 2.7 859 

Block Valve 10.4 235 

Control Valve 3.4 39 

Compressor Blowdown Valve 543.5 58 

Pressure Relief Valve 37.2 59 

Orifice Meter 14.3 12 

Other Meter 0.1 1 

Regulator 9.8 27 

Open-ended Line 21.5 72 

Table 4 – Methane emission factors for distribution 

Distribution Emission Factor 

(scf/hour/component) Component Count 

Leaker Emission Factors - Above Grade M&R Stations Components, Gas Service 

Connector 1.67 47 

Block Valve 0.557 34 

Control Valve 9.34 17 

Pressure Relief Valve 0.270 11 

Orifice Meter 0.212 4 

Regulator 0.772 6 

Open-ended Line 26.131 5 

Table 5 – Total hydrocarbon emission factor for processing after combining before and 

after the de-methanizer 

Emission Factor 

(scf/hour/component) Component Count

Leaker Emission Factors – Compressor Components, Gas Service

Valve 16.0 1198 

Connector 7.9 1003 

Open-ended Line 12.8 94 

Pressure Relief Valve 30.5 1 

Meter 36.2 4 

Leaker Emission Factors – Non-Compressor Components, Gas Service

Valve 6.4 1364 

Connector 5.7 1004 

Open-ended Line 11.3 192 

Pressure Relief Valve 2.0 9 

Meter 2.9 18 
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Table 6 – Total hydrocarbon emission factor for processing, transmission and 

underground storage 

Emission Factor 

(scf/hour/component) Component Count

Leaker Emission Factors – Compressor Components, Gas Service

Valve 15.07 1499 

Connector 5.68 1862 

Open-ended Line 17.54 166 

Pressure Relief Valve 40.27 60 

Meter 19.63 17 

Leaker Emission Factors – Non -Compressor Components, Gas Service

Valve 6.52 1364 

Connector 5.80 1004 

Open-ended Line 11.44 192 

Pressure Relief Valve 2.04 9 

Meter 2.98 18 
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Appendix A SOCMI Leaker Factors

Uncontrolled SOCMI Fugitive Emission Factors 

Equipment/Service SOCMI 

Average1 

SOCMI 

Without C2

2 

SOCMI With 

C2

2 

SOCMI NonLeaker3 

Valves 

 Gas/Vapor 0.0132 0.0089 0.0258 0.00029 

 Light Liquid 0.0089 0.0035 0.0459 0.00036 

 Heavy Liquid 0.0005 0.0007 0.0005 0.0005 

Pumps 

 Light Liquid 0.0439 0.0386 0.144 0.0041 

 Heavy Liquid 0.019 0.0161 0.0046 0.0046 

Flanges/Connectors 

 Gas/Vapor 0.0039 0.0029 0.0053 0.00018 

 Light Liquid 0.0005 0.0005 0.0052 0.00018 

 Heavy Liquid 0.00007 0.00007 0.00007 0.00018 

Compressors 0.5027 0.5027 0.5027 0.1971 

Relief Valve 

(Gas/Vapor) 

0.2293 0.2293 0.2293 0.0986 

Open-ended Lines4 0.0038 0.004 0.0075 0.0033 

Sampling Connections5 0.033 0.033 0.033 0.033 

Notes: All factors are in units of (lb/hr)/component. 

1. Factors are taken from EPA Document, EPA-453/R-95-017, November 1995, Page 2-12 

2. Factors with ethylene (C2) are TCEQ derived. 

3. Control credit is included in the factor; no additional control credit can be applied to these 

factors. AVO walk-through inspection required. 

4. The 28 series quarterly LDAR programs require open-ended lines to be equipped with an 

appropriate sized cap, blind flange, plug, or a second valve. If so equipped, open-ended lines 

may be given a 100% control credit. 

5. Use the SOCMI Sampling factor for Non-Leakier. Emission factor is in terms of 

(lbs/hr)/Sample Taken.