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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 Dehydrators 

Summary: This memo discusses the development of a minimum throughput of well pad gas 

dehydrators, below which emissions could be calculated using emission factors rather than rule 

specified equilibrium process models. 
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Equipment Threshold for Dehydrators 

This memo discusses the development of a minimum throughput of well pad gas dehydrators, 

below which emissions could be calculated using emission factors rather than rule specified 

equilibrium process models. Industry commenters pointed out correctly that many wellhead 

dehydrators have very small gas throughput and small methane vent emissions, for which 

application of the April 2010 proposed rule glycol dehydrator emissions model such as GRI’s 

GlyCalc® would be a substantial burden for little contribution to emissions. EPA determined 

that glycol dehydrators at well pads with gas throughput less than 0.4 million standard cubic feet 

per day (mmscf/day) represent a large fraction of well pad dehydrators while contributing a small 

fraction of total emissions from dehydrators. This memorandum describes the analysis that was 

conducted to determine this threshold. The dehydrator threshold developed is applicable to all 

dehydrators in subpart W. 

EPA examined thresholds from existing regulations and potential other equipment thresholds. 

Subsequently, EPA evaluated an acceptable equipment threshold using aggregated data from the 

HPDI database. As part of the analysis, it estimated the number of dehydrators that are above 

and below the threshold and their aggregate emissions contributions. 

Precedence for Glycol Dehydrator Thresholds 

The National Emission Standards for Hazardous Air Pollutants (NESHAP) regulations require 

operators to follow standards on installation, operation, monitoring of control devices for major 

sources of hazardous air pollutants. These standards are set forth in 40 CFR Part 63 Subpart HH1

for petroleum and natural gas production facilities, and in 40 CFR Part 63 Subpart HHH2 in 

petroleum and natural gas transmission and storage facilities. Under Subpart HH, owners or 

operator are exempt from standard set forth in §63.764 if 

(i) The actual annual average flowrate of natural gas to the glycol dehydration unit is 

less than 85 thousand standard cubic meters [3 million standard cubic feet] per day, as 

determined by the procedures specified in §63.772(b)(1) of this subpart; or 

 (ii) The actual average emissions of benzene from the glycol dehydration unit process 

vent to the atmosphere are less than 0.90 megagram per year, as determined by the 

procedures specified in §63.772(b)(2) of this subpart. 

This exclusion clause sets a threshold of 85,000 standard cubic meters per day (3 mmscf/day) 

throughput, below which it is not considered a major source of HAP emissions. 

Under Subpart HHH, standards set forth in §63.1275 only apply if the: 

 

1

 EPA. 40 CFR Part 63 63.760 – 63.779. Subpart HH—National Emission Standards for Hazardous Air Pollutants 

From Oil and Natural Gas Production Facilities. http://ecfr.gpoaccess.gov/cgi/t/text/textidx?c=ecfr&sid=2c6a4334a98427e866966423764af7f6&rgn=div6&view=text&node=40:10.0.1.1.1.8&idno=40 

2

 EPA. 40 CFR Part 63 63.1270 – 63.1289. Subpart HHH—National Emission Standards for Hazardous Air 

Pollutants From Natural Gas Transmission and Storage Facilities. http://ecfr.gpoaccess.gov/cgi/t/text/textidx?c=ecfr&sid=be33069d80bdecef68193057a216b329&rgn=div6&view=text&node=40:11.0.1.1.1.4&idno=40 
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(1) The actual annual average flow of gas to the glycol dehydration unit is less than 

283.0 thousand standard cubic meters [10 million standard cubic feet] per day, as 

determined by the procedures specified in §63.1282(a)(1); or 

(2) The actual average emissions of benzene from the glycol dehydration unit process 

vents to the atmosphere are less than 0.90 megagram per year as determined by the 

procedures specified in §63.1282(a)(2) of this subpart. 

This exclusion clause sets a threshold 283,000 standard cubic meters per day (10 mmscf/day) 

throughput, below which it is not considered a major source of HAP emissions. 

Quantitative Analysis of Threshold Options 

EPA applied the 80:20 criteria used for subpart W to select the top emission sources that 

contribute 80% of the emissions in a sector, while not including or minimizing the burden for 

sources that contribute the remaining 20% of emissions. 

Methodology 

HPDI database contains oil and gas production data from thousands of reporting properties 

across the U.S. EPA aggregated reporting properties by field, and the petroleum and natural gas 

production rates and well counts were estimated for each. 

The number of glycol dehydrators was estimated in the U.S. Inventory of Greenhouse Gas 

Emissions and Sinks (U.S. Inventory). EPA calculated the ratio of glycol dehydrators per well by 

dividing the total number of glycol dehydrators by the total number of gas producing wells. It 

used this ratio to determine the number of glycol dehydrators in each field, as characterized by 

HPDI. This analysis distributed dehydrators evenly to production properties by gas production 

to determine a size distribution of dehydrators. 

To fully characterize the uncontrolled emissions from glycol dehydrator units, emission factors 

for the absorbed methane released in the reboiler, and the natural gas used to power the glycol 

chemical circulation pneumatic pump were needed. Emission factors for both of these sources 

are available in the EPA/GRI3

 study Methane Emissions from the Natural Gas Industry, June 

1996. Under NESHAP regulations, onshore production operators of glycol dehydrators with 

throughputs greater than 3 mmscf/day are required to reduce their HAP emissions by 95%. As a 

result, to calculate the actual methane emissions, EPA scaled down the uncontrolled methane 

emissions from glycol dehydrators with a throughput greater than 3 mmscf/day by 95%. 

Results 

Table 1 shows the results of the quantitative analysis. To obtain 20% of the emissions 

determined by calculating with an emission factor, then the threshold should be set at 

approximately 0.386 mmscf/day. At the subpart HH NESHAP threshold of 3 mmscf/day, 97% of 

total emissions from glycol dehydrator vents would be calculated using an emission factor. At 

the 10 mmscf/day threshold, used in subpart HHH of NESHAP, 98% of total emissions from 

glycol dehydrator vents would be calculated using emission factors. 

 

3

 EPA/GRI. Methane Emissions from the Natural Gas Industry, Vol. 14. June 1996. Available online at: 

<http://epa.gov/gasstar/tools/related.html>. 
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Table 1: Options for thresholds on glycol dehydrator units. 

Percentage of 

Population 

Emissions 

Emissions 

Covered 

(scf/yr) 

Dehydrator 

Throughputs 

(scf/day) 

Dehydrator 

Count 

20% 1,620,755,208 386,909 22,644 

21% 1,689,438,814 400,000 23,022 

97% 7,851,805,696 3,000,000 33,423 

98% 7,973,038,944 10,000,000 37,035 

Total 8,114,537,522 37,816 

Emission Factor Development

EPA reviewed several data sources for emission factors suitable for glycol dehydrators with gas 

throughput less than 0.4 mmscf/day (rounded from 0.386 mmscf/day). It determined that the 

same emission factors it used to develop the threshold would be adequate to account for 

emissions from those dehydrators that fall below the threshold. The EPA/GRI3

 emission factors 

which were used for the analysis require the input for each dehydrator and its configuration (gas 

assisted glycol pump, electric pump, etc.) 

EPA determined it could reduce burden on reporters for determining emission from dehydrators 

falling below the threshold by developing a per-dehydrator emission factor, rather than requiring 

reporters to monitor the configuration and throughput. It divided the methane and carbon 

dioxide emissions covered at the 0.4 mmscf/day threshold in the above analysis by the total 

number of dehydrators that fall below that threshold. When adjusted for standard conditions 

(defined as 68 ̊F and 14.7 psia in subpart A), the resulting emission factors were 74.5 thousand 

standard cubic feet (mscf) per dehydrator per year of methane and 3.26 mscf/dehydrator/year of 

carbon dioxide. 

Conclusions

EPA determined that use of the glycol dehydrator throughput thresholds currently used under 

subpart HH or subpart HHH of the NESHAP regulations would result in collecting 97% or more 

of emissions data from this source by calculating with emission factors. This would be no better 

than the U.S. Inventory and not suitably provide data adequate to inform future policy. Using 

the principles of the 80:20 criteria, EPA determined that a minimum throughput (floor) of 0.4 

mmscf/day would require the largest 60% of wellhead dehydrators, representing 80% of 

emissions, to report using simulation software. This will allow nearly 40% of dehydrators, 

representing 20% of emissions, to report using an emission factor per dehydrator. This would be 

a substantial reduction in burden to reporters while maintaining the targeted data quality of this 

rule. 

EPA determined that the most appropriate emission factors for dehydrators below 0.4 mmscf/day 

throughput, are 74.5 mscf/dehydrator/year for methane and 3.26 mscf/dehydrator/year of carbon 

dioxide. These emission factors would provide sufficient estimates of emissions for these small 

dehydrators while diminishing burden to reporters.