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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. 

 
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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 – Petroleum 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 venting of tiny volumes such as instrument tubing when blowing 

out accumulated liquids and level indicator glasses. Including all these tiny blowdown sources 

would be a substantial burden for 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 study1

 that 

indicates this source to be significant. 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 equipment 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 equipment being blown down to the 

atmosphere. Blowdown reporting does not apply to equipment blowdown to flare or EOR 

operations other than the critical phase CO2 injection pumps. 

Background 

Emissions from equipment blowdowns were determined to be a significant source of greenhouse 

gas emissions in the U.S. GHG Inventory2. Blowdowns commonly occur from equipment such 

as compressors when they are shut down or started-up and from vessels and piping when it is 

taken out of service for maintenance. 

Commenters on the April 2010 proposed rule expressed concern over the blowdowns that are 

required to be recorded for purposes of calculating and reporting emissions. They pointed out 

that in the absence of a threshold 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 total volume between 

isolation valves of 50 cubic feet and greater. 

In addition, commenters expressed concern over the limited clarification of the kinds of 

equipment to be considered for reporting blowdown emissions. The final rule clarifies which 

segments of the industry and types of equipment 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>. 
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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 factor from compressor blowdowns in the EPA/GRI 

report3

, which is 3,774 standard cubic feet/compressor. For a typical 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 vent a volume of 3,774 standard cubic feet can be 

derived using the Ideal Gas Law (assuming equimolar and isothermal conditions in the system 

under operating pressure that vents to the atmosphere): 

P1V1 = P2V2

Thus, with P1 set at 14.7 psia (atmospheric conditions), V1 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), V2 (physical equipment internal volume contributing to 

blowdown through blowdown vent stacks) is calculated as: 

814.7

14.7 3,774

2

×

V =

resulting in V2 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 threshold 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 include reporting of blowdown emissions below 50 cubic feet of 

equipment internal volume between isolation valves. The rule clarifies that this volume includes 

all piping, compressor cases or cylinders, manifolds, suction and discharge bottles or any other 

gas-containing contiguous volume between the isolation valves. The rule does not include 

depressurizing to a flare, over-pressure relief, operating pressure control venting, and blowdown 

of non-GHG gases. For onshore production, blowdown is not required to be reported with 

exception of the critical phase CO2 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 valves for each process vessel 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 conditions 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>.