Attachment T

Compressors

Ned Jerabek

New Mexico Environmental Department
OIL AND NATURAL GAS SECTOR COMPRESSORS

6.0 CHARGE QUESTIONS FOR REVIEWERS 

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.

COMMENT: I found it difficult to understand the emission factor calculation methodology. As there 

is a very large difference in the emission factors for reciprocating and centrifugal compressors it 

seems logical that the emission factors would have been developed independently and applied 

independently instead of using the component method which “uses average emission factors for 

reciprocating and centrifugal compressor seals and the average number of reciprocating and 

centrifugal compressors per facility to estimate the average facility emissions.” The ICF study 

commissioned by EDF showed a 166% increase compared to the emissions calculations based on the 

EPA/GRI 1996 study. One concludes whatever the deficiencies of either study at a minimum 

methane emissions are at least as large as represented from the emission factors derived from the 

EPA/GRI 1996 study. Furthermore, as an air quality permitting manager from 1997 through 2013, I 

witnessed significant growth and expansion in the oil and gas producing and processing sectors such 

that I believe the updates and revisions made in the ICF study are appropriate and more 

representative. This conclusion is also supported by the URS/UT 2011 study.

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

emissions from compressors in the oil and gas sector? 

COMMENT: Yes, I think the studies and data sources were appropriately characterized, and perhaps 

that was the most workable approach, but it would be more accurate and easier to follow if we had an 

analysis that quantified vented emissions unique to each compressor type.

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.

COMMENT: The technologies discussed in the paper are low-tech easy to understand, and easy to 

see the value of capturing the vented methane emissions and routing them either back to the process 

or using them as a fuel on site. This is hard to argue against and sounds very much like a MACT 

floor baseline. The fact that the estimated payback periods are relatively short further strengthens the 

argument for immediate implementation. If there are other technologies out there, short of replacing 

the entire compressor, they are likely to be more technical and therefore more expensive and result in

higher maintenance costs. These compressors live in very harsh environments and therefore the more 

low-tech the solution the higher the probability that they will be reliable, long lived and well within 

the capabilities of existing maintenance personnel.
4. Did this paper appropriately characterize the emissions reductions achievable from the emissions 

mitigation technologies discussed for reciprocating compressors and wet seal centrifugal 

compressors?

COMMENT: Yes, the fact that the emission reduction strategies are comparable to existing 

mitigation technologies already in practice suggests they are achievable. For example, ducting the 

emissions to a Vapor Recovery Unit (VRU) is very similar to ducting Glycol Dehydrator emissions 

to a VRU, and therefore it is reasonable to expect the same recovery efficiency. 

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?

COMMENT: Industry leads that I interviewed believe there are significant operating costs for labor, 

installation, and ongoing maintenance that were under-represented. They point out that labor costs 

eighteen years ago (1996 study) were much less than they are now. They also pointed out that much 

of the installation work will likely be contracted out and there will be significant costs in 

transportation and per diem because the contractors are traveling to extremely remote locations. 

Section 4.1.2, Economic Rod Packing Replacement, states that “In some cases, the economic 

threshold for replacement can be as low as 30 scfh of gas leakage.” Industry representatives believe it 

is at least 3 times that. They also stated that coating the compressor rods with chrome or tungsten 

would be cost prohibitive. It should be pointed out that while they found fault with some of the 

details, they agreed there are opportunities for both resource conservation and cost savings.

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.

COMMENT: I am not aware of other emission mitigation options.

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

certain existing centrifugal compressors? 

COMMENT: I did get one comment whereby they thought that extremely large centrifugal 

compressors using wet seals benefitted from the oil lubrication thereby decreasing wear, but this 

reasoning is refuted by the very detailed explanation of how dry seals function in section 4.3.1.

8. Are there technical reasons why an operator would use a wet seal centrifugal compressor without a 

gas recovery system?

COMMENT: I did not receive any comments to that effect. Given the fact that tables 3-8, 3-10, and 

3-11 clearly show that the wet seal centrifugal compressor affords the greatest recovery opportunity 

for vented methane and the estimated payback period represented in section 4.5.3 for recovery 

systems is less than a year it would appear to be a win/win investment for industry.

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

reciprocating compressors impractical?

COMMENT: Not necessarily technical limitations, but it is my understanding that reciprocating 

compressors are often used in the field gathering system, natural gas production sector, where you 

have both low pressure and low gas volume. In that scenario a gas capture system would have 
minimal value. This appears to be supported by table 3-2 which lists an “Average Compressor 

Methane Emission Factor from Seals” at 9.48 Mscf/yr for reciprocating compressors in natural gas 

production. This is further supported by the discussion following table 3-3 which points out that the 

methane emissions from these units “...include emissions from compressor blowdowns, starter gas, 

and miscellaneous equipment associated with the compressor. Methane emissions from reciprocating 

compressor seals represent approximately 29% of the total emissions from reciprocating 

compressors. Note that the Eastern U.S. natural gas production did not include methane emissions 

from compressor seals in the reciprocating compressor emission factor, only emissions from 

associated equipment ...” I conclude this is not a good investment of monetary resources given the 

fact that the Processing, Transmission, and Storage sectors on average have reciprocating emissions 

that are 31 times greater than the Production sector.

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

COMMENT: I called five environmental managers in the oil and gas industry. Their comments are 

as follows:

For high pressure compression in the processing sector wet seal centrifugal compressors are favored 

because they believe the oil lubrication reduces wear. Of much greater concern is the liquids dump 

valve jamming open and blowing gas to the atmosphere as opposed to capturing gas from a wet oil 

seal system. They believe the emissions from wet seals are minimal. The overall opinion was that 

there are a lot more wet seal compressors out there than dry seal compressors which refutes the 

conclusions in the API/ANGA study. Very little compressor emissions mitigation is being 

implemented in the field because they have better opportunities to mitigate emissions on other 

components at the facility that give them a greater return on their investment. Again, by far the 

biggest source of methane emissions is from the liquids dump valve, either do to jamming from build 

up or improper programing governing their operation (i.e. set points). 

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?

COMMENT: As noted above in number 10 and follow up research, there are hybrid wet seal 

centrifugal compressors that have emissions comparable to dry seal systems. It was also industries

position that there is not a significant advantage both mechanical and in maintenance of dry seal 

centrifugal compressors when used for high compression loads.

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

COMMENT: I assume that to be a certainty. In order for local air agencies to implement new 

regulation to control VOC and methane emissions they will, by necessity, partner with industry and 

create workshop training programs to improve general understanding of how the emissions are 

emitted and how they are reasonably mitigated and controlled.