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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: Scrubber Dump Valves Memo 

Summary: This document provides background on transmission scrubber dump valve 

emissions and why this source warrants monitoring in the Final Rule. 
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Scrubber Dump Valves 

EPA has elected to include transmission storage tanks in Subpart W of the Greenhouse 

Gas Reporting Rule (GHGRP) as an emission source for reporting because there is 

information suggesting malfunctioning scrubber dump valves can vent a significant 

amount of gas through the compressor station condensate tanks. This document provides 

background on scrubber dump valve emissions and why this source warrants monitoring 

in the Final Rule. 

Scrubber dump valves, when stuck open, allow the gas feed to a compressor to blow 

through a condensate tank and out the tank vent. Scrubber dump valves were not 

identified in the EPA/GRI 1996 Study, and therefore, are not an emission source in the 

U.S. Inventory1

. Natural Gas STAR2 Partners have stated that scrubber dump valves 

represent one of the largest leaking components found at natural gas transmission 

stations. EPA believes that malfunctioning scrubber dump valves contribute significantly 

to transmission storage tank emissions similar to how malfunctioning gas-liquid separator 

dump valves may contribute to onshore production tank emissions. EPA expects the 

GHGRP will collect sufficient information and data on the significance of transmission 

compressor scrubber dump valve malfunctions to inform future policy at minimal 

additional burden to the petroleum and natural gas industry. 

I. The Scrubber Dump Valve Emission Source 

Scrubber dump valves open periodically to reduce the accumulation of condensate liquids 

in the scrubber. Scrubbers are located immediately before compressors to remove any 

liquids in the natural gas that may harm the compressor. The natural gas feed enters and 

contacts the cool metal wall or demister pad of the scrubber. The slightly cooled or 

filtered natural gas rises and exits the top of the scrubber while any liquids entrained in or 

condensed from the gas will drop out as condensate and accumulate at the bottom. When 

the accumulated condensate reaches a set level, a level controller opens the scrubber 

dump valve to allow the condensate to flow under scrubber pressure typically to an 

atmospheric tank. When the condensate falls to a set low level, a properly functioning 

dump valve closes, as diagramed in Exhibit 1. 

 

1

 EPA. (2007) Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006. Online at 

<http://epa.gov/climatechange/emissions/downloads/08_CR.pdf> 

2

 http://www.epa.gov/gasstar/ 
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Exhibit 1. Scrubber Diagram 

Some instances have been reported of dump valves remaining open after the liquid level 

has fallen below the set point, allowing natural gas from the scrubber to continuously 

flow through the dump valve ultimately passing through the storage tank’s atmospheric 

vent. This may occur because debris or an equipment malfunction prevents the scrubber 

dump valve from closing properly. An improperly closed scrubber dump valve can be a 

significant source of methane emissions because, for an extended period, a portion of the 

natural gas compressor feed is essentially vented to the atmosphere. 

Transmission scrubber dump valves are not included in the U.S. Inventory as an emission 

source. In the Technical Support Document (TSD) (EPA-HQ-OAR-2009-0923) the 

source selection analysis decision tree was not necessarily ideal for scrubber dump valves 

because it is known to be underestimated in current inventories. Therefore, the decision 

tree was overridden and this source was included in the reporting rule.. EPA’s 

knowledge of scrubber dump valves consists of presentations by Natural Gas STAR 

Partners. 

II. Evidence that Scrubber Dump Valves are a Significant Emission Source 

Natural Gas STAR Partners have experienced leaking scrubber dump valves as an 

emission source. One Natural Gas STAR Partner who inspected and quantified emissions 

from transmission stations using an IR camera and high volume sampler stated that 

scrubber dump valves leaking through condensate tanks were their largest source of 

emissions, exceeding emissions from reciprocating compressor rod packing and 
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blowdown valves.3

 In addition, Heath Consultants Inc. conducted a transmission 

compressor station measurement study that showed emissions from scrubber dump valves 

contributing more than 90 percent of the total emissions.4

 Exhibit 2 shows this result. 

Exhibit 2. Natural Gas STAR Transmissions Compressor Station Emissions 

Webcast

For transmission scrubber dump valves, as the emission point is distant from the 

malfunctioning valve, the gas is not detectible by sound, sight or smell (unless sour 

production gas) and remote, unmanned facilities may experience such undetected gas 

blow-through for long periods of time. Dump valves on scrubbers open when the liquid 

level in the vessel is above a set level and close when the liquid level in the vessel is 

below a set level, but still covering the drain nozzle. If the dump valve malfunctions and 

remains open, the gas entering the vessel will partially exit through the dump valve. And 

since scrubber dump valves typically allow liquid to pass to a storage tank, bypassed gas 

will also pass to the storage tank and ultimately through the tank vent, into the 

atmosphere. 

Based on this evidence, EPA chose to require natural gas transmission reporters to 

inspect for malfunctioning scrubber dump valves and quantify emissions from 

transmission storage tanks for which upstream scrubber dump valves have been 

determined to malfunction. 

 

3

 EPA. Methane Savings from Compressors. Compressors Technology Transfer Webcast. Presentation. 18 

Jan. 2006. Slide 36 

4

 EPA. Methane Savings from Compressors. Compressors Technology Transfer Webcast. Presentation. 18 

Jan. 2006. Slide 39 

Dump Valves

$14,651,510

Rod Packing

$1,034,090

Standardized 

Components

$207,170

Unit Valves

$445,320

Pressure Relief 

Valves

$62,980

Blow Down 

Valves

$223,582

Heath Consultants Inc. 2005

321 Leak Sources

1.6 Bcf

$10/Mcf
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III. Reporting Scrubber Dump Valve Emissions in Subpart W 

Subpart W requires onshore natural gas transmission reporters to annually view their 

storage tank vents with an optical leak imaging camera during the annual leak inspection. 

Only when emissions are viewed continuously for 5 minutes or through-valve leakage is 

detected, the reporter is required to directly measure the tank’s emissions with a meter or 

each dump valve’s through-valve leakage with an acoustic detection device which has 

correlation factors for through-valve leakage. Alternatively, operators can inspect the 

dump valves individually for leakage using an acoustic leak detection device. 

Summary 

In summary, EPA included scrubber dump valves via transmission storage tanks in the 

Final Rule because Natural Gas STAR Partners have demonstrated them to be a 

significant emission source in onshore natural gas transmission. EPA incorporated 

reporting scrubber dump valve emissions in subpart W while keeping the additional 

burden to reporters at a minimum. Given the highly uncertain, yet significant emissions 

from scrubber dump valves, EPA expects to gather sufficient information on the 

significance of this source’s emissions to inform future policy.