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Environmental Protection Agency (EPA) Methane White Paper 

Oil and Natural Gas Sector Compressors 

INGAA Comments 

1. Introduction 

The Interstate Natural Gas Association of America (INGAA), a trade association of the interstate 

natural gas pipeline industry, submits these comments on the EPA’s technical white paper, “Oil 

and Natural Gas Sector Compressors” (Compressor Paper). On April 15, 2014, the EPA released 

five technical white papers discussing potentially significant sources of emissions in the oil and 

natural gas sector. Specifically, the white papers focus on potential emissions and mitigation 

techniques for methane and volatile organic compounds (VOCs). INGAA is submitting separate 

comments on the three papers (Compressors, Leaks and Pneumatics) that address sources 

applicable to the interstate natural gas transmission and storage (T&S) sector. These comments 

address the Compressor Paper. 

In the Compressor Paper, EPA summarized its current understanding of vented VOC and 

methane emissions from this source category, and its understanding of available mitigation 

techniques and the cost, effectiveness and application of these techniques in the oil and natural 

gas sector. EPA has requested comment on 12 questions related to the contents and conclusions 

of the Compressor Paper. 

INGAA and its member companies have a long history of working collaboratively with a variety 

of stakeholders on numerous greenhouse gas (GHG) related issues, including methane 

quantification and mitigation. As such, INGAA welcomes the opportunity to contribute its 

knowledge of emissions and available control technologies. INGAA commends EPA for 

involving stakeholders and inviting comment on the methane white papers. 

 

Executive Summary 

INGAA has three general points in response to EPA’s white papers. First, there are significant 

differences in the methane emission estimates produced by EPA’s Inventory of U.S. Greenhouse 

Gas Emissions and Sinks (the National Inventory) and the Subpart W portion of its Greenhouse 

Gas Reporting Program (GHGRP).1

 There also are significant gaps in the information available. 

EPA needs to develop a systematic program to reconcile these differences and fill the gaps in 

order to support further discussion on methane reductions. Second, INGAA recommends 

additional discussion and analysis of the implications, including costs, of potential mitigation 

methods. Finally, EPA should recognize that new information will be available soon to 

supplement this discussion. 

                                                            

1 See http://www.epa.gov/ghgreporting/ and http://www.epa.gov/ghgreporting/reporters/subpart/w.html (Starting 

with the 2011 calendar year for Subpart W, the GHGRP requires pipeline companies to submit annual reports of 

GHG emissions for subject facilities. Pipelines must submit 2014 emission information for the fourth annual report 

by March 31, 2015). 
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In response to the Compressor Paper specifically, INGAA offers the following comments: 

1. EPA should review the data for centrifugal compressors. There is a significant disparity in 

reported emissions between the National Inventory and Subpart W for centrifugal 

compressors. EPA should resolve this issue before contemplating mitigation methods. 

2. EPA should consider additional mitigation approaches for rod packing. Specifically, INGAA 

recommends condition-based maintenance for rod packing as an alternative to maintaining or 

replacing rod packing on a prescribed schedule. 

3. Finally, EPA should acknowledge that almost all new compressor units use dry seals and 

therefore wet seals as a source of emissions should no longer be a focal point of the 

discussion. 

INGAA welcomes the opportunity to contribute to this discussion. 

Overview of INGAA Responses 

INGAA has attempted to answer each question to the best of its ability. Due to time and other 

limitations, however, INGAA has not answered all questions fully. For some questions, INGAA 

could not respond completely because this would require a significant long-term effort (e.g., a 

research program) that includes all stakeholders. For example, questions regarding technology 

prevalence, cost implications and technical feasibility impacts would entail an integrated 

research effort to provide a quality, validated and vetted answer. In most cases, however, 

INGAA has provided a thorough response with the information available while noting the gaps 

in existing data. Whenever possible, INGAA offers specific examples of field conditions and 

industry experience. Appendix A provides a quick reference to EPA’s questions on compressors 

and INGAA’s response. 

2. Establish an Open Process that Involves All Stakeholders to Further Examine Methane 

Reduction Issues 

Other than the Natural Gas STAR program, EPA has focused its oil and gas industry GHG 

efforts primarily on quantifying or estimating methane emissions. EPA must consider a number 

of issues, including economic, operations and environmental issues as it moves toward a broader 

methane strategy. 

INGAA envisions a process that is transparent and inclusive of industry and other stakeholders to 

examine issues and potential strategies. This has proven effective at other federal agencies and 

for other EPA regulations. For example, EPA could host technical conferences similar to those 

convened by the Federal Energy Regulatory Commission (FERC) to examine critical issues prior 

to initiating a rulemaking.2

 

                                                            

2

 FERC typically holds public technical conferences to gather information on important policy issues and to hear 

potentially competing positions in advance of issuing significant rulemakings. For example, in 2012-2013, FERC 

held a series of technical conferences to explore issues surrounding the increased use of natural gas for electric 

power generation and issues that may impede coordination between the gas and electric industries. After 

considering discussions raised by interested stakeholders at the 2012-2013 technical conferences, and subsequent 

public comments, the FERC initiated two rulemakings that propose to address concerns raised by participants at the 

technical conferences. Technical conferences provide the benefit of educating the regulator and other stakeholders 

on a particular issue and facilitating a structured dialogue where stakeholders can debate the effectiveness or need 
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This process would provide an efficient and effective means to advance the current 

understanding of methane emissions and how to design effective programs. As an example, the 

EPA GHGRP is intended to develop improved GHG inventories and to inform policy decisions.3

 

Subpart W of the GHGRP addresses T&S segment emissions from vents and equipment leaks 

and companies have submitted three years of data to date. Data is publicly available for the 2011 

and 2012 reporting years,4

 and indicate significant differences between Subpart W reporting and 

the National Inventory5 for T&S sources. These differences are due in part to differences in 

scope but also other reasons. It is very important that these differences are understood and that a 

single, accurate inventory is in place before EPA or industry can develop and implement 

effective and efficient mitigation strategies. These proposed technical conferences or 

workshops can provide EPA and industry the platform to discuss these important issues so that 

EPA’s future methane reduction policies reflect all relevant analysis. 

3. Sources of Information on Emissions and Reduction Opportunities6

 

EPA has requested comment on whether the Compressor Paper appropriately characterizes the 

different studies and data sources. INGAA agrees that it is important to understand the sources 

of methane emissions, how those emissions are measured and reported, what reduction 

opportunities exist (and what is the cost associated with these opportunities). While the 

Compressor Paper provides a summary of several sources of information, the paper does not 

reflect the full breadth of valuable information available to EPA on the topic. Further, while the 

studies have been summarized, they are neither compared nor contrasted with each other or with 

other sources of similar information. It would have been valuable for EPA to provide a tabular 

summary that presents the primary results from each study in common engineering units in order 

to identify data gaps and inconsistencies. While it may not have been the intent of the 

Compressor Paper to compare or contrast information, such an exercise is necessary to determine 

if a particular study appropriately characterizes the data available. Further, a comparison of the 

studies would help identify conflicts and data gaps in the information. 

Due to the limited comment period, INGAA did not have time to develop a definitive analysis of 

the various studies listed or a list of key studies that were not included. However, more detailed 

discussion below provides examples of data gaps or differences in the primary tools that are 

being used to inform the process (e.g., differences between Subpart W reporting and the National 

Inventory). 

In general, INGAA views the Compressor Paper as a summary of some information, but not the 

full range of studies and data sources currently available. EPA’s characterization that the studies 

                                                                                                                                                                                               

for prospective action prior to the issuance in a proposed rulemaking. Often technical conferences can reveal 

stakeholder support for certain initiatives, identify potential flaws in certain proposals, and identify alternative ways 

to address the regulator’s or stakeholders’ concerns. 

3 See 40 C.F.R. Part 98 (2014). 4

 EPA publishes GHGRP reporting results through its data publication tool, Facility Level Information on 

GreenHouse gases Tool (FLIGHT); see http://ghgdata.epa.gov/ghgp/main.do 5 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2012, EPA 430-R-14-003 (April 2014). This report 

presents the U.S. national GHG inventory from 1990-2012 for anthropogenic sources (i.e., associated with human 

activity) and is referred to as the “National Inventory” in these comments. The report is updated annually. 6

 This section generally responds to Compressor Paper questions 1 and 2.
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presented in the paper are pivotal suggests that EPA considers them key to understanding these 

issues. INGAA would caution EPA against placing excessive weight on this limited information 

since other studies, not referenced in the Compressor Paper, may be based on the same or similar 

data sets but reach different conclusions. 

INGAA also urges EPA to elaborate on how it arrived at its cost analysis for mitigation 

measures. Cost estimates are sensitive to assumptions and inherent data inconsistencies. Costs 

vary over time and with location and chosen technology. As presented by EPA, the 2014 study 

by ICF, Economic Analysis of Methane Emission Reduction Opportunities in the U.S. Onshore 

Oil and Natural Gas Industries, (ICF paper),

7

 appears to be the primary source of information on 

evaluating cost-effective emissions reduction opportunities. However, the ICF paper has several 

limitations such as only including emissions mitigation options for a subset of emission sources, 

only addressing one mitigation technology or work practice for these select sources (i.e., 

alternatives are not addressed), and generally not addressing uncertainties or presenting ranges 

associated with emission mitigation costs and performance. Further, several other sources of 

similar information have not been referenced. In fact, cost-effectiveness has been examined by a 

number of organizations such as the International Petroleum Industry Environmental 

Conservation Association (IPIECA)8

 and the Intergovernmental Panel on Climate Change 

(IPCC).9 Cost effectiveness has also been a topic during the United Nations Framework 

Convention on Climate Change (UNFCCC) events. Given the significance of understanding the 

costs associated with various methane reduction technologies in crafting a comprehensive costeffective reduction strategy, the Compressor Paper should expand the discussion to describe 

more fully the body of available information, including key parameters and assumptions that 

impact cost analyses. 

Other issues associated with the listed studies have not been fully vetted in the available time for 

comment. For example, the ICF paper makes assumptions to refine existing emission factors and 

update activity data counts and uses very coarse data (e.g., miles of pipe) to change activity data 

for compressor counts. Such unverified “guesstimates” introduce uncertainty across emissions, 

mitigation and cost discussions is differences between Subpart W data and the National 

Inventory for T&S sources. This is important because differences in the inventory affect 

potential achievable reductions and have implications for economic analysis. The Subpart W 

and National Inventory differences are discussed further below. 

New sources of information soon will become available that will significantly add to the 

understanding of methane emissions and reduction opportunities in the T&S sector. Specifically, 

the Pipeline Research Council International (PRCI)10 is completing a report summarizing GHG 

emission reduction technologies and work practices applicable to natural gas pipeline 

compressor stations. The PRCI work will identify existing and commercially available GHG 

emission reduction technologies and work practices for pipeline compressor stations, as well as 

                                                            

7 Economic Analysis of Methane Emission Reduction Opportunities in the U.S. Onshore Oil and Natural Gas 

Industries, ICF International, 2014. 8

 Oil and Natural Gas Industry Guidelines for Greenhouse Gas Reduction Projects, March 2007. 

9

 Climate Change 2007, the Fourth Assessment Report (AR4) of the United Nations Intergovernmental Panel on 

Climate Change (IPCC). 

10 The Pipeline Research Council International (PRCI) is the global collaborative research development organization 

of, by, and for the energy pipeline industry. 
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identify technologies in development. The report will evaluate technical and economic viability 

of existing and developmental GHG emission reduction technologies and work practices. While 

this report has yet to be released, PRCI has permitted INGAA to acknowledge the draft 

document for the purposes of our comments. More detail on compressor emissions reduction 

technologies and costs is provided in comments below.

4. National Estimates of Methane Emissions and Methane Emission Factors, Activity 

Data, and Calculation Methodologies11 

EPA has asked for comment on the national estimates of methane emissions and methane 

emission factors, activity data, and calculation methodologies. While there are several sources of 

information on methane emissions from the natural gas system, the two primary sources of GHG 

emission data are the National Inventory and the EPA GHGRP. The EPA compiles both a topdown estimate of GHG emissions (i.e., National Inventory activity data uses data such as facility 

count and miles of pipe to estimate emissions) and detailed bottom-up reporting (GHGRP). 

Using other common vernacular, the National Inventory is commensurate with a Tier 1 or Tier 2 

estimate (i.e., using activity data at the facility or large unit level), while Subpart W of the 

GHGRP uses Tier 3 (more granular source or component level activity data) or Tier 4 (detailed 

source level data or direct measurement) estimates. These two reports use different methods for 

estimating methane releases and do not include all of the same sources of releases. Accordingly, 

there currently is no single compilation that comprehensively and accurately addresses all 

sources of methane released by the T&S sector. 

While the National Inventory and the GHGRP produce consistent estimated emissions from 

some sources (e.g., the natural gas production segment), there are significant differences for the 

other segments of the natural gas system. There likely are multiple reasons for the disparity, 

including differences in the methodologies used in the two programs, and the specific sources 

being measured and included for reporting. For example, EPA updated key National Inventory 

estimates for the production segment based on more recent data, including Subpart W data. In 

contrast, EPA has not yet updated its key estimates for the T&S segment, and it does not appear 

that EPA has compiled or analyzed results based on thousands of measurements for T&S sources 

completed in response the GHGRP reporting requirements. INGAA suggests that the approach 

used for production can also be used to improve the estimates for the T&S sector and to serve as 

a more accurate basis for determining costs and mitigation opportunities. Work to reconcile 

these estimates remains an area of needed analysis before drawing any definitive conclusions 

about the total level of methane emissions from natural gas transmission or definitively 

addressing related questions about mitigation and costs. 

The need for more current data and analysis on methane emission levels is highlighted by the 

fact that most T&S emission factors used for estimating methane emissions are based on a 

limited set of data that is approximately two decades old. The emission factors that are used to 

calculate methane emissions for both the National Inventory Report and for some Subpart W 

sources have typically been in place since the 1990s and need to be updated. For T&S, the 

National Inventory (and other published papers and studies) primarily rely on emission factors 

                                                            

11 This section generally responds to Compressor Paper questions 1, 2, 4 (on mitigation technologies, reductions and 

costs), and 12 (ongoing or planned related studies). 
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derived from an EPA – Gas Research Institute (GRI) study12 that was conducted in the early 

1990s and published in 1996. These factors may not accurately reflect natural gas transmission 

methane emissions because operating practices, as well as the efficiency of operating equipment, 

have improved over the past 20 years. INGAA urges EPA to initiate a program to use the T&S 

data from 2011, 2012 and 2013 GHGRP reporting to support development of improved emission 

factors, and improved emission estimates for the T&S segments. 

The GHGRP also has some limitations, including reliance on dated emission factors and 

exclusion of smaller facilities and smaller sources at subject facilities. Subpart W also does not 

require reporting of known sources, such as rod packing emissions in “not operatingpressurized” (i.e., standby) mode, and centrifugal compressor emissions from dry seals. 

However, analysis of the reported data should significantly improve the understanding of 

methane emissions from the T&S sector, especially where measurement is required. Future 

improvements in estimates of aggregated methane emissions, along with use of Subpart W direct 

measurements to update emission factors, can lead to more accurate estimates overall. INGAA 

asks that EPA work with the industry to develop emission factors for the sources with new 

measurement data available. 

Compressor Emission Factors, Activity Data and Emission Estimates 

As noted above, while the emission studies listed in the Compressor Paper provide a sampling of 

available information, other valuable sources are available. For example, a number of papers 

from the late 1990s and early 2000s followed the publication of the EPA/GRI Report. As 

another example, in 2005 and 2006, INGAA (along with the American Petroleum Institute and 

American Gas Association) worked with EPA to prioritize data gaps and methane emission 

factors that required additional research. That effort developed a plan for a methane “emission 

factor reconciliation” project13 and initiated an effort to secure funding for field tests. The 

project would have implemented emissions testing projects to address data gaps that remain to 

this day, based on a prioritized list of emission sources collaboratively developed by EPA and 

the industry stakeholders. However, this project was not executed due in large part to EPA’s 

new responsibility to develop the GHGRP. 

A more comprehensive list of reports and emissions-related documents could be compiled, but a 

higher-priority endeavor might involve a better characterization of the strengths and weaknesses 

of available documents, identifying data or information gaps and defining a programmatic 

approach for resolving differences. INGAA cannot now provide a comprehensive response 

because of the limited time provided to respond to the white papers. INGAA believes there is a 

general need to develop a systematic approach to reconcile differences in available information 

because definitive data is not available or has not yet been analyzed (e.g., Subpart W data). 

                                                            

12 Methane Emissions from the Natural Gas Industry, EPA/GRI report (June 1996) at: 

http://www.epa.gov/gasstar/tools/related.html; This 15-volume compendium of reports is from an EPA/GRI study 

published in 1996 that remains a seminal reference on methane releases from natural gas systems. These reports and 

related data are referred to as the “EPA-GRI Study” in this paper. 

13 INGAA/API/AGA Natural Gas Systems GHG Emission Factor Comparison, Gap Assessment, & Improvement 

Collaborative Project, Executive Summary (May 17, 2006). 
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These comments will illustrate differences in the level of methane emissions by comparing the 

National Inventory with data from the first two years of Subpart W reporting – i.e., 2011 and 

2012 annual Subpart W reporting. Although, INGAA has not completed a thorough analysis of 

Subpart W data, a subset of this information has been reviewed. This review indicates that 

INGAA members have completed over 10,000 direct measurements of compressor emissions in 

the first two years of Subpart W reporting. Yet, it does not appear that EPA has completed an 

analysis of Subpart W measurement data. INGAA is working with PRCI on compilation and 

review of Subpart W data along with supplemental data on equipment, operations, and methods 

used for Subpart W measurements. Initial review indicates that emissions for compressors as 

reported pursuant to Subpart W are significantly lower than the National Inventory estimate of 

such emissions. EPA should analyze these more recent data. Additional information likely will 

be available as further analysis is completed, and INGAA will coordinate with PRCI to share that 

information with EPA. 

When comparing the data, it is important to understand the different nomenclature used to 

categorize emissions. For the white papers, INGAA supports the Compressor Paper approach 

that focuses on vented emissions – e.g., rod packing venting from reciprocating compressors. 

For Subpart W (and emission factors used in the National Inventory), “compressor” emissions 

include equipment leaks from unit isolation valves (when units are not operating and depressurized) and blowdown valves (e.g., when units are operating). EPA addresses those 

emissions in the Leaks White Paper, and that appears to be a reasonable approach. However, it 

is important to understand how emissions are reported and how various vented or leaking 

emissions are reflected in estimates. Since time was limited to comment, INGAA cannot provide 

concise analysis on this issue, but there certainly are inconsistencies. For example, in the 

National Inventory and Subpart W estimates discussed here, leaks along with compressor vented 

emissions are grouped and compared. However, when assessing mitigation it is inappropriate to 

use the same estimate because the emission rate may include rod pack venting (which is covered 

by the Compressor Paper and would be reduced by rod packing maintenance) and leaks (e.g., 

isolation valve leakage when the unit is shutdown, which should be covered in the Leaks paper 

and would not be affected by rod packing maintenance). 

Additional time would be required to segregate National Inventory emissions and Subpart W 

reported emissions into subsets based on emission source (e.g., rod packing, valve leak) and 

compressor operating mode. Both inventories present “rolled up” compressor emission estimates 

that aggregate some leak sources addressed in the Leaks Paper with vents addressed in the 

Compressor Paper, but the reported data from both programs is similar because certain leaks 

(e.g., isolation valve) and vented emissions are captured within the estimate for compressors. 

Since compressor emissions are reported on a similar basis, comparison is appropriate. Other 

than the emissions data used for the estimate (i.e., 1990s emissions factors versus recent Subpart 

W measurements), the primary difference is that only larger facilities that exceed the GHGRP 

threshold of 25,000 metric tons CO2 equivalent (CO2e) report for the GHGRP. For compressors 

in the transmission segment, comparisons indicate that: 

• Reciprocating compressor emissions are over seven times higher in the National Inventory 

than Subpart W; and 

• Centrifugal compressor emissions are over 30 times higher in the National Inventory than 

Subpart W. 
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These differences are due in part to the fact that many facilities were not required to submit a 

GHGRP report because only 30 percent or less of the transmission compressor stations exceeded 

the reporting threshold for the GHGRP. In contrast, the National Inventory uses “coarse” 

activity data (e.g., facility count for many sources, compressor count for compressor emissions). 

This can be reconciled by normalizing compressor emissions from Subpart W based on total 

emissions and total compressors (or facilities) for reporting facilities and then scaling using 

National Inventory activity data. Using data collected from INGAA members for 2011 GHGRP 

reporting, INGAA completed an initial analysis to normalize GHGRP data then scale the 

emissions based on the National Inventory activity data (i.e., compressor counts for the entire 

sector). This initial analysis to scale emissions shows that there remain significant differences 

for transmission compressors, including: 

• Reciprocating compressor emissions are over 50 percent higher in the National Inventory 

than based on Subpart W data; and 

• Centrifugal compressor emissions remain very disparate – 20 times higher in the National 

Inventory than based on Subpart W data. 

This is based on an initial analysis and provided as an example to demonstrate the significant 

uncertainty regarding compressor emission estimates. More detailed analysis should be 

completed by industry stakeholders at a later date, and INGAA comments on Subpart W have 

inquired about EPA plans to review Subpart W data, especially measurement data from T&S 

facilities. In addition, INGAA and others that are required to complete compressor vent 

measurements for Subpart W have asked EPA to use that data to develop emission factors for 

compressors (e.g., in recent comments on proposed Subpart W amendments). INGAA welcomes 

the opportunity to continue to explore how to best resolve these data inconsistencies and arrive at 

a reasonably accurate inventory that balances the considerable expense and burden incurred by 

INGAA members to implement Subpart W measurement and reporting requirements with the 

need for more consistent and accurate data. 

These comparisons clearly show large uncertainties regarding compressor emissions – especially 

for centrifugal compressors. There are significant and important data gaps. 

While some may presume that compressors are a primary source of emissions and mitigation 

measures are available, educated decisions about the need for, effectiveness of, and cost of 

mitigation cannot be made until the emission differences are resolved and there is a technically 

based understanding of compressor-related emissions. The contribution of various sources must 

be clearly understood before effective policies can be developed. Since GHGRP (and Subpart 

W) information is intended to inform policy decisions, it is imperative that program data be used 

to advance the current understanding of emissions and thus the potential for reductions. If 

Subpart W data shows significantly lower emissions from a particular source, then it would be 

appropriate to focus efforts on other sources. For example, focusing on emissions from 

centrifugal compressor emission sources, such as wet seal degassing vents, may be unproductive 

because these sources may account for lower emissions than indicated by the National Inventory 

and because new units will use dry seals rather than wet seals. 
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To summarize: 

• Two key available resources – GHGRP Subpart W reporting and that National Inventory – 

show very different emissions from compressors. 

• It is imperative that emissions and the associated sources (e.g., vents, leaks) be clearly 

defined, including understanding how differences in scope and approach affect the results 

from different programs. Significant data gaps may still remain. 

• If Subpart W is not meeting its intended objective, improvements in survey and reporting 

protocols should be considered, because the emissions data from this program is intended to 

inform policy decisions, such as methane reduction opportunities within the T&S sector. 

• “Compressor-related” emissions can include sources that are intended to vent (e.g., 

reciprocating compressor rod packing) and equipment leaks (e.g., unit isolation valve when 

the compressor is shut down and depressurized.) Data is not consistently “rolled up” for 

various reports listed by EPA, and consistent roll-up is needed to understand the actual 

emissions. The white papers intend for leak-related emissions (e.g., leaking valve) to be 

addressed by the Leaks Paper rather than the Compressor Paper. 

• Reported emissions from different studies may be based on emission factors that represent a 

different subset of emission sources – e.g., reciprocating compressor emissions factors 

include rod packing venting, and may also include leak sources (e.g., isolation valve 

leakage). Inconsistencies in different studies can introduce problems when comparing 

emissions or assessing mitigation effectiveness. For example, “average” times in different 

operating modes and both vented and leak sources are integrated into “composite” 

compressor emission factors in the original EPA-GRI study. Significant review of each 

study and emissions roll-up is needed to understand the data and implications better. This 

can influence emission reduction calculations – e.g., control effectiveness for rod packing 

maintenance should not be applied to an emission factor that includes other emission sources 

(compressor-related leaks). This is discussed further in the next section. 

• A systematic program needs to be developed and implemented to understand the data that is 

available through Subpart W, the National Inventory, and other sources to ensure that it is 

representative and consistent over the primary platforms being used by EPA to inform GHG 

policy. Leveraging Subpart W data and ongoing GHGRP requirements should be integral to 

that process. 

• Additional work is being completed. Still, a comprehensive, systematic approach is needed 

to understand fully the data generated by Subpart W, as well as how the accuracy and 

completeness of this data collection effort can be improved. Industry projects include an 

ongoing PRCI project to compile and analyze Subpart W data and related supplemental data 

provided by PRCI and INGAA members. In addition, several natural gas transmission 

companies are participating in a study with the Environmental Defense Fund that includes 

measurements at a number of T&S facilities, including on-site measurements and mobile 

monitoring using tracers. Results from that study should be available later this year. 
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5. Compressor Mitigation Techniques and Costs14

The Compressor Paper discusses available emission mitigation techniques and focuses on 

reducing vented emissions from reciprocating compressor rod packing and centrifugal 

compressor wet seal degassing. As noted in the emissions discussion above, INGAA’s 

understanding is that leak-related emissions (e.g., from a leaking isolation valve) are addressed in 

the Leaks Paper. As noted in earlier comments, the mitigation information presented is only a 

sampling of available technologies and work practices. A PRCI report is being completed that 

should be available this summer; that report is intended to provide a thorough list of technologies 

and work practices, and the associated performance and cost of such technologies and work 

practices. It includes both existing mitigation methods and developmental or conceptual 

approaches. The report includes all but one of the T&S mitigation techniques in the ICF paper 

and also includes many additional prospective mitigation technologies and work practices and 

their technical feasibility. While the PRCI report includes mitigation techniques that are 

currently in development or less common techniques, detailed appendices address the primary 

mitigation techniques. INGAA expects it will be able to share information from this report with 

EPA soon. 

INGAA is concerned with EPA’s estimates of potential reductions. As noted in Section 4, there 

is significant uncertainty about emissions, yet an assumed baseline emissions level is used to 

estimate potential reductions. Clearly, emissions data gaps need to be addressed before potential 

reductions can be understood. Section 4 also discusses the potential for errors in reduction 

calculations based on an unclear understanding of emission factors. For example, a reduction 

target for “compressors” applies a mitigation technique to reciprocating compressor rod packing. 

However, emission factors, such as EPA/GRI study emission factors used in the National 

Inventory, may represent emissions from several compressor-related sources, including leaks. 

These “typical” emission factors are a composite of different emission sources (e.g., rod packing 

venting, isolation valve leakage) and typical time in different compressor modes (e.g., operating, 

shutdown and depressurized). For a technology or practice that reduces rod packing venting, it is 

not appropriate to calculate the reduction using an emission factor that includes both rod packing 

and leak emissions. To understand the specific implications, significant additional review of the 

Compressor Paper material would be required (e.g., decouple the emissions factors and only 

apply rod packing reduction to the portion of the emission factor that applies to rod packing 

emissions). That was not possible within the comments schedule and may not be productive. 

Rather than delving into such nuance, EPA should consider identifying the differences in 

available data (e.g., comparing the National Inventory to Subpart W) and design and implement 

a systematic approach to resolve these differences and address data gaps. 

The Compressor Paper does not discuss cost sensitivities and factors that contribute to cost 

differences from facility to facility. The draft PRCI report, discussed above, will include several 

approaches for reviewing economic implications, and present cost ranges. Site-specific 

operating conditions and requirements can result in a wide range of mitigation costs. INGAA 

asks EPA to consider the additional information provided, when available, in the PRCI report. 

                                                            

14 This section generally responds to Compressor Paper questions 3, 4, 5, and 6 (on mitigation technologies, 

reductions and costs), 10 (on prevalence) and 12 (ongoing or planned related studies). 
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As with other issues, INGAA welcomes that opportunity to continue to engage with EPA on 

understanding mitigation performance and costs. As noted throughout these comments, a clearer 

understanding of emissions should be attained to reduce uncertainties in assessing mitigation 

performance and costs. 

Condition-Based Maintenance for Rod Packing 

The Compressor Paper fails to, and should, include condition-based maintenance for rod 

packing. This is used by some companies as an alternative to maintaining / replacing rod 

packing at a prescribed interval. Reliability studies have shown that many different mechanisms 

can affect the need for maintenance or contribute to the failure of a component (e.g., packing 

wear that increases emissions). Prescribed maintenance intervals are a relatively antiquated 

approach that is being replaced in many cases with predictive or condition-based maintenance, 

where operating conditions are monitored and maintenance decisions are based on performance 

(or defined “conditions”) rather than assuming a specific time basis (e.g., every 3 years) for 

maintenance. INGAA strongly recommends including condition-based maintenance for rod 

packing as a viable alternative to mitigate methane emissions. 

Condition-based maintenance practices may extend the operation of functional rod packing and 

preclude premature and wasteful rod packing maintenance/replacement. In other cases, it will 

identity rod packing where premature wear warrants maintenance on a more frequent basis than 

the prescribed interval. It also encourages the development of innovative rod packing 

technologies. This option considers current practices being used by operators, improvements to 

rod packing design, and the evolving technology. 

The EPA Natural Gas STAR program includes a lessons learned document “Reducing Methane 

Emissions from Compressor Rod Packing System,”15 which provides an example for conditionbased maintenance practices. Rod packing gas leaks are periodically monitored and the value of 

the incremental leaked gas (relative to post-maintenance/replacement leak rates) is compared 

with the discounted rod packing maintenance/replacement cost. When the incremental lost gas 

value exceeds the maintenance/replacement cost, the rod packing maintenance/replacement is 

cost-effective. This same general philosophy can be applied using a different basis for the repair 

decision, such as a defined leak rate or change in leak rate over time. 

Changing compressor rod packing at a set interval is discussed in the Compressor Paper as a 

means of reducing methane emissions. We support this approach provided the operating 

company is allowed the flexibility to establish a packing replacement interval coordinated with 

regular scheduled maintenance. The desired outcome of a rod packing replacement protocol is to 

prevent methane emissions to the environment, but the frequency of replacement must consider 

the blowdown emissions that occur each time the packing is replaced. 

Companies understand the value of rod packing monitoring and maintenance/replacement 

programs, and such programs have been instituted as part of safety and standard maintenance 

practices. EPA requests more details on mitigation techniques, including prevalence, but those 

answers are not known. For example, a survey would be required to understand the prevalence 

                                                            

15 http://www.epa.gov/gasstar/documents/ll_rodpack.pdf
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and common procedures for rod packing condition-based maintenance. In general, conditionbased maintenance grounded on periodically measuring rod packing leak rate typically will show 

a relatively flat leak rate over time, followed by an increase in leak rate when rod packing begins 

to fail. 

Low Emissions Rod Packing 

Question 3 from the Compressor Paper asks about low emissions packing for reciprocating 

compressors. While this technology is available, INGAA questions whether its effectiveness has 

been proven. INGAA has not developed detailed information on this topic, but is aware that 

some operators currently are assessing its performance by comparing conventional rod packing 

to low emission packing on similar units and monitoring the vent rate and changes over time. 

Such evaluations indicate that low emissions packing may emit at similar or higher levels than 

conventional packing. Thus, it is not apparent that this technology can significantly reduce 

vented rod packing emissions. At best, this technology requires additional study and 

improvement before its mitigation viability can be confirmed. 

6. Compressor Mitigation – Technology-Specific Questions16

The Compressor Paper includes several questions specific to control technology, especially 

related to centrifugal compressor wet seal emissions. This section provides some general 

responses. As noted in Section 4, there is a significant disparity in reported emissions from 

centrifugal compressors for Subpart W and the National Inventory. INGAA strongly 

recommends understanding and reconciling the differences and identifying, as appropriate, 

revisions to the National Inventory estimate or Subpart W methodologies. Mitigation-related 

questions cannot be addressed on an informed basis until that important data gap is addressed. It 

is possible that industry results from compilation and review of Subpart W data or the 

collaborative study with EDF will offer some insights, and those will be shared with EPA when 

available. 

Regarding centrifugal compressors with wet seals: 

• New turbine / centrifugal compressor installation are unlikely to include wet seals, as dry 

seals have been the norm for about two decades are longer. 

• Solar Turbines, the primary supplier of industrial-scale units used for natural gas 

compression, has not sold wet seal units for a number of years. A Solar Turbines 

representative indicated that it may consider a special request for wet seals but it is not aware 

of any such request in recent years. 

• According to a centrifugal compressor manufacturer, dry seal technology replaced wet seals 

to provide benefits that include increased safety, reliability and efficiency, and reduced 

operating, utility and maintenance costs. 

                                                            

16 This section generally responds to Compressor Paper questions 7, 8, 9, 10, and 11(specific technology questions 

such as questions regarding centrifugal compressor wet seals). 
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• An informal query of INGAA members indicates that most companies last installed a unit 

with wet seals in the late 1980s or early 1990s. All subsequent installations included dry 

seals. However, one company provided an example of three units with wet seals being 

installed in the mid-2000s. 

• Although there is an existing population of older units with wet seals, new units largely will 

be equipped with dry seals. 

• For the existing units, Subpart W data indicates that EPA has over-estimated the population 

of compressors with wet seals. For the transmission segment, the National Inventory 

assumes that 91 percent of centrifugal compressors include wet seals. Based on a review of 

the vast majority of Subpart W reports for 2011 and 2012, about 36 percent of units in 2011 

and 48 percent in 2012 indicated wet seals in the Subpart W report. At this time, it is unclear 

why there is such a difference between the two years, but in either case, the population is 

significantly lower than the National Inventory estimate. 

• As noted in Section 4, there is a 20-fold difference in the National Inventory estimate and an 

estimate scaled up from Subpart W data that uses the same activity data (total compressor 

count) as the National Inventory. The population difference and associated contribution from 

wet seals is only a part of that difference. Clearly, the National Inventory emission factor 

differs from Subpart W results to date. 

Initial review of 2011 Subpart W data (from a vast majority but not all reporting facilities in 

transmission) indicates about 70 measurements of wet seal degassing vents. The average 

value for annual emissions was 0.56 MMSCF or about 260 metric tons CO2e (287 short 

tons). The emission factor for wet seal compressors presented in Table 3-11 is 126 tons 

methane per compressor-year or 3,150 tons CO2e. This difference, along with the population 

count difference noted above, are most likely the primary contributors to the discrepancy in 

Subpart W and National Inventory estimates. Insight into the reason for this difference is not 

available at this time, but additional review, investigation, and analysis are clearly needed. 

• The Compressor Paper inquires about replacing wet seals with dry seals for existing wet seal 

units. INGAA’s understanding is that this practice is possible but is not common. INGAA 

does not have data on how frequently wet seals are replaced with dry seals. The Compressor 

Paper presents in Section 4.3.3 the capital costs for replacement and O&M savings costs in 

Table 4-4 that are much more favorable for dry seal replacement than similar costs noted for 

the ICF paper. The discrepancy is not acknowledged or explained. Compressor specific 

factors, such as compressor size, operating pressure, type of wet and dry seals, etc. also 

impact the costs for replacing a wet seal with a dry seal. One INGAA member company 

reported capital costs of $865,000 for replacement of a wet seal on a Solar Centaur 40 

T4702S centrifugal compressor, based on a May 2014 vendor proposal. This is more than 

double the cost in the ICF paper and more than an order of magnitude higher than the costs in 

Table 4-4. It also requires extensive compressor downtime of approximately 7 to 8 months 

to perform the replacement of wet seals with dry seals, which has economic implications that 

need to be considered. The upcoming PRCI report may also provide additional information 

on this issue for EPA’s consideration. 
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• Regarding vapor-recovery units (VRUs), INGAA believes that there are safety and 

operational concerns not addressed in the Compressor Paper. The use of VRUs to capture 

emissions from rod packing vents has rarely (if ever) been used in practice across the T&S 

sector and will require a feasibility or pilot study to determine the viability of the technology. 

Thus, INGAA does not support this control technique for compressor stations. 

As discussed in INGAA’s comments on the Leaks Paper, it supports the apparent consensus 

from the literature that a few sources/leaks contribute the vast majority of emissions (i.e., the 

80/20 or 90/10 rule) from compressor stations. Thus, approaches such as “directed 

inspection and maintenance” would be preferred for finding and mitigating larger emitters. 

Application of vapor recovery is unlikely to be a reasonable alternative. In addition, 

transmission facilities rarely include flares so on site use of the recovered gas is required. 

However, using the recovered gas may also introduce safety issues (e.g., pressure of vent line 

and system pressures) or may not be compatible with fuel if considered for that purpose. 

These bullets are intended to summarize information related to centrifugal compressor wet seal 

mitigation questions from the Compressor Paper. INGAA recommends that EPA reconcile the 

significant differences in estimated centrifugal compressor emissions between the National 

Inventory and Subpart W. Once emissions are better understood, it is possible that emissions 

from wet seals will be lower priority, especially since future centrifugal compressor installations 

will include dry seals. 

7. Coordinate Methane Initiatives across Agencies and Establish a Primary Point of Contact 

As part of the Obama administration’s Climate Action Plan, the Administration has initiated 

several efforts that address methane reductions, including revisions to the EPA Natural Gas 

STAR Program, launch of the Quadrennial Energy Review, revision of EPA’s GHGRP, the five 

methane white papers and plans to revise regulations to control VOC emissions and possibly 

methane emissions, from the oil and gas sector.17 This level of simultaneous activity on a 

common issue challenges the industry and other stakeholders, and coordination between industry 

and policymakers and across agencies is crucial. Therefore, INGAA recommends the 

Administration establish a primary point of contact responsible for ensuring coordination across 

the multiple federal agencies and departments tasked with methane-related initiatives impacting 

the T&S sector. Ideally, this point of contact would provide guidance and assistance in the 

development, interpretation and implementation of methane policies. It can be difficult for the 

pipeline industry to manage the numerous and sometimes duplicative requests for information 

from multiple agencies and departments. 

As questions arise about the overall climate change strategy, it would be extremely helpful to 

have this same primary point of contact within the Administration to discuss methane and other 

GHG reduction issues, opportunities and policies. 

                                                            

17 For example, potential revisions to 40 C.F.R., Part 60, Subpart OOOO. 
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8. Conclusion 

INGAA and its members are committed to reducing methane emissions in a prudent and 

environmentally responsible manner. INGAA and its members are undertaking initiatives to 

understand the sources of T&S sector methane emissions and to identify the most significant 

sources and the most cost-effective methane emission reduction strategies. 

In summary: 

1. There are significant differences in compressor emission estimates for the National Inventory 

and Subpart W of the GHGRP. This is due in part to a subset of sources reporting under the 

GHGRP, but the differences are not due solely to that fact. 

2. These differences must be reconciled and emissions data gaps addressed before conclusions 

can be made regarding the efficacy of mitigation measures or associated costs. 

3. A systematic program needs to be developed and implemented to address data gaps and reach 

consensus conclusions on compressor emissions. 

4. INGAA is aware of additional information that will be available from ongoing studies to 

supplement this discussion. This information may inform the process and provide some 

insight, but it is not expected that it will address all of the emissions data gaps. 

5. Once emissions are better understood, INGAA recommends additional discussion on the 

implications, including costs, of potential mitigation methods. 

6. Compressor Paper mitigation discussions focus on reciprocating compressor rod packing 

venting and centrifugal compressor wet seals. For rod packing, condition-based maintenance 

should be included an alternative to maintaining or replacing rod packing on a prescribed 

schedule. Centrifugal compressor emissions reported for Subpart W are significantly lower 

than the National Inventory. In addition, new units are equipped with dry seals, which has 

been the standard for over a decade. Thus, it may be inappropriate to continue to focus on 

wet seal emissions. 

INGAA appreciates the opportunity to comment on the Compressor Paper and will continue to 

provide EPA with additional information as it becomes available. 

Sincerely, 

Lisa S. Beal 

Vice President, Environment and Construction Policy 

Interstate Natural Gas Association of America 

 
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Appendix A 

Compressor Charge Question and INGAA Response Quick Reference 

1. Please comment on the national estimates of methane emissions and methane emission factors 

for vented compressor emissions presented in this paper. Please comment on the activity data 

and the methodologies used for calculating emission factors presented in this paper. 

Responses are found in sections three and four of the INGAA comment document. 

2. Did this paper appropriately characterize the different studies and data sources that quantify 

vented emissions from compressors in the oil and gas sector? 

Responses are found in sections three and four of the INGAA comment document. 

3. Did this paper capture the full range of technologies available to reduce vented emissions from 

reciprocating compressors and wet seal centrifugal compressors at oil and gas facilities? In 

particular, are there other options for reducing emissions at existing reciprocating or centrifugal 

compressors? For example, the EPA is aware of “low emissions packing” for reciprocating 

compressors but has no detailed information on this technology. 

Responses are found in section five of the INGAA comment document. 

4. Did this paper appropriately characterize the emissions reductions achievable from the 

emissions mitigation technologies discussed for reciprocating compressors and wet seal 

centrifugal compressors? 

Responses are found in sections four and five of the INGAA comment document. 

5. Did this paper appropriately characterize the capital and operating costs for the technologies 

discussed for reduction of vented emissions from reciprocating compressors and wet seal 

centrifugal compressors? 

Responses are found in sections five of the INGAA comment document. 

6. If there are emissions mitigation options for reciprocating and centrifugal compressors that 

were not discussed in this paper, please comment on the pros and cons of those options. Please 

discuss the efficacy, cost and feasibly for both new and existing compressors. 

Responses are found in sections five of the INGAA comment document. 

7. Are there technical limitations that make the replacement of wet seals with dry seals 

impractical at certain existing centrifugal compressors? 

Responses are found in section six of the INGAA comment document.8. Are there technical 

reasons why an operator would use a wet seal centrifugal compressor without a gas recovery 

system? 
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Responses are found in section six of the INGAA comment document. 

9. Are there technical limitations that make the installation of gas capture systems at certain 

reciprocating compressors impractical? 

Responses are found in section six of the INGAA comment document. 

10. Please comment on the prevalence of the different emission mitigation options in the field. 

Responses are found in sections five and six of the INGAA comment document. 

11. Given the substantial benefits of dry seal systems (e.g., lower emissions, less maintenance, 

and higher efficiency), are you aware of situations where new wet seal centrifugal compressors 

are being installed in the field? If so, are there specific applications that require wet seal 

compressors? 

Responses are found in section six of the INGAA comment document. 

12. Are there ongoing or planned studies that will substantially improve the current 

understanding of vented VOC and methane emissions from reciprocating and centrifugal 

compressors and available techniques for increased product recovery and emissions reductions? 

Responses are found in sections four and five of the INGAA comment document.