### § 98.233(p) Reciprocating compressor rod pa cking 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 duri ng a reporting period:

1. Operating

2. Standby pressurized

3. Not operating, depressurized

El Paso recognizes and mentioned in its’ comments 1 

### to the EPA proposed reporting rule last year, that the operati ng mode of compressor units imp acts 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 EP A in the Subpart W proposal creates enormous challenges and will result in greenhouse gas emi ssions (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 c onsider challenges associated with atypical modes of operation. Currently, Subpart W does not take into account that the following modes of operation are considered atypi cal 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, pr essurized” 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 c onsider that requiring measurement of both typi cal and atypical modes of operation would result in unintended GHG emi ssions from unit blowdowns, disruption of operations and supply, and non-repr esentative conditions. Furthe rmore, monitoring compressor units in atypical modes has the additional challe nge of tracking the operating time in these modes given it is not a common practice by most co mpanies today, and would be a major undertaking not justified by the level of impr ovement in capturing emissions.

1 Docket ID No EPA–HQ–OAR–2008–0508

### To achieve the EPA’s goal of collecting meanin gful 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 a pplicable 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 modes 2



### .

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 us ed to calculate emissions factors for each key component category in each mode. Total unit em issions 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 develope d 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 developm ent of emission factors, incl uding 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 vent s along with other compressor unit vents and Section 98.233(q)(3) which indicates that compresso r blow down vents should be surveyed using

an IR camera and then the “leaker” emissi on 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 prov ide 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 addi tional 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 we re already blown down, it would have to be pressurized and put on line, and then after the m easurements were made, it would be taken back

off line and blown down. This would also in crease 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 a nd as a result, already blown down. So all of these units would have to be pressurized and put on lin e to make a measurement before being put

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

would have to be taken off line in order to measure either the “sta ndby pressurized” mode and the “not operating – depressurized” mode.

2. Putting a unit on line temporarily may not pr ovide a representative measurement since the rod packings may not be properly warmed up or it may have to be placed in a recirculating

mode 3 

### if there is no extra gas capacity in the pipeline.

3. It may not be feasible to ta ke a unit off line due to pipeli ne demand – requiring a unit to come off line might disrupt gas supply and opera tions. This may be especially true in the production sector or at transmi ssion stations with only centrifuga l units where there may not be any additional units w ith which to switch.

4. Generally, companies in the oil and gas s ector keep track only of the compressor operating hours, so that the only current reliabl e information is whether a compressor unit is operating or off-line. It is El Paso’s understanding that ther e 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 wa s to blow down the unit when off- line, then it would be assumed that all off-line hours were sp ent depressurized. Sim ilarly, if the standard procedure was to leave the unit pressurized when off-line, it would be assumed that all off-line hours were pressurized. Modify ing 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 disc harge pressures are essentially equal in recirculating mode. During typical compressor operations, the difference betwee n 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 un its 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 pressu rized” 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 typi cally 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 pressuri zed” 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 substant ially 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 St ar 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 documen t are personal communi cations and better data is currently available. For instance, the orig inal GRI survey of comp ressor stations (Indaco, 1995) 5 

### found a 19% increase in idle versus runn ing 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 ra te than running rod packings.

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

outside contractors and ha s approximately ten times as many meas urements 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 pressu rized 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 Lear ned 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 sm aller data set than GRI it was a more diverse set of stations). Finally, El Paso ’s data set (which has ove r four times more data points for unit valves than the GRI study) indicates a le ak 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 compre ssor 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 acro ss the blow down valve and through the rod packings. When depressurized, the blow down valv e is open and leakage measured from that vent is due to leakage across the unit isolation valves.

Table 1 summarizes the available emissions fa ctors and the results of the model using the

available data.

Table 1: Emission Factors and Projected Leak Rates from Re lated 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 Leakage 1



### 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 mi ght expect idle/pressurized units to leak on average between 5% and 43% more than running units. Depressuri zed 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 pr ovide an excellent representation of total leakage.

### Calculation of System-wide Emi ssions 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 se als and across the blow down valv e and pressure relief valve, which are closed when the unit is operating. Mu ch 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 tota l leakage from the key vented components while in the operating mode is calculated per Equation 1:

LR Operating = (N CS-Operating x EF CS--Operating ) + (N BDV x EF BDV ) + (N PRV x EF PRV ) (Eq. 1)



### where:

LR Operating = Leak rate from a compressor unit in “Operating” mode (scfh CH 4 )

N

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

EF CS—Operating = Emission factor for compressor seals on “Operating” compressors (scfh CH 4 /seal)

N

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

EF BDV = Emission factor for blow down valves (scfh CH 4 /blow down valve)

N

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

EF PRV = Emission factor for pressure relief valves (scfh CH 4 /pressure relief valve)



### For most compressor units, the num ber 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 diffe r between units that are in the “Operating” mode and in the “Standby Pressurize d” mode, i.e., these factors are not impacted by

mode.

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

mode occurs at the same sources as in the “Operating” mode ex cept that the l eakage through the rod packing seals may differ between the two an d 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 = (N CS-Standby Pressurized x EF CS—Standby Pressurized ) + (N BDV x EF BDV ) + (N PRV x EF PRV ) (Eq. 2)



### where:

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

N

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

EF CS—Pressurized = Emission factor for compressor seal s on “Standby Pressurized” compressors

N

BDV = Number of blow down valv es on the “Standby Pressurized” unit

EF BDV = Emission factor for blow down valves

N

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

EF PRV = Emission factor for pressure relief valves.

### Figure 3 represents the leakage from a recipr ocating 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. Leak age through these valves travels through the open blow down valve

and is measured at the blow down vent. The l eakage in this case is calculated pursuant to Equation 3:

LR Not Operating-- Depressurized = N Unit Isolation Valve Pairs x EF Unit Isolation Valve Pairs (Eq. 3)

where:

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

N Unit 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 be cause 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, st orage facilities may have mo re 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:

LR Annual = ( α Operating × LR Operating ) + ( α Standby Pressurized. × LR Standby Pressurized )

. (Eq. 4a) or

LR Annual = ( α Operating × LR Operating ) + ( α Not Operating--Depressurized × LR Not Operating—Depressurized ) (Eq. 4b)

where:

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

α Operating = Amount of time compressor unit is “Operating”

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

α

Not Operating--Depressurized = Amount of time compressor unit is “Not Operating—Depressurized”.Figure 1: Leakage from ReciprocatingCompressor Unit in “Operating” ModeStandard Component Leakage(Flanges, Fuel Fittings, Valve Caps)Typically < 10% of Total andAddressed bySection 98.233(q)Pressure Relief Valve LeakagePressure Relief ValveEmission FactorBlow Down Vent --Leakage Across ClosedBlow Down ValveBlow Down Valve Emission FactorCompressor Seal Vent“Operating” Emission Factor xNumber of Compressor SealsReciprocating Compressor Unit – “Operating” ModeFigure 2: Leakage from Reciprocating Compressor Units in “Standby Pressurized” ModeStandard Component Leakage(Flanges, Fuel Fittings, Valve Caps)Typically < 10% of Total and Addressed bySection 98.233 (q)Pressure Relief Valve LeakagePRV Emission Factor xNumber of PRV’sBlow Down Vent --Leakage Across Closed Blow Down ValveBlow Down Valve Emission Factor xNumber of Blow Down ValvesCompressor Seal VentStandby/Pressurized Emission Factorx Number of Compressor SealsReciprocating Compressor Unit – “Standby Pressurized” ModePressure Relief Valve LeakageZero When Unit DepressurizedStandard Component Leakage(Flanges, Fuel Fittings, Valve Caps)Zero When Unit DepressurizedFigure 3: Leakage from Reciprocating CompressorUnits in “Not Operating--Depressurized” ModeUnit Isolation Valves --Leakage Across ClosedUnit Isolation Valves and ThroughOpen Blow Down ValveUnit Isolation Valve Pair Emission Factor xNumber of Isolation Valve PairsCompressor Seal VentZero When Unit DepressurizedReciprocating Compressor Unit – “NotOperating--Depressurized” ModeExample Calculations of LeakRates Using Emission FactorsTable 2 presents a sample calculation of the annual leak rate from a reciprocating compressor unit that is left pressurized when takenoff line. In this example, hypotheticalemission factors are used to represent thekey sources. This example also assumes that thecompressor unit in question has four compressor rod packing seals, one blow down valve,one pressure reliefvalve, and one pairofunit isolation valves. Finally,the compressor unit is assumed tobe “Operating” 75% of the year (6570 hours) and “Not Operating—Depressurized” the remaining 25%of the year (2190 hours).Therefore, as shown in Table 2, to calculate the contribution of rod packing sealleakage while the compressor unit is operating, thehypothetical emission factor for “Operating” rod packing seals (99.0 scfh CH4) is multiplied by the number of rod packing seals (4)and again multiplied by the activity factor (6570 hours per year that the compressor is in“Operating” mode). This result wasthendivided by 1000 to convert to Mcf/yr (Mcf= 1000 cubic feet in US natural gas industry terminology) resulting in 2602 Mcf CH4/year.Similar calculations are made for each of the other component categories, with the results fromeach category being totaled tocalculate the overall annual compressor unit leak rate.Note that in practice, emissions will becalculated by using a combination of unit specific measurement data and operating companyemission factors. Actual measurement data will be used for eachunit as available, and the emission factors will be used to projectleakage from the mode which was not measured. Thiswill 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 Not Operating(Basis: 75% Utilization)ComponentComponent EmissionFactor(scfh Methane)No. ofComponentsActivity Factor(Hours Leaking Per Year)AnnualEmissions(McfCH4/Year)Compressor Status: OperatingRod Packing Seal – Operating 99.0 4 6570 Hours (Operating) 2602Blow Down Valve 102.6 1 6570 Hours (Operating) 674Pressure Relief Valve 10.1 1 6570 Hours (Operating) 66Compressor Status: Standby PressurizedRod Packing Seal – Standby Pressurized 144.6 4 2190 Hours (Standby Pressurized) 1267Blow Down Valve 102.6 1 2190 Hours (Standby Pressurized)225Pressure Relief Valve 10.1 1 2190 Hours (Standby Pressurized)22Total Annual Emissions (Mcf CH4/year)4856Table 3: Sample Calculation of Emissions from a Reciprocating Compressor Unit that is Depressurized When Not Operating(Basis: 75% Utilization)ComponentComponent EmissionFactor(scfh Methane)No. ofComponentsActivity Factor(Hours Leaking Per Year)Annual Emissions(Mcf CH4/Year)Compressor Status: OperatingRod Packing Seal – Operating 99.0 4 6570 Hours (Operating) 2602Blow Down Valve 102.6 1 6570 Hours (Operating) 674Pressure Relief Valve 10.1 1 6570 Hours (Operating) 66Compressor Status: Not Operating--DepressurizedUnit Isolation Valve Pair 1344 12190 Hours (Depressurized) 2943Total Annual Emissions (Mcf CH4/year)6286

### Development of Emission Factors

To develop emission factors, each key comp ressor 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 de pendant population emissi on 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 component s surveyed in that year. In addition to the average leak rate (which is the population emission factor), the standard

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

per component category.

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

1) Blow Down Valve Vent (combine “Opera ting” and “Standby Pressurized” modes ) 2) Pressure Relief Valve Vent (combine “Ope rating” 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 Applicab le 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 w ould most likely result in too few da ta points to be st atistically valid, while grouping data industry wide would increas e the uncertainty due to wider differences in equipment and operating practices.