Page 1 of 4 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY WASHINGTON, D.C. 20460

OFFICE OF

AIR AND RADIATION

Petroleum and Natural Gas Systems (Subpart W) of the Greenhouse Gas Reporting Rule Docket: EPA-HQ-OAR-2009-0923

Title: Compressor Modes and Thresholds

Summary: This memo discusses the compressor m onitoring requirements required in 40 CFR Part 98 Subpart W – Petroleum and Natural Gas Systems of the Greenhouse Gas Reporting Rule, which has been modified from the April 2010 pr oposed rule in response to comments on the burden and potential adverse consequences of implementing the proposed rule.Page 2 of 4 Compressor Modes and Threshold

EPA determined through experi ence reported by industry partners in the Natural Gas STAR Program 1 that different and significa nt vented emissions occur from large gas compressors in three different operational modes: emissions fr om drive shaft seals while compressors are operating, leakage through blowdow n valves when closed and co mpressors are either operating or standby under pressure, and un it isolation valve leak s during time when compressors are shut down and depressurized. This memo discusses th e compressor monitoring requirements required in 40 CFR Part 98 Subpart W – Petroleum a nd Natural Gas Systems of the Greenhouse Gas Reporting Rule, which has been modified from the April 2010 proposed rule in response to comments on the burden and potential adverse conseque nces of implementing th e proposed rule.

Background of Three Modes of Compressors and Reporting Requirements Vented emissions from compressors are differe nt, and from different components during the three modes of operation. These three modes ar e: (1) operating, (2) standby under pressure, and (3) standby and de-pressurized. Reciprocating compressors may spend significant time in all three modes, while centrifugal compressors ar e more commonly in either the operating or standby and de-pressurized modes. Centrifugal compressor means any equipment that increases the pressure of a pro cess natural gas or CO 2 by centrifugal action, employing rotating movement of the driven shaft. Reciprocating compressor means a piece of equipment that increases the pressure of a proce ss natural gas or CO 2 by positive displacement, employing linear movement of a shaft driving a piston in a cylinder. When a compressor is fully pressurized, natural gas can

leak from the closed blowdown valve and co mpressor drive shaft seals (piston rods on reciprocating compressors, rota ting drive shaft on centrifugal co mpressors). When compressors are shut down but not depre ssurized, the blowdown valve is closed but can still leak. Furthermore, reciprocating compressor rod packi ng is a dynamic seal, which can leak more when the compressor is shutdown under pressure. Centri fugal compressor seals “close” when the shaft stops spinning and seal oil (in wet s eal compressors) stops circulating.

When compressors are shut down for long periods of time in stand-by, or for maintenance, they are commonly blown down (depressurized by opening a blowdown valve to vent the high pressure gas in the compressor case or cylinders , suction and discharge bo ttles for reciprocating compressors and piping between the unit isolation va lves). This gas is typically vented out of a stack on the roof of a compressor building, or a high vent stack in outdoor compressors for safety

reasons. . The unit isolation valves are large and seal against a hi gh pressure differential between suction or discharge pressure and atmospheri c pressure, typically between 300 and 1000 psig. During this shutdown-depressurize d period leakage through the large unit isolation valves can be very large and go unnoticed as the emission is through the blowdown vent stack on a compressor building roof or outdoor elevated blowdown ve nt stack. If the comp ressor is shutdown for internal maintenance, leaking unit isolation valves will be “blinded” (flat, metal plate inserted between the flanges of the isolation valve on the compressor side ... a positive seal against gas leaking into the compressor during maintenance) . Obviously unit isolatio n valve leakage cannot

be detected or measured with a blind in place.

1 http://www.epa.gov/gasstar/documents/savings.pdfPage 3 of 4 Requirement for Emissions Measurement in Each Compressor Mode Based on comments received on the April 2010 proposed rule, compressors are not shutdown and depressurized on a forward schedule every year, but are taken o ffline for scheduled maintenance at least once during a three year period. EPA received co mments indicating that coordinating the once per year compressor emissi ons analysis with unscheduled shutdown for measuring emissions in the not-operating, de -pressurized mode would be difficult and burdensome. They pointed out that their operati ng commitments could no t necessarily be met by shutting down each compressor when the emissions measurement personnel were onsite. Further, since detecting and measuring the unit valve l eakage must be done wit hout blind flanges in place, this activity must be pre-planned and m obilized well ahead of a compressor shutdown for maintenance. EPA did not intend for reporters to perform unscheduled s hutdown of compressors simply for monitoring emissions in this made. Ther efore, today’s final rule clarifies that reporters

are allowed to measure vented emissions of each compressor in the mode found during an annual measurement, but must measure each compressor in the shutdown-depressurized mode at least once during any three consecutive year period. Em issions measurement in other modes must be performed each year in the mode that comp ressors are found during the annual measurement.

Hence, reporters are not requi red to shutdown compressors dur ing the reporting period for the purposes of monitoring under subpart W. Howeve r, reporters will be expected to schedule monitoring in conjunction with planned compre ssor shutdowns for maintenance such that unit isolation valves are tested for through-valve leakage before blind flanges are swung. EPA has

added a more cost effective alternative for this testing by allowing the us e of an acoustic leak

detection instrument that is calibrated for estimating through-valve leakage from the acoustic signal, valve type and size, and stream contents and pressure. This instrument can do the leak detection and quantification at on e time at the unit valve and not involve insertion meters or endof-pipe measurements on the blowdown vent stack.

Reporter Level Emission Factors for Emissions Measurement With the above provision in the final rule, wh erein not every compresso r vented emissions will be measured in every mode of operation each cal endar year, EPA is requir ing that reporter-level emission factors be developed for each mode of operation. All measurements made in a given mode of operation during the re porting year and two preceding repo rting years will be summed and divided by the number of compressors measure d. This reporter specific emission factor, in cubic feet per hour, will be used to estimate and report the emissions for all compressors not measured in that mode in the current reporting y ear, for the time in that mode. The reporter will still report emissions for compressors measured in the reporting year based on the compressor’s

measured vented emissions and duration in that mode. This way, if reporters make repairs (not required by the MRR) to leaking unit isolation valves, they can take advantage of that eliminated emission on that compressor which is measured.

For a hypothetical example, wh ere one reporter has four co mpressor stations (i.e. four transmission compressor facilities), each with five compressors for a total of 20 compressors, he conducts monitoring of each compressor in each compressor station the first year in the “asfound” mode. This monitoring finds that 75% of centrifugal compressors were in the operating mode, while the remaining 25% of centrifuga l compressors were in the not-operating, depressurized mode. In this cas e the reporter does not have complete emissions data to representPage 4 of 4 both modes of operation for each individual cen trifugal compressor in each station. EPA requires that the reporter deve lop an average per-hour emission factor for centrifugal compressor wet seal degassing vents in the operating mode and a separate per-hour emission factor for centrifugal compressors unit isolation valve leak age in the not-operating depressurized mode. Then, the reporter will use these average emission factors for those centrifugal compressors not measured in the associated mode by multiplying the factor by the cumulative time that compressors in each station were in the mode bu t not measured in that mode in this reporting year. The compressors measured in a mode will use their unique measurement for the time they were in that mode in that year . In a subsequent year, a compresso r measured in one of the modes in the previous year, but not in the present re porting year will use the average emission factor.

Emission Factor for Onshore Production Compressors The onshore production sector has many small comp ressors dispersed over hundreds of miles at production well-sites. These smaller compressors are typically recipr ocating type and not “spared” (i.e. a single compressor that does not have a standby sp are). Hence, they have a high operating factor and because of their type and si ze, have smaller process emissions (vented and equipment leaks) primarily from rod packing. By contrast the proces sing, transmission, gas storage, LNG storage and LNG im port/export terminal sectors gene rally have larger compressors within a single facility boundary or adjacent to that boundary. These large compressors are of different types (reciprocating, ce ntrifugal wet seal and centrifuga l dry seal). They are almost always spared, and therefore have lower operating factors (more time in the standby depressurized mode) and higher emissions, partic ularly from wet seal centrifugals.

Insufficient publicly available data was available for EPA to relia bly quantify the distribution of onshore production compressors by sizes and services to facilitate the de finition of a threshold between “smaller” and “larger” wellhead compresso rs to make it possible to provide an emission factor for “small” compressors and require meas urements from “large” compressors. As such, today’s final rule requires the us e of a process emissions (vented and equipment leaks) factor for all onshore petroleum and natu ral gas production compressors. For reciprocating compressors, the Final Rule provides process emission factor s which were adopted from the Gas Research Institute study 2 (Volume 8, 1994; available online at th e EPA GasStar site). For wet seal degassing emissions from centrifugal compressors , the Final Rule provides process emission factors which were adopted from a study reported at the 24 th World Gas Conference 3 which aggregated all available measurement data in to three factors: high, medium and low. For conservative estimation, EPA is using the low emission factor from this study.

2 GRI study available at http://epa.gov/gasstar/tools/related.html

3 “ Methane’s Role in Promoting Sustainable Deve lopment in the Oil and Natural Gas Industry”, http://www.epa.gov/gasstar/doc uments/best_paper_award.pdf