Centrifugal Compressors1

This memo investigates potential improvements to the centrifugal compressor emissions 

estimation methodologies used in the U.S. Inventory. Recent data collected and released 

publicly by industry and the Natural Gas STAR Program have demonstrated that the 

emissions characterized from centrifugal compressor wet seals are incorrectly 

characterized by the U.S. Inventory. Additionally, the Inventory incorrectly projects the 

number of wet seals in operation by not adjusting for the trend toward dry seals in new 

centrifugal compressors. 

Background 

Centrifugal compressors are widely used in production and transmission of natural gas. 

Seals on the rotating shafts prevent the high pressure natural gas from escaping the 

compressor casing. Traditionally, these seals used high-pressure oil as a barrier against 

escaping gas. Very little gas escapes through the oil barrier; considerably more gas is 

absorbed by the oil under the high pressures at the “inboard” (compressor side) seal 

oil/gas interface, thus contaminating the seal oil. Seal oil is purged of the absorbed gas 

(using heaters, flash tanks, and degassing techniques) and re-circulated. The recovered 

methane is commonly vented to the atmosphere. 

Experiences that companies have shared with the Natural Gas STAR Program show that 

methane emissions from wet seal degassing vents typically range from 40 to 200 standard 

cubic feet per minute (scfm). 

Another common type of centrifugal compressor seals, called “dry seals”, use high 

pressure gas to seal the compressor and emit typically 6 scfm or less of methane. These 

types of seals have become increasingly common in the U.S. natural gas industry. 

The current Inventory does not differentiate between these seal types and underestimates 

the overall emissions. It has recently been determined that once Natural Gas STAR 

Reductions are subtracted from the Inventory emissions estimates for this source, the net 

impact on emissions from centrifugal compressors seals in the transmission segment is a 

negative 6.44 Bcf. The purpose of this memo is to more accurately account for methane 

emissions from centrifugal compressors in the U.S. Inventory. 

Current Inventory Methodology 

The U.S. Inventory currently bases its estimates on the GRI/EPA report2

 which collected 

sample data in 1992. GRI/EPA noted the trend toward dry gas seal technology; however, 

it was a recent trend for new installations and was estimated to have negligible impact on 

emissions from compressor seals in 1992. The U.S. Inventory deducts Natural Gas 

STAR reported reductions from the conversion of wet seals to dry seals, but that only 

represents conversions – not the addition of new compressors with dry seals. The 

Inventory does not account for this trend toward dry seals in new compressors. 

 

1

 Developed with technical support from ICF International. 

2

 EPA/GRI. Methane Emissions from the Natural Gas Industry. Volume 8, June 1996. 

<http://epa.gov/gasstar/documents/emissions_report/8_equipmentleaks.pdf>. 
The emission factors for centrifugal compressors consider all leaking components on the 

compressor itself or immediately adjacent to it. The leaks were detected using tools such 

as organic vapor analyzers and toxic vapor analyzers and measured using high volume 

samplers. The average number of leaking components and the average emissions of the 

sample were combined to develop the emission factors in Exhibit 1. 

Exhibit 1. GRI/EPA Centrifugal Compressor Emission Factors 

Processing 

Component Component Emission Factor 

(Mcf/component/year) Average Component Count 

Compressor Blowdown OpenEnded Line 6,447 1 

Compressor Starter Open-ended 

Line 1,341 1 

Compressor Seal 228 1.5 

Miscellaneous 31 1 

TOTAL (Mcf/compressor/year) 7,750 

Note: total emission factor adjusted for time pressurized (43.6%) and the 11.1% of compressors which have 

sources routed to flares per EPA/GRI 1996 report, see Table 4-14 of Volume 8. 

Transmission 

Component Component Emission Factor 

(Mcf/component/year) Average Component Count 

Compressor Blowdown OpenEnded Line 9,352 1 

Compressor Starter Open-ended 

Line 1,440 1 

Compressor Seal 165 1.5 

Miscellaneous 18 1 

TOTAL (Mcf/compressor/year) 11,100 

Underground Storage 

Component Component Emission Factor 

(Mcf/component/year) Average Component Count 

Compressor Blowdown OpenEnded Line 10,233 1 

Compressor Starter Open-ended 

Line 1,440 0.5 

Compressor Seal 126 1.5 

Miscellaneous 17 1 

TOTAL (Mcf/compressor/year) 11,160 

EPA noted that the compressor seal emission rate for wet seals is lower than the rates 

documented by industry through the Natural Gas STAR Program and their publicly 

available measurement studies. For comparison, the EPA/GRI rate of 1 scfm (which is 

126 Mcf/year divided by the number of minutes EPA/GRI assumes the compressor to be 

pressurized – 24.2% of the year) is lower than the 40 to 200 scfm observed by industry by 

one to two orders of magnitude. Since the EPA/GRI study measured only the compressor 

and adjacent components for leaking emissions, EPA considers that the 1 scfm is 
indicative of leakage at the seal face on the compressor. The majority of the emissions 

from degassing the pressurized seal oil is vented through a stack. 

For the activity factor, a survey of the number of centrifugal compressors and compressor 

stations provided EPA/GRI with a sample ratio of centrifugal compressors per 

compressor station. This was extrapolated to the national level using the total number of 

compressor stations (the total national count of the number of compressor stations is 

extrapolated via pipelines miles; so effectively the number of centrifugal compressors is 

extrapolated by pipeline miles as well.) 

New Centrifugal Compressors EFs 

A recent World Gas Conference paper published that 48 sample measurements of wet 

seal degassing vents account for 14,860 thousand m3 of methane per year. Converted to 

English units at standard conditions, this is equivalent to nearly 525,000,000 scf each 

year. Since these samples were from processing plants, EPA converted the average 

annual emissions to emission per minute using the average operating factor for 

processing compressors that is reported by EPA/GRI, 43.6%. The resulting factor is 47.7 

scfm, well within the industry observed range. 

Exhibit 2. Development of Wet Seals Emission Factor 

scfm hour

hours

year

compressors

scf year EFwetseals 7.47

%6.43

1

60min

1

760,8

1

48

525 000, 000, /

= × × × =

EPA assumed that the midpoint (3 scfm) of the reported Lessons Learned3

 range of up to 

6 scfm adequately characterized dry seals. Since there are typically 2 dry seals per 

compressor, the total per-compressor emissions rate from dry seals is 6 scfm. 

These per-minute seal rates were then converted to annual emissions rates using the 

operating factors for processing (43.6%), transmission (30%4), and underground storage 

(22.4%) respectively, as reported in EPA/GRI. The resulting factors were used to replace 

the average seal emissions rates used in the development of the EPA/GRI per-compressor 

emission factors to develop more accurate ones for use in the inventory as demonstrated 

in Exhibit 3. 

Exhibit 3. Revised Centrifugal Compressor Inventory Emission Factors 

Note: sum products of individual components and counts may not match the final total factor due to 

independent rounding 

Processing – Dry Seals 

Component Component Emission Factor 

(Mcf/component/year) Average Component Count 

Compressor Blowdown OpenEnded Line 6,447 1 

Compressor Starter Open-ended 1,341 1 

 

3

 EPA. Replacing Wet Seals with Dry Seals in Centrifugal Compressors. Lessons Learned from Natural 

Gas STAR Partners. EPA430-B-03-012, October 2006. 

<http://epa.gov/gasstar/documents/ll_wetseals.pdf>.

4

 This is time pressurized – both operating and pressurized and not-operating and pressurized. 
Line 

Compressor Seal (dry) 687 2 

Miscellaneous 31 1 

TOTAL 9,194 

Processing – Wet Seals 

Component Component Emission Factor 

(Mcf/component/year) Average Component Count 

Compressor Blowdown OpenEnded Line 6,447 1 

Compressor Starter Open-ended 

Line 1,341 1 

Compressor Seal (wet) 10,931 1 

Miscellaneous 31 1 

TOTAL 18,750 

Transmission – Dry Seals 

Component Component Emission Factor 

(Mcf/component/year) Average Component Count 

Compressor Blowdown OpenEnded Line 9,352 1 

Compressor Starter Open-ended 

Line 1,440 1 

Compressor Seal (dry) 473 2 

Miscellaneous 18 1 

TOTAL 11,756 

Transmission – Wet Seals 

Component Component Emission Factor 

(Mcf/component/year) Average Component Count 

Compressor Blowdown OpenEnded Line 9,352 1 

Compressor Starter Open-ended 

Line 1,440 1 

Compressor Seal (wet) 7,521 1 

Miscellaneous 18 1 

TOTAL 18,331 

Underground Storage – Dry Seals 

Component Component Emission Factor 

(Mcf/component/year) Average Component Count 

Compressor Blowdown OpenEnded Line 10,233 1 

Compressor Starter Open-ended 

Line 1,440 0.5 

Compressor Seal (dry) 353 2 

Miscellaneous 17 1 

TOTAL 11,676 

Underground Storage – Wet Seals

Component Component Emission Factor 

(Mcf/component/year) Average Component Count 
Compressor Blowdown OpenEnded Line 10,233 1 

Compressor Starter Open-ended 

Line 1,440 0.5 

Compressor Seal (wet) 5,616 1 

Miscellaneous 17 1 

TOTAL 16,586 

New Centrifugal Compressors AFs 

EPA used the activity factors for centrifugal compressors in 1992; however, as the report 

notes, dry seals were an emerging trend. A presentation in 2003 on centrifugal 

compressors in the natural gas industry5 shows that vendors of compressors report that 

90% of new compressors sold are equipped with dry seals. Since the emissions rates are 

substantially different between centrifugal compressors equipped with wet seals and 

those equipped with dry seals, EPA has deemed it appropriate to account for each 

separately in the national Inventory. 

The new approach used the base year activity data collected in the EPA/GRI report, and 

also used the methodology to drive the total count of centrifugal compressors to other 

years (using transmission pipeline miles.) It updated the split between those equipped 

with wet seals and dry seals and also the associated emission factors. 

Using the EPA/GRI report, this analysis assumed that in 1992 and earlier, all centrifugal 

compressors were equipped with wet seals. It drives the total number of centrifugal 

compressors each year using the ratio of centrifugal compressors in 1992 to pipeline 

miles in 1992 and multiplying it by the total number of pipeline miles (reported to 

DOT6

), as shown in Exhibit 4 below. 

Exhibit 4. Equation for Total Centrifugal Compressor Count 

x x PipelineMiles

PipelineMiles

TotalCompressors TotalCompressors = ×

1992

1992

Given that 90% of new centrifugal compressors since 2003 are equipped with dry seals, 

and that there were 0 dry seal compressors in 1992; Exhibit 5 demonstrates how the 

number of dry seal compressors was estimated for each year. Between 1992 when there 

were negligible dry seal compressors and 2003 when 90% of new centrifugal 

compressors were equipped with dry seals, EPA interpolated a straight-line estimate of 

the percentage of new compressors that were equipped with dry seals. That is, the 

percentage of new compressors equipped with dry seals increases linearly between 0% in 

1992 and 90% in 2003. In years where the estimated total number of centrifugal 

compressors decreased, it was assumed that old compressors were retired, not newly 

 

5

 EPA. Methane Savings from Compressors. Oklahoma City, OK. May 14, 2009. 

<http://epa.gov/gasstar/documents/workshops/okcity2009/icf_compressors.pdf>. 

6

 DOT. PHMSA Distribution, Transmission, and Liquid Annual Data. 

<http://www.phmsa.dot.gov/portal/site/PHMSA/menuitem.ebdc7a8a7e39f2e55cf2031050248a0c/?vgnextoi

d=a872dfa122a1d110VgnVCM1000009ed07898RCRD&vgnextchannel=3430fb649a2dc110VgnVCM100

0009ed07898RCRD&vgnextfmt=print>. 
installed compressors. Thus, when compressors counts decreased, it was assumed that 

those compressors were equipped with wet seals. This is demonstrated in Exhibit 5. 

Companies that retrofit their wet seals are accounted for when voluntary reductions are 

subtracted from the emissions estimate and were not accounted for in the activity factor. 

Exhibit 5. Equation to Estimate the Number of Centrifugal Compressors Equipped 

with Dry Seals and Wet Seals after 1992. 

x x DrySealsx WetSeals = TotalCompressors −

1992 1992 WetSeals = TotalCompressors

If the total count of compressor increases between 1992 and 2003: 

( )

(2003 1992)

1992

1 90%

−

−

= − − × ×

x

DrySeals TotalCompressors TotalCompressors x x x

If the total count of compressor increases from 2003 or after: 

DrySealsx = (TotalCompressorsx −TotalCompressorsx−1)×90%

If the total count of compressors decreases: 

DrySealsx = DrySealsx−1

Where, 

WetSealsx = the number of compressors with wet seals in year x 

DrySealsx = the number of compressors with dry seals in year x 

TotalCompressorsx = the total number of centrifugal compressors in year x 

CompressorsRetrofittedx = the number of compressors retrofitted from wet seals to have dry seals in year x

x = calendar year 

The U.S. Inventory already estimates the number of total centrifugal compressors for 

each year. The resulting time series of activity factors is displayed in Exhibit 6 below, 

based on the 1990-2008 final Inventory data. 

Results 

The revised emissions factors were multiplied by the revised activity to yield the 

emissions displayed in Exhibit 7. The total revised centrifugal compressor emissions 

were compared to the published 2008 Inventory estimates in Exhibit 8. In each segment, 

the largest increase in emissions is for the years 1990 – 1992 before the dry seals 

technology begun penetrating the market. As dry seals penetrated the market further, the 

difference between the revised emissions estimates and the previous methodology are 

smaller. 

Some companies have retrofitted existing centrifugal wet seal compressor with dry seals 

as a voluntary emission reduction practice. These reductions are reported to the Natural 

Gas STAR Program and already accounted for in the previous methodology when 

Natural Gas STAR segment total reductions are subtracted from the Inventory segment 

total emissions estimates. After this improvement, there is a net positive contribution to 

emissions from centrifugal compressor seals. 

Conclusions 
EPA has implemented this improvement in the 1990- 2009 Inventory update cycle. It 

more accurately represents the emissions from centrifugal compressors, the types of 

centrifugal compressors that are in industry. 
Exhibit 6. Centrifugal Compressor Activity Factors 1990 – 2008 

Processing Compressor Counts 

1990 1991 1992 1993 1994 1995 1996 1997 

Wet seal compressors 665 660 665 674 694 688 695 696 Dry seal compressors - - - 1 5 5 8 9 

2000 2001 2002 2003 2004 2005 2006 2007 

Wet seal compressors 696 700 674 675 656 635 637 640 Dry seal compressors 20 32 32 37 37 37 54 80 Transmission Compressor Counts 

1990 1991 1992 1993 1994 1995 1996 1997 

Wet seal compressors 699 703 698 702 718 707 696 699 Dry seal compressors - - - 0 4 4 4 6 

2000 2001 2002 2003 2004 2005 2006 2007 

Wet seal compressors 696 675 681 677 677 671 671 671 Dry seal compressors 20 20 46 46 49 49 49 50 Underground Storage Compressor Counts 

1990 1991 1992 1993 1994 1995 1996 1997 

Wet seal compressors 104 108 111 117 114 114 121 114 Dry seal compressors - - - 1 1 1 4 4 

2000 2001 2002 2003 2004 2005 2006 2007 

Wet seal compressors 108 102 102 103 98 97 86 87 Dry seal compressors 11 11 14 18 18 18 18 25 
Exhibit 7. Revised Centrifugal Compressor Emissions Estimates 1990 – 2008 (Bcf) 

Processing Emissions 

1990 1991 1992 1993 1994 1995 1996 1997 

Wet seal compressors 12.48 12.37 12.47 12.65 13.01 12.91 13.03 13.05 Dry seal compressors - - - 0.01 0.04 0.04 0.07 0.08 

2000 2001 2002 2003 2004 2005 2006 2007 

Wet seal compressors 13.05 13.13 12.64 12.65 12.30 11.90 11.94 11.99 Dry seal compressors 0.18 0.29 0.29 0.34 0.34 0.34 0.49 0.74 Transmission Emissions 

1990 1991 1992 1993 1994 1995 1996 1997 

Wet seal compressors 12.81 12.89 12.79 12.86 13.17 12.96 12.75 12.81 Dry seal compressors - - - 0.00 0.04 0.04 0.04 0.07 

2000 2001 2002 2003 2004 2005 2006 2007 

Wet seal compressors 12.75 12.38 12.48 12.41 12.41 12.30 12.29 12.30 Dry seal compressors 0.24 0.24 0.54 0.54 0.57 0.57 0.57 0.59 Underground Storage Emissions 

1990 1991 1992 1993 1994 1995 1996 1997 

Wet seal compressors 1.72 1.79 1.84 1.95 1.90 1.90 2.01 1.89 Dry seal compressors - - - 0.01 0.01 0.01 0.04 0.04 

2000 2001 2002 2003 2004 2005 2006 2007 

Wet seal compressors 1.79 1.69 1.70 1.70 1.62 1.60 1.42 1.44 Dry seal compressors 0.13 0.13 0.16 0.21 0.21 0.21 0.21 0.30 Note: Tables compare the 2008 Inventory to 2009 “revised” methods. In addition to the revised methods, some small differences exist 

revised historical values from the published sources. 
Exhibit 8. Comparison of Revised Estimates to Former Estimates 1990 – 2008 (Bcf) 

Processing Emissions 

1990 1991 1992 1993 1994 1995 1996 1997 

2008 Inventory 5.16 5.12 5.16 5.24 5.42 5.38 5.45 5.47 Revised Estimates 12.48 12.37 12.47 12.65 13.01 12.91 13.03 13.05 

2000 2001 2002 2003 2004 2005 2006 2007 

2008 Inventory 5.55 5.68 5.48 5.52 5.38 5.21 5.36 5.53 Revised Estimates 13.05 13.13 12.64 12.65 12.30 11.90 11.94 11.99 Transmission Emissions 

1990 1991 1992 1993 1994 1995 1996 1997 

2008 Inventory 7.74 7.79 7.73 7.78 8.00 7.87 7.75 7.81 Revised Estimates 12.81 12.89 12.79 12.86 13.17 12.96 12.75 12.81 

2000 2001 2002 2003 2004 2005 2006 2007 

2008 Inventory 7.93 7.70 8.05 8.00 8.04 7.97 7.96 7.98 Revised Estimates 12.75 12.38 12.48 12.41 12.41 12.30 12.29 12.30 Underground Storage Emissions 

1990 1991 1992 1993 1994 1995 1996 1997 

2008 Inventory 1.16 1.21 1.24 1.95 1.90 1.90 2.01 1.89 Revised Estimates 1.72 1.79 1.84 1.93 1.88 1.88 2.00 1.88 

2000 2001 2002 2003 2004 2005 2006 2007 

2008 Inventory 1.79 1.69 1.70 1.70 1.62 1.60 1.42 1.44 Revised Estimates 1.85 1.75 1.78 1.84 1.76 1.74 1.56 1.66 Note: Tables compare the 2008 Inventory to 2009 “revised” methods. In addition to the revised methods, some small differences exist 

revised historical values from the published sources.