Attachment R

Compressors

Environmental Defense Fund

Ramón Alvarez, Robert Harriss, David Lyon
Oil and Natural Gas Sector: Compressors 

Peer Review Responses of Environmental Defense Fund

June 16, 2014 

I. EXECUTIVE SUMMARY

• Compressors are collectively a substantial source of methane emissions. Emissions 

from reciprocating and centrifugal compressors – including emissions from seals and 

non-seal components – are significant. 

o Emissions from compressor seals. Because Subpart W requires direct 

measurement of compressor emissions, it provides valuable estimates for both 

reciprocating and centrifugal seal emissions (though likely conservative because it 

does not measure all operating modes). Based on this data, average emissions 

from reciprocating compressor rod-packing are likely moderately higher than the 

emission factor used in EPA’s Annual Inventory. Centrifugal compressor wet 

seal emissions are likely substantially higher than the EPA emission factor. 

o Skewed distribution. Compressor emissions exhibit a highly-skewed or fat tail 

distribution, with about 20% of compressors producing approximately 80% of 

total rod packing and wet seal emissions.

o Non-seal emission. Non-seal emissions are an important component of overall 

compressor emissions. The ICF Report noted, for instance, that non-seal, fugitive 

emissions from reciprocating compressors account for approximately 75% of 

overall emissions from those sources. 

o National Emissions. The best available data suggests that national emissions from 

compressors (all non-exhaust sources) are likely on the order of 2,000 Gg 

(2,000,000 metric tons) of methane, underscoring that this is a substantial 

emissions source relative to others in the inventory.

• Compressor emissions can be effectively mitigated using readily-available, costeffective technologies. For both reciprocating and centrifugal compressors, mitigation 

technologies include both abatement and capture technologies. 

o Regular rod-packing replacement is an effective, low-cost way to mitigate 

emissions, which ICF projected would reduce emissions by 3.6 billion cubic feet 

(bcf) of methane (approximately 69,000 metric tons) at a cost of $6.11/mcf 

reduced. Rod-packing replacement can be optimized by inspecting for excess seal 

emissions as part of a regular leak detection and repair program, enhancing 

control effectiveness. 

o Rod-packing gas capture systems can have greater effectiveness than rod-packing 

replacement and, in many cases, result in beneficial use of otherwise wasted gas. 

o Wet-seal degassing capture systems are feasible, result in substantial reductions, 

and are highly-cost effective. If broadly applied at centrifugal compressors not 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

2 

currently subject to the NSPS, ICF projected that such capture systems would 

reduce methane by 19.1 billion cubic feet (over 360,000 metric tons) while 

achieving net savings of $3.07/mcf of methane reduced.

o LDAR and gas capture technologies can also effectively mitigate non-seal 

compressor emissions. 

II. QUANTIFYING THE PROBLEM OF METHANE & VOC LEAKS FROM 

COMPRESSORS (QUESTIONS 1, 2, 12) 

At various points throughout EPA’s Compressor White Paper (“Compressor Paper”),1 the 

agency presents component level emissions factors for compressors (i.e., seal emissions), 

aggregate emissions factors for compressors (i.e., factors for wet seal centrifugal compressors 

and reciprocating compressors), and national emissions estimates for compressor emissions 

across segments. Each one of these emissions estimates is important for different reasons. For 

instance, component level emissions factors provide an indication of the relative importance of 

different compressor emissions sources like vented seal emissions and leaks. These componentlevel factors also help identify technologies available to mitigate emissions described later in the 

paper. Aggregate compressor emissions factors and national emissions estimates, conversely, 

help to characterize the magnitude of emissions from compressors and the emissions reductions 

achievable by deploying various technologies. 

Accordingly, Part II.A analyzes and critiques compressor emissions factors, with 

particular emphasis on developing a clear understanding of component-level emissions factors 

(seals, leaks, etc.) and identifying the gross-emitter or fat-tail distribution that many compressor 

data sets exhibit. Part II.B analyzes and critiques national emissions estimates, with emphasis on 

recent findings from ICF indicating current inventories may understate the number of large 

compressors in existence in the upstream segment. 2 Part II.C discusses forthcoming work. 

We conclude that: 

• Because Subpart W requires direct measurement of compressor emissions, it provides 

valuable estimates for both reciprocating and centrifugal seal emissions (though likely 

conservative because it does not measure all operating modes). Based on this data, average 

emissions from reciprocating compressor rod-packing are likely moderately higher than the 

emission factor used in EPA’s Annual Inventory. Centrifugal compressor wet seal emissions 

are likely substantially higher than the EPA emission factor. 

 

1

 EPA, Report for Oil and Natural Gas Sector Compressors (2014) [hereinafter “Compressor Paper”], available at 

http://www.epa.gov/airquality/oilandgas/pdfs/20140415compressors.pdf. 

2

 ICF International, Economic Analysis of Methane Emission Reduction Opportunities in the U.S. Onshore Oil and 

Natural Gas Industries 3-7 (Mar. 2014) [hereinafter “ICF Cost Curve Report”], available at 

http://www.edf.org/sites/default/files/methane_ cost_curve_report.pdf. 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

3 

• Non-seal emissions are an important component of overall compressor emissions. The ICF 

Report noted, for instance, that non-seal, fugitive emissions from reciprocating compressors 

account for approximately 75% of overall emissions from those sources. 

 

• Compressor emissions exhibit a highly-skewed or fat tail distribution, with about 20% of 

compressors producing approximately 80% of total rod packing and wet seal emissions. 

• The best available data suggests that national emissions from compressors (all non-exhaust 

sources) are likely on the order of 2 gigatons of methane, underscoring that this is a 

substantial emissions source relative to others in the inventory. 

A. Analysis and Critique of Emissions Factors (Questions 1, 2) 

We first provide our analysis of available data concerning compressor seal emissions, 

noting the skewed distribution of these emissions. We then provide specific critiques on the 

studies EPA cites. 

1. Compressor Seal Emissions 

The Compressor Paper summarizes previous work on compressor seal emissions, but the 

paper does not integrate the findings in a way that allows for easy comparison among sources. In 

this section, we describe why we believe Subpart W represents a particularly valuable data 

source for estimating seal emissions (though it likely underestimates these emissions); we 

synthesize data presented in the Compressor Paper and the referenced sources; and we discuss 

the distribution of compressor seal emissions – an important feature not addressed in EPA’s 

Compressor Paper. 

Compressor seal emissions measurements reported in the EPA Greenhouse Gas 

Reporting Program (GHGRP Subpart W) are the most robust dataset available, due to the large 

number of direct measurements of compressor seal emissions (over 2,500 compressors were 

directly measured and reported in 2012). Notwithstanding these direct measurements, Subpart 

W very likely understates compressor seal emissions because it does not require reporting in 

every operating mode during which emissions can occur. Specifically, Subpart W requires rod 

packing emissions to be reported only in the "operating" mode, but both the GRI/EPA and 

URS/UT3

 study show that rod packing emissions can be significant while a compressor is 

pressurized, but idle (a “not-operating, pressurized” mode)4 and that transmission compressors 

spend 34% of their time in this mode.5

 

3

 URS Corp. et al., Natural Gas Industry Methane Emission Factor Improvement Study (2011).

4

 The URS/UT study also reported emissions from idle, depressurized compressors; GRI/EPA assumed such 

emissions would be negligible. URS/UT did not discuss the cause emissions, but it is possible that they may 

originate from another component such a leaking isolation valve. 

5

 The GRI/EPA study reported that Transmission compressors are in operating mode 45% of the time, and 34% in 

the not-operating pressurized mode; they also indicate that reciprocating compressors in processing are pressurized, 

but we could not find a distinction between operating and idle, not-operating modes. 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

4 

With this in mind, EPA GHGRP data can be used to derive average emissions from 

compressor seals (both reciprocating and centrifugal) that can be compared to other reported 

values (see Tables 1-4 and discussion below). Tables 3 and 4 compare the compressor seal 

emissions reported by the various sources reviewed in the Compressor Paper. Estimates for rod 

packing emissions generally appear to agree to a greater extent than for wet seal emissions, 

though care must be taken when comparing these estimates since they do not always represent 

the same thing. The main differences among estimates are how operating hours in each 

operating mode are accounted for and whether an estimate includes emissions from non-seal 

emissions sources: 

• Operating Hours/Modes. As discussed above, reciprocating compressor rod packing 

emissions are only reported to Subpart W for the annual hours that a compressor is in 

”operating” mode (the same applies for centrifugal compressor wet seal emissions). As a 

result, rod packing emissions reported under Subpart W represent a lower bound on total 

rod packing emissions from a compressor. The URS/UT study reported annual emissions 

assuming a compressor would be in the observed mode for all hours in the year. The 

GRI/EPA study accounted for hours in different operating modes for reciprocating 

compressors, as did all the other referenced studies, which derive their estimates from 

GRI/EPA. 

• Seals vs. Non-Seal Emissions. All references except the GHG EI report emissions for 

compressor seals exclusive of other compressor emission sources. The GHG EI reports a 

single average emission factor for all compressor fugitives; the GRI/EPA report indicated 

that the seal emissions are about 25% of total compressor emissions for reciprocating 

compressors and less than 5% for centrifugal compressors, though below, we describe 

why the centrifugal conclusion is flawed.6

 

Table 3 shows that Subpart W data for rod packing emissions are lower than GRI/EPA 

estimates by 5% to 60%, depending on the sector. However, this difference may not persist if 

rod packing emissions from compressors in the not-operating (idle-pressurized) mode were 

included. 

On the basis of GHGRP measurements, Table 4 strongly suggests that the GRI/EPA 

estimates for centrifugal wet seal emissions are underestimated – by as much as factors of 5 and 

15 times for processing and transmission, respectively (very few values are available for 

storage). The wet seal emissions data in the URS/UT study are somewhat higher than the 

GHGRP data, but support the conclusion that the GRI/EPA estimates for wet seal emissions are 

too low. 

The GHGRP data for centrifugal compressors indicates that there are roughly equal 

numbers of compressors with wet seals and dry seals in the Transmission, Storage, and 

Processing segments, although the distribution varies among segments. See Table 5. The percent 

of centrifugal compressors that have wet seals in the GHGRP for these 3 segments is 

 

6

See Compressor Paper at 12-13. 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

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intermediate to the values reported in the GHG EI (80%) and the API/ANGA report7 (21% for 

processing only). 

Data presented in the ICF report indicate that more large compressors are in use in the 

gathering segment, with emission factors comparable to the average rates for storage and 

processing compressors in the GHGRP. The URS/UT study provides actual measurements that 

are not quite as high as the ICF estimate, but much larger than the assumed value for 

production/gathering segment emissions in the GRI/EPA study, suggesting average emissions 

from this segment are indeed larger. 

Table 1. Summaries of direct measurements reported under EPA GHGRP Subpart W for 

Reciprocating Compressor Rod Packing Emissions in 2012. 

Reciprocating Compressor 

Rod Packing (2012) 

Compressors Mscfy CH4 Metric tons CH4 / yr 

# Mean Median Mean Median 

Transmission 851 1,247 367 24 7.1 

Processing 1,269 882 160 17 3.1 

Underground Storage 116 535 151 10 2.9 

Table 2. Summaries of direct measurements reported under EPA GHGRP Subpart W 

2012 Centrifugal Compressor Rod Packing Emissions - EPA GHGRP Subpart W. 

Centrifugal Compressor 

Wet Seals (2012) 

Compressors Mscfy CH4 Metric tons CH4 / yr 

# Mean Median Mean Median 

Transmission 99 997 16 19 0.3 

Processing 167 5,366 154 103 3.0 

Underground Storage 5* 8,658* 14,074* 166* 270* 

*Note: With emissions only reported for 5 storage compressors, we do not believe the values for this 

segment have statistical significance; they are provided here only for completeness. 

 

7

 Am. Petroleum Inst. & America’s Natural Gas Alliance, Characterizing Pivotal Sources of Methane Emissions 

from Natural Gas Production: Summary and Analysis of API and ANGA Survey Responses (2012).
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

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Table 3. Summary of Reciprocating Compressor Rod Packing Emissions reported in EPA 

Compressor Paper. 

Reciprocating 

Compressor Rod 

Packing Emissions 

mscfy CH4 (per compressor) 

EPA GHG RP 

(Operating 

Mode Only) 

GRI/EP

A 

URS/U

T (OP)* 

URS/U

T 

(NOP)* 

URS/U

T 

(NOD)* 

ICF NSPS 

GHG EI 

(whole 

compressor 

- seal plus 

non seal 

potential) 

Processing 882 1,125 N/A N/A N/A 2,799 1,102 4,087 

Transmission 1,247 1,307 5,201** 12,238† 29,602‡ 3,801 1,282 5,550 

Underground Storage 535 1,350 N/A N/A N/A 5,279 1,334 7,707 

Gathering-Boosting N/A 

9.5 

241 N/A N/A 723 582 

104-5,950 

Production N/A N/A N/A N/A N/A 9.4 

*Assumes compressor operating in this measured mode for the entire year; OP = Running (operating); 

NOP = Not Operating (Idle), Pressurized; NOD = Not Operating (Idle), Depressurized 

** Recalculated from raw data in report to account for possible transcription errors in original report (see

attached spreadsheet). 

†This value becomes 8,741 if include 2 reported measurements of zero. 

‡This value was reported for operating compressors, but we believe that was an error. Their appendix 

indicates these emissions were from idle-depressurized compressors. We have reclassified these two 

measurements as NOD. 

Table 4. Summary of Centrifugal Compressor Wet Seal Emissions reported in EPA 

Compressor Paper. 

Cenrifugal 

Compressor Wet 

Seal Emissions 

mscfy CH4 (per compressor) 

EPA GHG RP 

(Operating 

Mode Only) 

GRI/EPA

† 

URS/UT 

(2011) ICF NSPS 

GHG EI 

(whole 

compressor - 

seal plus non 

seal potential) 

Processing 5,366 342 

8,137* 

N/A 10,783 18,750 

Transmission 997 248 N/A 5,959 18,331 

Underground Storage 8,658** 189 N/A N/A 5,959 16,586 

Gathering/Boosting N/A N/A N/A 12,000 N/A N/A 

†See discussion below: it appears this factor does not capture vented emissions from seal oil degassing 

* Assumes compressor operating in this measured mode for the entire year. Value is reported to be an

evaluative measurement, with potential measurement errors discussed; this value in original report 

seems to be in error as it is inconsistent with the reported measurements (appears it should be ~8,800). 

**Note: only 5 storage compressors reported seal emissions, so this value has little statistical significance.
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

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Table 5. Summary of Centrifugal Compressors Reporting to GHGRP Subpart W, by Seal 

Type. 

Segment Wet Dry Unclassified Total Wet %

Processing 274 153 26 453 60% 

Transmission 291 433 3 727 40% 

Storage 23 16 1 40 58% 

Total 588 602 30 1,220 48% 

Finally, the distribution of reported compressor seal emissions is critical for identifying 

the most effective mix of technologies to reduce these emissions. The distributions of 

compressor seal emissions reported under Subpart W are highly skewed, with roughly 20% of 

compressors accounting for 80% or more of the total emissions from rod packing and wet 

seals.(see Figures 1 and 2). Accordingly, focusing on the highest emitting compressors and 

reducing their emissions to the level of the lowest emitting devices could produce significant 

reductions. Figures 1A and 1B illustrate the skewed nature of the emissions distributions. For 

example, Figure 1B shows that rod packing emissions from the 20% highest emitting 

compressors at Processing Sites are greater than about 1,000 Mscfy, whereas 50% emit less than 

160 Mscfy and the lowest emitting 20% emit less than 1 Mscfy. EPA’s Compressor Paper does 

not discuss these distributional issues, and we urge the agency to incorporate and assess them 

going forward. 

 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

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Figure 1. Distributions of 2012 reciprocating compressor rod packing emissions 

measurements reported under Subpart W.

0

0.1

0.2

0.3

0.4

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0.6

0.7

0.8

0.9

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1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

Cumulative Probability

2012 Rod Packing Emissions (Mscf CH4 - log scale)

A. Rod Packing Emissions - Cumulative Distribution Function 

Transmission

Processing

Underground Storage

0.0

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fraction of cumulative emissions

fraction of compressors

B. ~20% of Compressors Produce ~80% of Total Rod Packing Emissions 

Transmission

Processing

Underground Storage
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

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Figure 2. Distributions of 2012 centrifugal compressor wet seal emissions measurements 

reported under Subpart W.

0

0.1

0.2

0.3

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0.6

0.7

0.8

0.9

1

1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

Cumulative Probability

2012 Wet Seal Emissions (Mscf CH4)

A. Wet Seal Emissions - Cumulative Distribution Function 

Transmission

Processing

0.0

0.1

0.2

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1.0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

fraction of cumulative emissions

fraction of compressors

B. ~20% of Compressors Produce 80-95% of Total WEt Seal Emissions 

Transmission

Processing
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

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2. Critique of the Compressor Paper's Summaries of Specific Studies 

GRI/EPA. The values presented in Table 3-2 accurately reflect the values in the 

GRI/EPA report. The Compressor Paper also correctly summarizes the GRI/EPA assumption 

that annual per-seal reciprocating and centrifugal compressor emissions are similar; we 

hypothesize that this assumption may partially explain why GHGRP estimates of centrifugal 

compressor wet seal emissions are so much higher than the GRI/EPA value. In addition, the 

URS/UT at Section 1.3.3. examines this issue and concludes that the GRI/EPA estimate of the 

wet seal emissions factor does not reflect the full suite of emissions. Specifically, the URS/UT 

report indicates that wet seal emissions have two components: fugitives at the seal face and 

vented emissions from the circulating oil. The URS/UT report further indicates that the 

GRI/EPA study factor included only the fugitive component, as the vented emissions from oil 

degassing were included as part of the station venting estimates. Finally, after reviewing 

available data in their Table 1-4, the URS/UT report concludes that only an estimate from the 

Natural Gas STAR program includes both the fugitive and vented component. The Natural Gas 

STAR emission factor of roughly 21,000-105,000 Mscf/year per seal, is even higher than the 

other estimates in our Table 4 and supports the conclusion that the GRI/EPA factor does not 

completely characterize wet seal emissions. We thus suggest that the GRI/EPA methodology 

for determining compressor level wet seal emissions improved and updated to reflect more fully 

the extensive volume of emissions. 

URS/UT. We have identified a number of weaknesses and potential errors in Table 3-4 of 

the Compressor Paper, which should include several modifications to better reflect the 

measurements in the URS/UT report (presented in Tables 3-3 and 3-4 of the URS/UT report). 

These are needed because there are apparent mismatches in the URS/UT report between the 

measurements classified in their Table 2-2, the measurement results reported in Table B-3, and 

the resulting average values presented in their Table 3-4. We provide a spreadsheet that seeks to 

correct some of the apparent discrepancies, though additional consultation with URS/UT may be 

helpful to confirm whether these apparent discrepancies are real, and update the summary tables 

as needed. In particular: 

• The reported annual emissions factors should be clarified to state that they are based on 

scaling up the measurements assuming operation in the observed mode for all hours in 

the year. 

• The Gathering/Boosting reciprocating compressor emissions data should be clarified to 

reflect the data was collected with the compressor in "run," or operating, mode. 

• For Transmission compressors, the first entry for "Average Rod Packing - Idle + 

depressurized" should be for "Average Rod Packing - Idle + pressurized". Additionally, 

the reported average of 12,236 Mscfy appears to exclude two zero values (measured, no 

emissions according to Table 2-2) for Site 2 reciprocating compressors 1 and 2. 

Including these 2 measurements of zero emissions, results in an average of 8,741 Mscfy 

for 7 reciprocating compressors. 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

11 

• For Transmission compressors, the second entry for "Average Rod Packing" should be 

corrected to reflect that the 29,603 Mscfy value corresponds to two measurements of 

emissions at Site 5, reciprocating compressors 6 and 7 reported in Table B-3. 

• For Transmission Compressors, the Table in the original paper did not include 4 

measurements of compressors in run mode (Site 4, reciprocating compressors 2,4,6,8 

which were measured according to Table 2-2 but entered incorrectly into Table B-3). We 

suggest the WP add these results, which are "Average Rod Packing -Run Mode", 4 

measurements, 5,201 Mscfy. 

• The value for wet seal emissions seems to be in error as it is inconsistent with the 

reported measurements (appears it should be ~8,800 - Table B-2 shows 12 measurements 

including one value of zero). 

• The GRI/EPA value for wet seals should be 248 instead of 396, according to the 

GRI/EPA report. 

GHGRP. As indicated in the Compressor Paper's summary of the GRI/EPA and 

URS/UT study, reciprocating compressors can emit in the "idle and pressurized mode". Because 

this mode is missing from the GHGRP data, the GHGRP estimates of compressor rod packing 

emissions underestimate actual emissions. The URS/UT paper also reports rod packing 

emissions from the "idle-depressurized" mode (GRI/EPA assumed that these emissions would be 

negligible). The cause of these emissions should be attributed to the appropriate source category 

(for example, the emissions measured from the rod packing in the depressurized mode may come 

from a leaking isolation valve). 

The information presented in Tables 3-5 and 3-6 of the Compressor Paper should be more 

clearly described. It appears that the reported emissions are total compressor emissions, not just 

seals. At a minimum, it would be helpful if the text and table headings describe what is included 

in the reported emission totals. Better would be to report emissions by type, and then the sum 

totals. Such an analysis would provide a check on the GRI/EPA breakdown of seal vs. non-seal 

emissions. (Seal emissions were estimated to be about 25% of total compressors emissions for 

reciprocating compressors and less than 5% for centrifugals.)8

 The Paper should also clarify that 

not all facilities report emissions to the GHGRP, so the activity factors and emissions reported in 

Tables 3-5 and 3-6 do not reflect national totals. 

US GHG Inventory. As described in the ICF report (see section 3.7), the compressor EFs 

in the GHG EI contain both seal and non-seal emissions. ICF assumes the seal emissions are 

~25% of the total compressor emissions in the inventory; this is supported by the discussion of 

the GRI/EPA report in Section 31 that reciprocating seal emissions are 24-28% of total emissions 

from reciprocating compressors (see, e.g., Table 4-18). The Compressor Paper discussion about 

the GHG inventory needs to clearly indicate that the emission factors in the GHG EI are 

aggregate compressor emissions - not just seals. This clarification should also be made in Tables 

3-8 and 3-9 to avoid confusion. 

 

8

See Compressor Paper at 12-13. 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

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The last paragraph of Section 3-4 refers to "net 2012 methane emissions" but the values 

presented appear to be potential emissions (which is what the caption for Table 3-9 correctly 

indicates); table A-141 of the GHG EI shows net emissions are only reported for recip 

compressors - not for centrifugal. Thus it is impossible to determine total compressor seal 

emissions for each segment. However, reciprocating net totals are possible for all sectors except 

production from Table A-141. 

A final general comment. We encourage EPA to ensure that all sources of emissions 

from compressors are analyzed, whether in the Compressor Paper or the Leaks Paper. 

B. Analysis and Critique of National Emissions Estimates (Questions 1, 2) 

EPA’s Compressor Paper also requests comment on national estimates of compressor 

emissions, which provide an overall sense of the importance of compressor emissions when 

compared to other significant sources. National estimates traditionally combine an emissions 

factor with an activity factor to establish overall national emissions. In the case of compressors, 

the emission factor in the national inventory includes not only seal emission (discussed in Part 

I.A, above), but also vented emissions blowdown vent stacks, open-ended lines, and leaks from 

various components. Some of these additional compressor emission sources are significant. For 

instance, the 1996 EPA/GRI data, ICF estimated that only about a quarter of total reciprocating 

compressor emissions come from rod-packing. Nonetheless, we focus here on the combined 

emissions factors for centrifugal and reciprocating compressors because nearly all of the 

emissions from these sources can be addressed by the technologies we describe in Part III, 

below. 

The Compressor Paper presents several studies that touch on compressor activity factors 

and national emissions estimates. We highlight several considerations relevant to national 

emissions, below: 

• The ICF Report surveyed specific states and found that there were a greater number of 

larger compressors in upstream segments, leading to a 166% upward revision of 

compressor emission in the gathering sector. 

• The GHGRP data can be used to provide an independent estimate of the type of seals 

used in centrifugal compressors. As shown in Table 5, about 50% of all the centrifugal 

compressors in the Transmission, Storage and Processing segments that were reported to 

the GHGRP have wet seals. This value is intermediate to the values reported in the GHG 

EI (80%) and the API/ANGA report (21% - processing only). 

Ultimately, the Greenhouse Gas Emissions Inventory provides a good benchmark for 

national compressor emissions and that source suggests that emissions are extensive (see Table 

3-9 indicating emissions greater than 2,000 Gg CH4). We note that the Inventory potential 

emission factors appear to be higher than would be consistent with the total compressor 

emissions factors derived from the GHGRP. Figure 3 shows the GHGRP (non-combustion) total 

emissions for 462 Transmission compressor stations, averaging 20,445 Mscf/yr. As noted 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

13 

earlier, however, the GHGRP data may not fully account for all emissions such as rod packing in 

the not-operating, pressurized mode. In either case, available data suggests national emissions 

from compressors and compressor stations are an extensive source of emissions. 

Figure 3. Subpart W Transmission and Storage Compressor Station Emissions.

C. Additional Studies (Question 12) 

Colorado State University and Carnegie Mellon University are leading two studies 

designed to characterize methane emissions from specific natural gas industry segments: one 

focuses on Transmission and Storage and the other on Gathering and Processing. Both are part 

of the series catalyzed by EDF and are being done in partnership with natural gas operators. 

Both studies will report emission measurements taken at representative facilities where 

compressors are a significant source of emissions and are on track to submit papers for 

publication in peer-reviewed scientific journals by late summer 2014. 

Another project sponsored by EDF that focuses on emissions from oil and gas operations 

in the Barnett Shale will result in facility-level emission data from a limited number of 

midstream and transmission facilities in the area. Papers summarizing the results of this work 

should be submitted for publication in peer-reviewed scientific journals by late 2014.

 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.E-01 1.E+00 1.E+01 1.E+02 1.E+03

cumulative probability

transmission facility emission rate (kg CH4 / hr)
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

14 

III. MITIGATION TECHNOLOGY: EFFECTIVENESS & COST 

EPA’s Compressor Paper presents a number of different mitigation technologies, 

discusses the effectiveness of those technologies, and provides information on cost. Broadly 

speaking, the technologies EPA presents can be classified as either emissions avoidance 

(reciprocating rod-packing replacement and centrifugal compressor dry seal retrofit) or emissions 

capture (rock packing venting capture and wet seal degassing capture). Part III.A of our 

response discusses reciprocating compressor mitigation technology, including both rod packing 

replacement and reciprocating compressor gas capture. Part III.B discusses centrifugal 

compressor mitigation technology, with focus on wet seal degassing capture technologies and the 

charge questions EPA presents around this technology. These two sections respond generally to 

charge questions 4 and 5 and likewise include specific responses to questions 7, 8, 9, and 10. 

EPA has also requested comment on additional technologies capable of reducing vented 

emissions at compressors (charge questions 3 and 6), and in response, Part III.C discusses leak 

detection and repair (“LDAR”). 

We conclude: 

• Regular rod-packing replacement is an effective, low-cost way to mitigate emissions, 

which ICF projected would reduce emissions by 3.6 billion cubic feet (bcf) of methane 

(approximately 69,000 metric tons) at a cost of $6.11/mcf reduced. Rod-packing 

replacement can be optimized by inspecting for excess seal emissions as part of a regular 

leak detection and repair program, enhancing control effectiveness. 

• Rod-packing gas capture systems can have greater effectiveness than rod-packing 

replacement and, in many cases, result in beneficial use of otherwise wasted gas. 

• Wet-seal degassing capture systems are feasible, result in substantial reductions and are 

highly-cost effective. If broadly applied at centrifugal compressors not currently subject 

to the NSPS, ICF projected that such capture systems would reduce methane by 19.1 

billion cubic feet (over 360,000 metric tons) while achieving net savings of $3.07/mcf of 

methane reduced. 

• LDAR and gas capture technologies can also effectively mitigate non-seal compressor 

emissions. 

A. Reciprocating Compressor Mitigation Technology; Effectiveness; and 

Cost (Charge Questions #4, 5) 

This section addresses reciprocating compressor rod-packing mitigation technology, 

effectiveness, and cost, focusing on rod-packing replacement and also briefly discussing 

capturing of vented gas. 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

15 

1. Rod-Packing Replacement 

EPA’s Compressor Paper describes how rod-packing replacement can mitigate emissions 

as well as the costs of replacing rod-packing. We note that rod-packing replacement costs are 

very low (on the order of $1,620 per cylinder, occurring on average every four years9) but 

otherwise do not have specific comments on EPA’s description of the technology or presentation 

of costs. Instead, we focus on mitigation potential and discuss the analysis EPA presents in the 

Compressor Paper (drawing from the TSD for NSPS OOOO

10). We also present an analysis 

from ICF’s Cost Curve Report, which is referenced in the Compressor Paper but not fully 

characterized. Finally, we note the benefits an LDAR program could have in optimizing the 

cost-effectiveness of rod-packing replacement. 

The ICF Cost-Curve Report provides a helpful description of rod-packing emissions and 

the way in which replacing rod-packing can help mitigate emissions. Without fully recounting 

EPA’s discussion in the Compressor Paper, we emphasize a few points key to determining the 

mitigation potential of this practice: 

• EPA concludes that even newly-installed rod-packing emits methane, but these emissions 

increase over time as rod-packing wears out. 

• Replacing rod packing can lower emissions by ensuring worn-out, higher-emitting seals 

are replaced with lower emitting seals. This is not, however, a one-time action, and to be 

effective, rod-packing must be replace regularly over time (EPA suggests every 26,000 

operating hours or approximately 3 years). 

EPA estimated the effectiveness of rod-packing replacement in the NSPS OOOO TSD 

for most segments by taking the average rod-packing emissions factor from the 1996 EPA / GRI 

study and subtracting the emissions factor for newly-installed rod-packing, which EPA 

determined was 11.5 scf/h per seal. EPA performed this analysis focused on the likely impacts 

of rod-packing replacement at new compressors. The results are summarized in Table 4-1 on 

page 31 of the Compressor Paper, and for transmission sector reciprocating compressors, the 

analysis yields a control efficiency of approximately 80%. 

IFC took a different approach to estimating the effectiveness of rod packing replacement. 

EPA’s Compressor Paper cites ICF’s assumed control effectiveness of 98% for timely rodpacking replacement, which appears in the appendix material of the report. In the body of the 

report, however, ICF makes more detailed assumptions with different results, summarized in 

Table 3-5 from the ICF report, reproduced below. 

 

9

 EPA, Natural Gas STAR Program, Lessons Learned: Reducing Methane Emissions from Compressor Rod Packing 

Systems (2006). 

10 EPA, Technical Support Document for NSPS Subpart OOOO (2012).
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

16 

In particular, ICF assumes that new rod-packing emits 11.5 scf/h (consistent with NSPS 

OOOO). ICF also adopts EPA’s average emissions factor for transmission compressor seals (57 

scf/h) and assumes that, after 3 years, rod-packing emissions will increase linearly to this value, 

and, if not replaced, will continue to increase in a linear fashion every year beyond that. Finally, 

ICF assumes that rod-packing would be replaced every five years and takes the difference 

between three- and five-year replacements averaged over the course of 15 years as the per-year 

effectiveness of a replacement strategy. 

Though different, both of these approaches provide helpful insights into the effectiveness 

of rod-packing replacement program. Similar to other regular maintenance programs like 

LDAR, the first time facilities undertake such programs, emissions reductions will likely be 

greater given that facilities may not have performed these activities at regular intervals (or at all)

in the past. After instituting a regular maintenance program, ongoing maintenance is critical to 

ensure emissions do not return to pre-program levels, though per-year reductions will be less 

than initial reductions given the established mitigation baseline. Accordingly, EPA’s averagevalue based approach more closely approximates substantial reductions that could be expected 

from a first round of rod-packing replacement at existing compressor facilities, and ICF’s 

analysis assumes some baseline level of control and reductions that could be achieved in addition 

to that level, reflecting the effectiveness of ongoing maintenance. 

Though we discuss the importance of LDAR at compressor stations more fully below, 

here, we note that regular LDAR at compressor stations can help optimize both the effectiveness 

and cost of rod-packing replacements. The rate of wear on cylinder rod-packing is not uniform, 

and, while regular replacement will yield benefits, a LDAR program will enhance these benefits 

in several important ways. First, it will allow operators to perform regular maintenance – short 

of rod-packing replacement – that keeps rod-packing emissions as close to 11.5 scf/h as possible. 

Second, it will allow operators to pinpoint the time rod-packing must be replaced, whether before 

or after the 26,000 hours identified in NSPS OOOO. Finally, it will allow operators to more 

swiftly identify equipment failures that result in significant rod-packing emissions. This type of 

smart rod-packing replacement could, in principle, result in an effectiveness of close to 100%, 

assuming a smart approach could maintain emissions at levels close to 11.5 scf/h. These benefits 

are, of course, in addition to the ancillary benefits LDAR would have for reducing leaks from 

other components at compressor facilities. 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

17 

This type of smart rod-packing replacement program could also improve costs by 

allowing for less costly repairs, in lieu of replacement, and leveraging rod-packing replacement 

costs when necessary. Moreover, in addition to conventional LDAR approaches, there are 

several examples of flow-meter technologies that can inexpensively and accurately monitor rodpacking vents. For instance, Cook Compression offers portable, inline, and insertion meters 

that can measure and track packing case vent flow with an accuracy of ±1% reading, ± 0.5% full 

scale.11 Hoerbiger also offers rod packing monitor kits that can help operators determine when 

to change the packing.12

 

Ultimately, we emphasize that there are real and significant benefits to a regular rodpacking replacement program, though underscore that deploying LDAR alongside such a 

program will only enhance the effectiveness of rod packing replacement and potentially reduce 

the costs of such a program. 

2. Rod-Packing Gas Recovery (Charge Question #9) 

EPA’s Compressor Paper also presents information on the possibility of capturing gas 

from rod-packing vents that would otherwise be wasted, focusing on one technology that would 

capture gas and reroute it back to the compressor for beneficial use and another that would 

leverage existing capture infrastructure by connecting to site vapor recovery units. As a 

threshold matter, we note that the gas capture systems described in the Compressor Paper are 

more effective at reducing emissions than rod packing replacements (which, even when newlyinstalled, leak methane). Also, these systems can capture vented emissions from other points at 

the compressor stations, so encourage beneficial use of the vented gas and can have emission 

reduction benefits beyond those described in the Compressor Paper. Accordingly, we urge EPA 

to strongly consider these technologies. 

Moreover, the two technologies EPA presents – VRUs and REM Technology’s capture 

system – are complementary and can potentially address emissions at the vast majority of 

compressor sites. EPA’s Compressor Paper notes that the incremental costs of manifolding 

reciprocating compressor rod packing vents to an existing site VRU are small. In instances 

where a VRU isn’t present, however, adding a VRU along with needed additional compression 

could have significant cost impacts, but the REM technologies system provides an important 

alternative. Because the REM system reroutes vent gas to the compressor air intake, no 

additional compression is needed and the technology is therefore capable of capturing emissions 

where onsite compression and VRUs are unavailable. Accordingly, the gas capture technologies 

are capable of addressing substantial emissions from compressor stations, and we encourage 

EPA, when it considers the cost of these systems, to assess them in light of the full suite of 

vented emissions sources they can address. 

 

11 SentrixTM Portable, Inline & Insertion Meters, Cook Compression, available at 

http://www.cookcompression.com/index.cfm/lev2/1869/Sentrix%E2%84%A2.Gas.Flow.Measurement. 

12 U.S. Dept. Of Energy, Methane Mitigation Workshop, http://www.arpa-e.energy.gov/sites/default/ 

files/documents/files/Methane_Mitigation_Workshop_McCoy.pdf. 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

18 

In response to EPA’s charge question, we are not aware of any technical limitations to 

installing this technology. 

B. Centrifugal Compressor Mitigation Technology; Effectiveness; and Cost 

(Questions 4, 5, 7, 8, 9, 10) 

EPA’s Compressor Paper also present centrifugal compressor mitigation technologies, 

including dry seal retrofits and wet seal capture systems. The benefits of dry seal systems are 

manifest, and we agree with the Compressor Paper’s conclusion that, given these substantial 

benefits and the small incremental costs of installing dry seal systems, dry seals are preferable 

for new installations. Indeed, the ICF Report estimates that 90% of new compressor installations 

are equipped with dry seals. Moreover, ICF estimates that, even in retrofit applications with 

higher upfront costs, dry seal retrofits have a payback period of 29 months. 

In this section, however, we focus on wet seal capture systems, given their relatively 

lower costs and the substantial emissions that come from these devices, as described in Part II, 

above. 

1. Wet Seal Degassing Capture 

The ICF Cost-Curve Report has classified wet-seal gas capture technologies as one of the 

most significant emissions mitigation opportunities as well as one of the most cost-effective 

(with payback periods of 3 months and less depending on the size of the facility). The ICF 

Report provides and in-depth analysis of retrofitting wet seal systems, and EPA has appropriately 

characterized that analysis in the Compressor Paper. Here, we only emphasize that wet seal 

degassing capture systems are feasible in retrofit applications and that a variety of uses are 

available for the captured gas. 

In addition, BP recently presented information at a Natural GasStar Technology Transfer 

Workshop, indicating that 100 centrifugal compressors it owns in Alaska operate using wet seal 

degassing technology, which achieves a 99% emission reduction and is extremely reliable (no 

failures in 30 years of operation).13 Further, the technology has a payback period of less than a 

month, assuming a gas price of $3 mscf. 

EDF has asked a consultant to perform further analyses of system design and cost with 

several technology vendors and have learned that capture systems have been installed in retrofit 

applications. We are working to refine cost estimates of a design similar to the BP system along 

with other possible designs. 

C. Importance of LDAR at Compressor Stations (Question 6) 

EPA has requested comment on additional technologies capable of achieving compressor 

reductions. In Part III.A, above, we described how leak detection and repair practices could help 

 

13 BP, Centrifugal Compressor Wet Seal: Sea Oil Degassing & Recovery (Feb. 11, 2014), http://www.epa.gov/ 

outreach/gasstar/documents/workshops/denver-2014/Seal_Oil_Degassing.pdf. 
ENVIRONMENTAL DEFENSE FUND • JUNE 16, 2014 

19 

optimize rod-packing replacement. Though we discuss equipment leaks in a separate paper, 

here, we emphasize that LDAR at compressor stations can achieve substantial, additional 

reduction by minimizing equipment leaks and can do so in a very cost-effective fashion. Several 

reports have included data supporting these conclusions: 

• The ICF report found LDAR for compressor stations (storage) to be cost effective, with 

potential methane reductions of 1.5 bcf per year at an annualized cost of -$4.5 million per 

year and initial capital costs of $2.8 million (-$3.03/mcf methane reduced). The report 

also found LDAR for compressor stations (transmission) to be cost effective, with 

potential methane reductions of 2.8 bcf per year at an annualized cost of $7.7 million per 

year and initial capital costs of $5.3 million ($2.79/mcf methane reduced).14

• The Carbon Limits report found an average net present value for performing an LDAR 

survey at a compressor station of $3,376, with aggregate abatement costs of -$1.7/tCO2e 

in the production segment and -$8.3/tCO2e in the transmission segment.15

• The Clearstone II analysis found that methane leaks from gathering compressor stations 

averaged 97 tonnes per year per facility, resulting in $49,018 in lost gas value from the 

leaking components.16 

D. Errata 

1) In the Compressor Paper, there are two typos in the second paragraph of Section 3.7. The 

Section numbers referenced should be 3.7.1 and 3.7.2. 

2) Table 3-5 in the Compressor Paper is incorrectly referred to as Table 3-4.

 

14 ICF Cost Curve Report at 4-4. 

15 Carbon Limits, Quantifying Cost-effectiveness of Systematic Leak Detection and Repair Programs Using Infrared 

Cameras at 17, 30 (Mar. 2014). 

16 Clearstone Engineering, Cost-Effective Directed Inspection and Maintenance Control Opportunities at Five Gas 

Processing Plants and Upstream Gathering Compressor Stations and Well Sites at 28 (2006).