### Centrifugal Compressors 1



### This memo investigates potential improvements to th e centrifugal compressor emissions estimation methodologies used in the U.S. Inventory . Recent data collected and released publicly by industry and the Natural Gas STAR Progr am 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 f or the trend toward dry seals in new centrifugal compressors.

Background Centrifugal compressors are widely used in producti on and transmission of natural gas. Seals on the rotating shafts prevent the high press ure 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 o il 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 techniqu es) and re-circulated. The recovered methane is commonly vented to the atmosphere.

Experiences that companies have shared with the Nat ural Gas STAR Program show that methane emissions from wet seal degassing vents typ ically 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 typica lly 6 scfm or less of methane. These types of seals have become increasingly common in t he U.S. natural gas industry.

The current Inventory does not differentiate betwee n these seal types and underestimates the overall emissions. It has recently been determ ined that once Natural Gas STAR Reductions are subtracted from the Inventory emissi ons estimates for this source, the net impact on emissions from centrifugal compressors se als 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 report 2 

### which collected sample data in 1992. GRI/EPA noted the trend towar d 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 co mpressors with dry seals. The Inventory does not account for this trend toward dr y seals in new compressors.

1 Developed with technical support from ICF Internat ional.

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 co nsider all leaking components on the

compressor itself or immediately adjacent to it. T he leaks were detected using tools such as organic vapor analyzers and toxic vapor analyzer s 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 factor s 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 press urized (43.6%) and the 11.1% of compressors which h ave sources routed to flares per EPA/GRI 1996 report, s ee 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 fo r 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/G RI 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 compr essor. The majority of the emissions

from degassing the pressurized seal oil is vented t hrough 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 ce ntrifugal compressors per compressor station. This was extrapolated to the n ational level using the total number of compressor stations (the total national count of th e number of compressor stations is extrapolated via pipelines miles; so effectively th e 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 m 3 

### of methane per year. Converted to English units at standard conditions, this is equiv alent to nearly 525,000,000 scf each year. Since these samples were from processing pla nts, EPA converted the average annual emissions to emission per minute using the a verage 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 Facto r

scfm hour

hours year

s compressor

year scf EF wetseals 

### 7.47 %6.43 1

min 60 1

760,8 1

48 / 000, 000, 525

= × × × =

EPA assumed that the midpoint (3 scfm) of the repor ted 

### Lessons Learned 3 

### range of up to 6 scfm adequately characterized dry seals. Since t here 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%), transmiss ion (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 develo pment 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 co unts 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 C ompressors . 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 pres surized 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 CountCompressor 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 compr essors in 1992; however, as the report notes, dry seals were an emerging trend. A present ation in 2003 on centrifugal compressors in the natural gas industry 5 

### shows that vendors of compressors report that 90% of new compressors sold are equipped with dry s eals. Since the emissions rates are substantially different between centrifugal compres sors equipped with wet seals and those equipped with dry seals, EPA has deemed it ap propriate to account for each separately in the national Inventory.

The new approach used the base year activity data c ollected 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 upda ted the split between those equipped with wet seals and dry seals and also the associate d emission factors.

Using the EPA/GRI report, this analysis assumed tha t in 1992 and earlier, all centrifugal compressors were equipped with wet seals. It drive s the total number of centrifugal compressors each year using the ratio of centrifuga l compressors in 1992 to pipeline miles in 1992 and multiplying it by the total numbe r of pipeline miles (reported to

DOT 6 

### ), as shown in Exhibit 4 below.



### Exhibit 4. Equation for Total Centrifugal Compress or Count

x x les PipelineMi les PipelineMi essors TotalCompr

essors TotalCompr ×

=

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 ea ch year. Between 1992 when there were negligible dry seal compressors and 2003 when 90% of new centrifugal compressors were equipped with dry seals, EPA inter polated a straight-line estimate of the percentage of new compressors that were equippe d with dry seals. That is, the percentage of new compressors equipped with dry sea ls 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 comp ressors were retired, not newly

5 EPA. Methane Savings from Compressors . Oklahoma City, OK. May 14, 2009. <http://epa.gov/gasstar/documents/workshops/okcity2 009/icf_compressors.pdf>.

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

. <http://www.phmsa.dot.gov/portal/site/PHMSA/menuite m.ebdc7a8a7e39f2e55cf2031050248a0c/?vgnextoi d=a872dfa122a1d110VgnVCM1000009ed07898RCRD&vgnextch annel=3430fb649a2dc110VgnVCM100 0009ed07898RCRD&vgnextfmt=print>.

### installed compressors. Thus, when compressors coun ts decreased, it was assumed that

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

Companies that retrofit their wet seals are account ed 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 Cent rifugal Compressors Equipped with Dry Seals and Wet Seals after 1992.

x x x 

### DrySeals

essors TotalCompr WetSeals

−

=

1992 1992 

### essors TotalCompr WetSeals = If the total count of compressor increases between 1992 and 2003:



## ( ) ) 1992 2003 ( 1992 % 90 1



### −

− × ×

− =

− 

### x essors TotalCompr essors TotalCompr DrySeals

x x x

If the total count of compressor increases from 200 3 or after:



## ( ) % 90 1 

### ×

− =

− x x x 

### essors TotalCompr essors TotalCompr DrySeals

If the total count of compressors decreases:

1

− 

### =
x x 

### DrySeals DrySeals

Where,

WetSeals

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

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

x = the total number of centrifugal compressors in y ear x CompressorsRetrofitted

x = the number of compressors retrofitted from wet s eals 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 f actors is displayed in Exhibit 6 below, based on the 1990-2008 final Inventory data.



### Results The revised emissions factors were multiplied by th e revised activity to yield the emissions displayed in Exhibit 7. The total revise d centrifugal compressor emissions were compared to the published 2008 Inventory estim ates 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 se als penetrated the market further, the

difference between the revised emissions estimates and the previous methodology are

smaller.

Some companies have retrofitted existing centrifuga l 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 p revious methodology when Natural Gas STAR segment total reductions are subtr acted 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- 2 009 Inventory update cycle. It

more accurately represents the emissions from centr ifugal compressors, the types of centrifugal compressors that are in industry.Exhibit 6. Centrifugal Compressor Activity Factors1990 – 2008Processing Compressor Counts1990199119921993199419951996199719981999Wet seal compressors 665 660 665 674 694 688 695 696 698 691Dry seal compressors - - - 1 5 5 8 9 11 11200020012002200320042005200620072008Wet seal compressors 696 700 674 675 656 635 637 640 644Dry seal compressors 20 32 32 37 37 37 54 80 117Transmission Compressor Counts1990199119921993199419951996199719981999Wet seal compressors 699 703 698 702 718 707 696 699 709 693Dry seal compressors - - - 0 4 4 4 6 16 16200020012002200320042005200620072008Wet seal compressors 696 675 681 677 677 671 671 671 671Dry seal compressors 20 20 46 46 49 49 49 50 55Underground Storage Compressor Counts1990199119921993199419951996199719981999Wet seal compressors 104 108 111 117 114 114 121 114 103 105Dry seal compressors - - - 1 1 1 4 4 4 7200020012002200320042005200620072008Wet seal compressors 108 102 102 103 98 97 86 87 87Dry seal compressors 11 11 14 18 18 18 18 25 29Exhibit 7. Revised Centrifugal Compressor Emissions Estimates 1990 – 2008 (Bcf)Processing Emissions1990199119921993199419951996199719981999Wet seal compressors 12.48 12.37 12.47 12.65 13.0112.91 13.03 13.05 13.10 12.96Dry seal compressors - - - 0.01 0.04 0.04 0.07 0.08 0.10 0.10200020012002200320042005200620072008Wet seal compressors 13.05 13.13 12.64 12.65 12.30 11.90 11.94 11.99 12.07Dry seal compressors 0.18 0.29 0.29 0.34 0.340.34 0.49 0.74 1.07Transmission Emissions1990199119921993199419951996199719981999Wet seal compressors 12.81 12.89 12.79 12.86 13.1712.96 12.75 12.81 13.00 12.71Dry seal compressors - - - 0.00 0.04 0.04 0.040.07 0.18 0.18200020012002200320042005200620072008Wet seal compressors 12.75 12.38 12.48 12.41 12.41 12.30 12.29 12.30 12.31Dry seal compressors 0.24 0.24 0.54 0.54 0.570.57 0.57 0.59 0.65Underground Storage Emissions1990199119921993199419951996199719981999Wet seal compressors 1.72 1.79 1.84 1.95 1.90 1.90 2.01 1.89 1.71 1.75Dry seal compressors - - - 0.01 0.01 0.01 0.040.04 0.04 0.08200020012002200320042005200620072008Wet seal compressors 1.79 1.69 1.70 1.70 1.621.60 1.42 1.44 1.44Dry seal compressors 0.13 0.13 0.16 0.21 0.210.21 0.21 0.30 0.34Note: Tables compare the 2008 Inventory to 2009 “revised” methods. In addition to the revised methods, some small differences exist in the activity data due torevised historical values from the published sources.Exhibit 8. Comparison of Revised Estimates to Former Estimates 1990 – 2008 (Bcf)Processing Emissions19901991199219931994199519961997199819992008 Inventory 5.16 5.12 5.16 5.24 5.42 5.38 5.45 5.47 5.51 5.45Revised Estimates 12.48 12.37 12.47 12.65 13.01 12.91 13.03 13.05 13.10 12.962000200120022003200420052006200720082008 Inventory 5.55 5.68 5.48 5.52 5.38 5.21 5.36 5.53 5.98Revised Estimates 13.05 13.13 12.64 12.65 12.3011.90 11.94 11.99 12.07Transmission Emissions19901991199219931994199519961997199819992008 Inventory 7.74 7.79 7.73 7.78 8.00 7.87 7.75 7.81 8.03 7.85Revised Estimates 12.81 12.89 12.79 12.86 13.17 12.96 12.75 12.81 13.00 12.712000200120022003200420052006200720082008 Inventory 7.93 7.70 8.05 8.00 8.04 7.97 7.96 7.98 7.81Revised Estimates 12.75 12.38 12.48 12.41 12.4112.30 12.29 12.30 12.31Underground Storage Emissions19901991199219931994199519961997199819992008 Inventory 1.16 1.21 1.24 1.95 1.90 1.90 2.01 1.89 1.71 1.75Revised Estimates 1.72 1.79 1.84 1.93 1.88 1.88 2.00 1.88 1.70 1.762000200120022003200420052006200720082008 Inventory 1.79 1.69 1.70 1.70 1.62 1.601.42 1.44 1.44Revised Estimates 1.85 1.75 1.78 1.84 1.76 1.74 1.56 1.66 1.71Note: Tables compare the 2008 Inventory to 2009 “revised” methods. In addition to the revised methods, some small differences exist in the activity data due torevised historical values from the published sources.