§ 98.233(p) Reciprocating compressor rod packing venting: El Paso recommends 

monitoring in “As Found” mode. 

Section 98.233 (p) requires that all compressor vents be surveyed in each of the operational 

modes that occur during a reporting period: 

1. Operating 

2. Standby pressurized 

3. Not operating, depressurized 

El Paso recognizes and mentioned in its’ comments1

 to the EPA proposed reporting rule last 

year, that the operating mode of compressor units impacts leakage from key compressor 

components such as rod packing seals, blow down valves, and unit isolation valves. However, a 

survey of all three modes as required by the EPA in the Subpart W proposal creates enormous 

challenges and will result in greenhouse gas emissions (GHG) that otherwise, would not have 

occurred under normal operations. 

To alleviate these problems, as the first step, the EPA should take into account that compressor 

units will primarily fall under one of the following operating scenarios: 

Typical Mode 1 Typical Mode 2 

Scenario 1 Operating Standby pressurized 

Scenario 2 Operating Not operating, depressurized 

The proposed monitoring requirements do not consider challenges associated with atypical 

modes of operation. Currently, Subpart W does not take into account that the following modes 

of operation are considered atypical and probably occur less than 7% of the time (see Appendix 

I): 

1. Scenario 1 - For units that are normally blown down when off line, only a very short 

amount of time is spent in the “standby, pressurized” mode prior to being blown down 

(“not operating, depressurized” mode). 

2. Scenario 2 - Units that are normally in the “standby, pressurized” mode when off line will 

only be depressurized for maintenance or during emergency conditions. 

 Additionally, it does not consider that requiring measurement of both typical and atypical modes 

of operation would result in unintended GHG emissions from unit blowdowns, disruption of 

operations and supply, and non-representative conditions. Furthermore, monitoring compressor 

units in atypical modes has the additional challenge of tracking the operating time in these modes 

given it is not a common practice by most companies today, and would be a major undertaking 

not justified by the level of improvement in capturing emissions. 

 

1

 Docket ID No EPA–HQ–OAR–2008–0508 
To achieve the EPA’s goal of collecting meaningful and characteristic emissions data, the 

abovementioned operating scenarios should be the focal point of the EPA’s monitoring 

proposal. A survey of each unit located at an applicable facility, in the mode in which is it 

found, i.e. “As Found” should be required as the “As Found” approach will result in a 

large sample size of all modes2

. 

In practice, each compressor unit would be surveyed in the mode in which it was found when the 

survey took place. This data would then be used to calculate emissions factors for each key 

component category in each mode. Total unit emissions would then be calculated as follows: 

1. For the mode in which the unit was surveyed, the emissions will be based on actual 

measurements. 

2. For the other typical mode, the emissions will be based on the number of key 

components, and emission factors developed for this mode based on company-wide 

sampling results in this mode. 

3. The atypical mode will be rolled up with one of the typical modes as described in 

Appendix I Item 4. 

Details regarding the development of emission factors, including sample calculations, are 

presented in Appendix I. 

Note: A clarification is needed between Section 98.233(p)(2)(i) which requires the 

measurement of compressor blow down vents along with other compressor unit vents and 

Section 98.233(q)(3) which indicates that compressor blow down vents should be surveyed using 

an IR camera and then the “leaker” emission factor applied to calculate leakage. 

 

2

 Over the past three years (2007 – 2009), compressor units in the El Paso transmission system spent over 50% of their time 

in each of the modes as follows: Operating (28.6%), Standby, Pressurized (34.7%), and Depressurized – Offline (36.8%). 

Consequently, “As Found” sampling should provide a representative sample of each mode. 
APPENDIX I 

Support Documentation for “As Found” Monitoring Proposal 

Monitoring Challenges

1. Surveying all three modes will require an additional blow down event per unit, because if 

a unit is on line it would have to be taken off line and then blown down to survey the 

depressurized mode. This would result in an additional blowdown event that would increase 

GHG emissions. On the other hand, if a unit were already blown down, it would have to be 

pressurized and put on line, and then after the measurements were made, it would be taken back 

off line and blown down. This would also increase GHG emissions. Hence, the monitoring 

requirement will result in increased greenhouse gas emissions that otherwise would not have 

occurred under normal operations as per the case shown below: 

At least 50% of compressor units in El Paso’s transmission system are currently 

depressurized when off line and as a result, already blown down. So all of these units 

would have to be pressurized and put on line to make a measurement before being put 

back off line and blow down a second time. All units found operating during the survey 

would have to be taken off line in order to measure either the “standby pressurized” mode 

and the “not operating – depressurized” mode. 

2. Putting a unit on line temporarily may not provide a representative measurement since 

the rod packings may not be properly warmed up or it may have to be placed in a recirculating 

mode3

 if there is no extra gas capacity in the pipeline. 

3. It may not be feasible to take a unit off line due to pipeline demand – requiring a unit to 

come off line might disrupt gas supply and operations. This may be especially true in the 

production sector or at transmission stations with only centrifugal units where there may not be 

any additional units with which to switch. 

4. Generally, companies in the oil and gas sector keep track only of the compressor 

operating hours, so that the only current reliable information is whether a compressor unit is 

operating or off-line. It is El Paso’s understanding that there is no current standardized industry 

practice or method of determining whether an off-line compressor is pressurized or 

depressurized. Usually, standard operating procedures are employed to estimate the hours when 

the unit is off-line. If the standard procedure was to blow down the unit when off- line, then it 

would be assumed that all off-line hours were spent depressurized. Similarly, if the standard 

procedure was to leave the unit pressurized when off-line, it would be assumed that all off-line 

hours were pressurized. Modifying the compressor’s Remote Terminal Units (RTU’s) for all 

 

3

 In a recirculating mode, the gas leaving the discharge side of the compressor flows through a bypass valve and into the 

suction side of the compressor. As a result, suction and discharge pressures are essentially equal in recirculating mode. 

During typical compressor operations, the difference between suction and discharge pressures are at least 200 psig 
affected units to track hours in atypical modes would be a major undertaking not justified by the 

level of improvement in capturing emissions. 

5. Units that are usually depressurized when off line might spend a very short amount of 

time pressurized prior to being blown down, and units that are left pressurized while off line 

might be depressurized for short periods for maintenance. However, these are atypical 

conditions. For instance, in 2009, compressor units in El Paso’s transmission system that are 

typically left in “standby pressurized” mode when off-line were “operating” 31.6% of the time, 

in “standby pressurized mode 61.9% of the time, and “unavailable” 6.5% of the time. This 

“unavailable” designation comes from El Paso RTU data and the unit may or may not be 

depressurized during this time, so in the worst case the “not operating – depressurized” mode 

(the atypical mode for units typically left pressurized when off line) only occurs for 6.5% of the 

time. 

For units which are typically depressurized when off-line, detailed data on the amount of time 

spent in “standby pressurized” (the atypical condition) is not available. However, in most cases 

these units are only left in the “standby pressurized” for a short period (less than one hour) prior 

to being depressurized. 

Effect of Operational Mode on Leakage 

Although the mode of a compressor unit can substantially affect the leakage for individual units, 

the average differences may not be as great as indicated in the EPA source document mentioned 

in the technical support document for the proposed rule. This Star program “Lessons Learned” 

publication (EPA, 2004)4

 states that rod packings on idle units may leak at four times the rate of 

rod packings on running units. 

However, the primary references for this document are personal communications and better data 

is currently available. For instance, the original GRI survey of compressor stations (Indaco, 

1995)5

 found a 19% increase in idle versus running rod packings, while in the PRCI study 

(Howard et al., 1999 – cosponsored by PRCI, GRI, and EPA), idle rod packings were found to 

leak on average at a 46% greater rate than running rod packings. 

A more extensive data set has been compiled by El Paso. This data was primarily collected by 

outside contractors and has approximately ten times as many measurements as the PRCI data set. 

This data indicates an average of only 6% increase in leakage from seals in idle pressurized 

mode compared to seals in running mode. Consequently, the difference in leakage between 

running units and idle but pressurized units is probably not as significant as indicated in 

the EPA Lessons Learned source. 

 

4

 EPA, 2004. Reducing Emissions when Taking Compressors Off-Line. EPA430-B-04-001, February 2004. 

5

 Indaco, 1995. Leak Rate Measurements at US Natural Gas Transmission Compressor Stations. GRI-94/0257.37. 

Gas Research Institute, 8600 West Bryn Mawr Avenue, Chicago, IL 60631. 
For the depressurized units, the EPA Lessons Learned document suggests an average leak rate of 

23.3 scfm. This is in line with the original GRI study of 26.1 scfm. However, the PRCI study 

indicated a leak rate of 6.78 scfm (although this was based on a smaller data set than GRI it was 

a more diverse set of stations). Finally, El Paso’s data set (which has over four times more data 

points for unit valves than the GRI study) indicates a leak rate of 9.21 ± 2.64 scfm. 

To estimate the differences between modes, let’s assume a model of a reciprocating compressor 

unit that has one blow down valve, four compressor cylinders and therefore four rod packing 

seals, and one pair of unit isolation valves. When either running or in standby pressurized mode, 

there is potential leakage across the blow down valve and through the rod packings. When 

depressurized, the blow down valve is open and leakage measured from that vent is due to 

leakage across the unit isolation valves. 

Table 1 summarizes the available emissions factors and the results of the model using the 

available data. 

Table 1: Emission Factors and Projected Leak Rates from Related Leak Studies 

Component EPA 

 (EPA, 2004) 

GRI 

(Indaco, 1995) 

PRCI 

(Howard, 1999) 

El Paso (1998 – 

present) 

 

Rod Packing – Running 0.3 1.02 ± 0.51 1.66 ± 0.48 1.96 ± 0.23

Rod Packing – Idle, Pressurized 1.3 1.21 ± 0.47 2.42 ± 1.05 2.08 ± 0.33

Blow Down Valve 7.5 2.53 ± 1.07 0.39 ± 0.33 2.50 ± 0.72

Unit Isolation Valve Pair 23.3 26.1 ± 9.41 6.78 9.21 ± 2.64

 

Total Unit Leakage1 

 Unit Running 8.75 6.61 7.03 10.3

 Unit Idle – Pressurized 12.7 7.37 10.1 10.8

 Unit Depressurized 23.3 26.1 6.78 9.21

 

Ratio to Leakage from Running 

Units 

 Unit Running 1.0 1.0 1.0 1.0

 Unit Idle – Pressurized 1.43 1.11 1.43 1.05

 Unit Depressurized 2.67 3.96 0.96 0.89

1 Assumes four rod packings, one blow down valve, and one pair of unit isolation valves

Based on these projections, we might expect idle/pressurized units to leak on average between 

5% and 43% more than running units. Depressurized units might leak on average slightly less 

than running units to as much as four times greater. 

 

The El Paso and PRCI data indicate there is little difference between operating modes on 

average, although for individual units there may be substantial differences. If this is the case, 

sampling units in an “As Found” mode should provide an excellent representation of total 

leakage. 
Calculation of System-wide Emissions Using Emission Factors 

To aid in understanding the relative contributions of various leak sources to total emissions, a 

simple model of compressor station fugitive methane emissions can be constructed based on the 

major sources of leakage. This model is illustrated in Figures 1, 2, and 3. 

Figure 1 shows the leak sources for a unit in the “Operating” mode. In this mode leakage may 

occur through the rod packing seals and across the blow down valve and pressure relief valve, 

which are closed when the unit is operating. Much smaller leakage may also occur at standard 

components (such as connectors and valves) on the compressors themselves but this leakage is 

addressed by Section 98.233(q) of Subpart W. For this case, the total leakage from the key 

vented components while in the operating mode is calculated per Equation 1: 

LROperating = (NCS-Operating x EFCS--Operating) + (NBDV x EFBDV)+ (NPRV x EFPRV) (Eq. 1) 

where: 

LR Operating = Leak rate from a compressor unit in “Operating” mode (scfh CH4)

NCS-Operating = Number of compressor seals on the “Operating” unit 

 EFCS—Operating = Emission factor for compressor seals on “Operating” compressors (scfh CH4/seal)

NBDV = Number of blow down valves on the “Operating” unit 

EFBDV = Emission factor for blow down valves (scfh CH4/blow down valve) 

NPRV = Number of pressure relief valves on the “Operating” unit 

EFPRV = Emission factor for pressure relief valves (scfh CH4/pressure relief valve) 

For most compressor units, the number of blow down and pressure relief valves will be one of 

each, but there are some units that have more. Additionally, the emission factors for both blow 

down valves and pressure relief valves are not expected to differ between units that are in the 

“Operating” mode and in the “Standby Pressurized” mode, i.e., these factors are not impacted by 

mode. 

Figure 2 shows the leakage for a unit in the “Standby Pressurized” mode. The leakage in this 

mode occurs at the same sources as in the “Operating” mode except that the leakage through the 

rod packing seals may differ between the two and consequently is represented by a different 

emission factor. For this case, the total leakage from the key vented components while in the 

standby pressurized mode is calculated per Equation 2: 

 LR Standby Pressurized = (NCS-Standby Pressurized x EFCS—Standby Pressurized) + (NBDV x EFBDV)+ (NPRV x EFPRV) (Eq. 2) 

where: 

LR Standby Pressurized = Leak rate from a compressor unit in “Standby Pressurized” mode

 NCS-Standby Pressurized = Number of compressor seals on the “Standby Pressurized” unit 

 EFCS—Pressurized = Emission factor for compressor seals on “Standby Pressurized” compressors

NBDV = Number of blow down valves on the “Standby Pressurized” unit 

EFBDV = Emission factor for blow down valves 

NPRV = Number of pressure relief valves on the “Standby Pressurized” unit 

 EFPRV = Emission factor for pressure relief valves.
Figure 3 represents the leakage from a reciprocating compressor unit that is in the “Not 

Operating – Depressurized” mode. When a unit is depressurized (blown down), the only source 

of leakage will be through the unit isolation valves, which are closed in order to isolate the unit 

from the main pipeline. Leakage through these valves travels through the open blow down valve 

and is measured at the blow down vent. The leakage in this case is calculated pursuant to 

Equation 3: 

 LRNot Operating-- Depressurized = NUnit Isolation Valve Pairs x EF Unit Isolation Valve Pairs (Eq. 3) 

where: 

LRNot Operating-- Depressurized = Leak rate from a compressor unit in “Not Operating – Depressurized” mode 

NUnit Isolation Valve Pairs = Number of unit isolation valve pairs 

 EF Unit Isolation Valve Pairs = Emission factor for unit isolation valve pairs. 

The unit isolation valves are counted as a pair because there will always be at least one suction 

valve and one discharge valve and measurements at the blow down vent cannot distinguish 

which valve might be leaking. Typically there will be only one pair per unit at transmission 

compressor stations; however, storage facilities may have more than one pair per unit. 

Annual leakage from a compressor unit can then be calculated using its activity factors (the 

amount of time it spends in each mode – “Operating”, “Standby Pressurized”/“Not Operating—

Depressurized” and the leak rate in two of the three modes per Equation 4a or 4b: 

LRAnnual = (αOperating × LROperating) + (αStandby Pressurized. × LRStandby Pressurized). (Eq. 4a) or 

LRAnnual = (αOperating × LROperating) + (αNot Operating--Depressurized × LRNot Operating—Depressurized) (Eq. 4b) 

where:

LRAnnual = Annual leak rate from the compressor unit (scf) 

αOperating = Amount of time compressor unit is “Operating” 

αStandby Pressurized = Amount of time compressor unit is “Standby Pressurized” 

αNot Operating--Depressurized = Amount of time compressor unit is “Not Operating—Depressurized”. 

 
Figure 1: Leakage from Reciprocating Compressor Unit in “Operating” Mode 

Standard Component Leakage 

(Flanges, Fuel Fittings, Valve Caps) 

Typically < 10% of Total and 

Addressed by Section 98.233 (q)

Pressure Relie

Pressure R

Emissio

Blow Down Vent -- 

Leakage Across Closed 

Blow Down Valve 

Blow Down Valve Emission Factor 

Compressor Seal Vent 

“Operating” Emission Factor x 

Number of Compressor Seals 

Reciprocating Compressor Unit – “Operating” Mode 
Figure 2: Leakage from Reciprocating Compressor Units in “Standby Pressurized” Mode 

Standard Component Leakage 

(Flanges, Fuel Fittings, Valve Caps) 

Typically < 10% of Total and Addressed by 

Section 98.233 (q)

Pressure Relie

PRV Emis

Number 

Blow Down Vent -- 

Leakage Across Closed Blow Down Valve 

Blow Down Valve Emission Factor x 

Number of Blow Down Valves 

Compressor Seal Vent 

Standby/Pressurized Emission Factor 

x Number of Compressor Seals 

Reciprocating Compressor Unit – “Standby Pressurized” M

Pressure Relief Valv

Zero When Unit Dep

Standard Component Leakage 

(Flanges, Fuel Fittings, Valve Caps) 

Zero When Unit Depressurized 
Figure 3: Leakage from Reciprocating Compressor Units in “Not Operating--Depressurized” Mode 

Unit Isolation Valves -- 

Leakage Across Closed 

 Unit Isolation Valves and Through 

Open Blow Down Valve 

Unit Isolation Valve Pair Emission Factor x 

Number of Isolation Valve Pairs 

Compressor Seal Vent 

Zero When Unit Depressurized 

Reciprocating Compressor Unit – “Not Operating--Depressurized” M
Example Calculations of Leak Rates Using Emission Factors

Table 2 presents a sample calculation of the annual leak rate from a reciprocating compressor unit that is left preoff line. In this example, hypothetical emission factors are used to represent the key sources. This example also compressor unit in question has four compressor rod packing seals, one blow down valve, one pressure relief val

unit isolation valves. Finally, the compressor unit is assumed to be “Operating” 75% of the year (6570 hours) an

Depressurized” the remaining 25% of the year (2190 hours). Therefore, as shown in Table 2, to calculate the contribution of rod packing seal leakage while the compressor unhypothetical emission factor for “Operating” rod packing seals (99.0 scfh CH4) is multiplied by the number of ro

and again multiplied by the activity factor (6570 hours per year that the compressor is in “Operating” mode). Th

divided by 1000 to convert to Mcf/yr (Mcf = 1000 cubic feet in US natural gas industry terminology) resulting in 

Similar calculations are made for each of the other component categories, with the results from each category becalculate the overall annual compressor unit leak rate. Note that in practice, emissions will be calculated by using a combination of unit specific measurement data and emission factors. Actual measurement data will be used for each unit as available, and the emission factors will leakage from the mode which was not measured. This will make the calculations more facility specific. Table 3 presents a similar example for a reciprocating unit that is depressurized when off-line. 
Table 2: Sample Calculation of Emissions from a Reciprocating Compressor Unit --Pressurized When No

 (Basis: 75% Utilization) 

Component 

Component Emission 

Factor 

(scfh Methane) 

No. of 

Components 

Activity Factor 

 (Hours Leaking Per Ye

Compressor Status: Operating 

Rod Packing Seal – Operating 99.0 4 6570 Hours (OperatingBlow Down Valve 102.6 1 6570 Hours (OperatingPressure Relief Valve 10.1 1 6570 Hours (Operating

Compressor Status: Standby Pressurized 

Rod Packing Seal – Standby Pressurized 144.6 4 2190 Hours (Standby PressuBlow Down Valve 102.6 1 2190 Hours (Standby PressuPressure Relief Valve 10.1 1 2190 Hours (Standby Pressu

Total Annual Emissions (Mcf CH4/year) 
Table 3: Sample Calculation of Emissions from a Reciprocating Compressor Unit that is Depressurized W

 (Basis: 75% Utilization) 

Component 

Component Emission 

Factor 

(scfh Methane) 

No. of 

Components 

Activity Factor 

 (Hours Leaking Per Yea

Compressor Status: Operating 

Rod Packing Seal – Operating 99.0 4 6570 Hours (OperatingBlow Down Valve 102.6 1 6570 Hours (OperatingPressure Relief Valve 10.1 1 6570 Hours (Operating

Compressor Status: Not Operating--Depressurized 

Unit Isolation Valve Pair 1344 1 2190 Hours (Depressuriz

Total Annual Emissions (Mcf CH4/year) 
Development of Emission Factors

To develop emission factors, each key compressor component (blow down valve vent, rod 

packing vent, and unit isolation valve pair) would be measured in the operating condition found 

at the time of the measurement. Time dependant population emission factors would be 

calculated for the mode in which each component was not surveyed, by averaging the leakage 

found in each component category over the total number of components surveyed in that year. 

In addition to the average leak rate (which is the population emission factor), the standard 

deviation, and the minimum and maximum observed leak rates should be reported for each factor 

per component category. 

The following are the suggested categories for emission factors and are summarized in Table 4: 

1) Blow Down Valve Vent (combine “Operating” and “Standby Pressurized” modes ) 

2) Pressure Relief Valve Vent (combine “Operating” and “Standby Pressurized” modes) 

3) Rod Packing Seal Vent(s) on Reciprocating Compressors 

a. “Operating” Mode 

b. “Standby Pressurized” Mode 

4) Unit Isolation Valve Pairs – “Not Operating” mode only 

Table 4: Summary of Emission Factor Categories for Compressor Units 

Component Operating Mode 

“Operating” “Standby Pressurized” “Not Operating—

Depressurized” 

Blow Down Valve Use measurements in either mode to develop combined factor Not Applicable 

Pressure Relief Valves Use measurements in either mode to develop combined factor Not Applicable 

Rod Packing Seals Individual Factor Individual Factor Not Applicable 

Unit Isolation Valve Pairs Not Applicable Not Applicable Individual Factor 

Although emission factors could be calculated by grouping data by units, facilities, operating 

companies, or industry, El Paso recommends grouping data at the operating company level. 

Grouping data by facility would most likely result in too few data points to be statistically valid, 

while grouping data industry wide would increase the uncertainty due to wider differences in 

equipment and operating practices.