Page 1 of 3 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: Equipment Threshold for Blowdowns

Summary: This memo discusses the development of a minimum volume threshold for reporting blowdown emissions in 40 CFR part 98 Subpart W – Petroleum and Natural Gas Systems of the Greenhouse Gas Reporting Rule.Page 2 of 3 Equipment Threshold for Blowdowns

This memo discusses the development of a minimum volume threshold for reporting blowdown emissions in 40 CFR part 98 Subpart W – Petrol eum and Natural Gas Systems of the Greenhouse Gas Reporting Rule . Commenters pointed out that the April 2010 proposed rule taken literally could be interpreted to cover ve nting of tiny volumes such as instrument tubing when blowing out accumulated liquids and level indicator glas ses. Including all these tiny blowdown sources

would be a substantial burden fo r little contribution to emissions . This was not EPA’s intent nor consistent with the types of “purge and blow” data collected by the EPA/GRI 1996 study 1 that indicates this source to be signi ficant. EPA determined that the industry proposed 50 cubic feet from compressor and vessel blowdowns is a suitable minimum threshold for reporting.

Today’s final rule has set a physical volume threshold for e quipment blowdowns, below which reporters are exempt from reporting. The final rule sets this threshold at 50 cubic feet for the

total internal volume between isolation valves of equipm ent being blown down to the atmosphere. Blowdown reporting does not apply to equipment blowdown to flare or EOR operations other than the critical phase CO 2 injection pumps.

Background Emissions from equipment blowdowns were determin ed to be a significan t source of greenhouse gas emissions in the U.S. GHG Inventory 2 . Blowdowns commonly occur from equipment such as compressors when they are shut down or starte d-up and from vessels a nd piping when it is

taken out of service for maintenance.

Commenters on the April 2010 proposed rule e xpressed concern over the blowdowns that are required to be recorded for purposes of calcula ting and reporting emissi ons. They pointed out

that in the absence of a threshol d that would limit the recording of blowdowns, this portion of the subpart W would be unduly burdensome in the frequency of a number of small blowdown events that result in insignificant volumes of natural gas, such as blowdowns from sight glasses and small tubing. Commenters have further suggested requiring emissions reporting from all compressor blowdowns and from process vessels and piping with a to tal volume between

isolation valves of 50 cubic feet and greater.

In addition, commenters expressed concern ove r the limited clarification of the kinds of equipment to be considered for reporting blow down emissions. The final rule clarifies which segments of the industry and types of equipmen t are included for the purposes of subpart W.

1 EPA/GRI. (1996) Methane Emissions from the Natural Gas Industry <http://www.epa.gov/gasstar/tools/related.html>

2 EPA. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 – 2008 . April 2010, U.S. EPA # 430-R-10- 006. <http://epa.gov/climatechange/emissions/usinventoryreport.html>.Page 3 of 3 Derivation of Equipment Threshold for Reporting Blowdown Emissions To determine whether the proposed threshold of 50 cubic feet is a reasonable limit for reporting,

EPA did a back-calculation of the emission fact or from compressor blowdowns in the EPA/GRI

report 3 , which is 3,774 standard cubi c feet/compressor. For a t ypical large processing or transmission compressor operating at a pressure of 750 psig to 800 psig, the physical volume of equipment between isolation valves that would ve nt a volume of 3,774 standa rd cubic feet can be derived using the Ideal Gas Law (assuming equimo lar and isothermal conditions in the system under operating pressure that vents to the atmosphere):

2 2 1 1 V P V P =

Thus, with P 1 set at 14.7 psia (atmospheric conditions), V 1 set at 3,774 cubic feet (under standard conditions) and with P2 considered at 800 psig (=814.7 psia; actual pressure in the compressor isolated volume before blowdown), V 2 (physical equipment internal volume contributing to blowdown through blowdown vent stacks) is calculated as:

7

. 814 774

, 3 7

. 14

2 ×

= V

resulting in V 2 of ~ 68 cubic feet.

This value is not far removed from commenters’ suggested 50 cubic feet of physical equipment volume. Hence, EPA deemed it appropriate to assign a reporting thres hold of 50 cubic feet between isolation valves for reporting emissions from equipment blowdowns, as a reasonable threshold based on analysis of the EPA/GRI report cited above. This is expected to significantly reduce reporting burden, while sustaining the necessary quality of data to inform future policy.

Equipment Threshold for Blowdown Vent Stacks in Final Rule Today’s final rule does not in clude reporting of blowdown emi ssions below 50 cubic feet of equipment internal volume between isolation valves. The rule clarifies that this volume includes all piping, compressor cases or cylinders, manifold s, suction and discharge bottles or any other gas-containing contiguous volume between the is olation valves. The rule does not include depressurizing to a flare, over-p ressure relief, operating pressure control venting, and blowdown of non-GHG gases. For onshore production, blowdown is not requi red to be reported with exception of the critical phase CO 2 pumps in EOR operations. The final rule stipulates the

calculation of the total volume between isolation valves may be determined using engineering

estimates based on best available data.

The total volume contained between isolation va lves for each process ve ssel and the number of times it was blown down in the calendar year is the actual volume of emissions; reporters will

convert the actual volume to standard conditio ns using equations provided in subpart W.

3 EPA/ GRI. Methane Emissions from the Natural Gas Industry , Vol. 7: Blow and Purge Activities. 1996. Available online at <http://epa.gov /gasstar/tools/related.html>.