What is EPA Subpart W for midstream operations?

EPA 40 CFR Part 98 Subpart W covers greenhouse gas reporting for petroleum and natural gas systems. Facilities emitting 25,000 metric tonnes CO2e per year or more must report annually. Sources include combustion equipment, venting, flaring, fugitive emissions, pneumatic devices, dehydrators, and compressor seals.

What GWP values should I use for methane?

EPA currently requires IPCC AR4 values: methane GWP of 25 and N2O GWP of 298 for Subpart W reporting. IPCC AR5 uses GWP of 28 for methane. The 20-year GWP for methane is 80-84, reflecting its stronger short-term warming effect.

How are fugitive emissions estimated for midstream facilities?

EPA provides default emission factors per component type and service. For example, gas service valves emit 0.0268 kg/hr, connectors 0.0006 kg/hr, and open-ended lines 0.0165 kg/hr. Multiply by component count and 8,760 hours/year for annual emissions. LDAR programs can reduce these by 40-90%.

What is the EPA reporting threshold for greenhouse gas emissions?

The reporting threshold under Subpart W is 25,000 metric tonnes of CO2e per year. Facilities exceeding this threshold must report to EPA annually via the electronic Greenhouse Gas Reporting Tool (e-GGRT). Even below the threshold, state regulations may impose additional requirements.

GHG Emissions Estimator | Midstream Greenhouse Gas Calculator

Emission Source

Source Type

Methane GWP Basis

Combustion Parameters

Fuel Type

Fuel Consumption Rate

MSCF/day

Operating Hours per Year

hr/yr

Higher Heating Value (HHV)

Btu/scf

Auto-fill: NG = 1,020 Btu/scf | Propane = 2,516 Btu/scf | Diesel = 137,000 Btu/gal

Venting Parameters

Vent Gas Flow Rate

MSCF/day

Methane Content

mol%

Hours of Venting per Year

hr/yr

Flaring Parameters

Flare Gas Flow Rate

MSCF/day

Flare Destruction Efficiency

Methane Content

mol%

Operating Hours per Year

hr/yr

Fugitive Emission Components

Service Type

Number of Valves

count

Number of Connectors

count

Number of Open-Ended Lines (OELs)

count

Number of PRVs

count

Number of Pump Seals

count

Number of Compressor Seals

count

Pneumatic Devices

High-Bleed Devices

count

EPA: 37.3 scf/hr CH₄ per device

Low-Bleed Devices

count

EPA: 1.39 scf/hr CH₄ per device

Intermittent Devices

count

EPA: 13.5 scf/hr CH₄ per device

Operating Hours per Year

hr/yr

Dehydrator Vent Parameters

Gas Throughput

MMSCF/day

Flash Gas Rate

MSCF/day

Methane Content in Flash Gas

mol%

Operating Hours per Year

hr/yr

Understanding GHG Emissions from Midstream Operations

EPA Subpart W
40 CFR Part 98 Subpart W requires annual GHG reporting for petroleum and natural gas facilities emitting ≥25,000 metric tonnes CO₂e/year. Covers gathering, boosting, processing, transmission, and storage.

Key Emission Sources
Combustion (engines, heaters), venting (tanks, blowdowns), flaring (routine/emergency), fugitive (component leaks), pneumatic devices, and glycol dehydrator vents.

Global Warming Potentials (GWP)
CO₂ = 1 (reference) | CH₄ = 25 (AR4) / 28 (AR5) / 80 (20-yr) | N₂O = 298. Methane is a potent short-lived greenhouse gas; reducing methane emissions is a key strategy for near-term climate benefits.

Learn the Theory

Understand GHG reporting regulations, Subpart W requirements, emission factors, and reduction strategies

Read Engineering Guide →

Key Emission Factors

CO₂e = CO₂ + CH₄ × GWP + N₂O × 298

Natural Gas Combustion: 53.06 kg CO₂/MMBtu

Diesel Combustion: 73.96 kg CO₂/MMBtu

Propane Combustion: 63.07 kg CO₂/MMBtu

Methane Density: 0.0423 lb/scf at 60°F

Flare Default DE: 98%

Standards & References

EPA 40 CFR Part 98 Subpart W
Petroleum and Natural Gas Systems GHG Reporting
API Compendium (2009)
Compendium of GHG Emissions Methodologies for Oil & Gas Industry
EPA AP-42
Compilation of Air Pollutant Emission Factors
IPCC AR4 & AR5
Global Warming Potentials for GHG Reporting
EPA GHGRP Technical Support
Subpart W Calculation Methodology

Engineering Notes

Reporting Threshold: 25,000 MT CO₂e/yr triggers EPA Subpart W reporting
Methane GWP: EPA requires AR4 (GWP=25) for current reporting; AR5 (28) for voluntary
Flare Efficiency: 98% is EPA default; actual may be 95-99.5% depending on operation
Fugitive LDAR: Leak Detection and Repair programs can reduce fugitive emissions 40-90%
Pneumatic Conversion: Replacing high-bleed with low-bleed devices reduces emissions by ~96%
This tool provides screening-level estimates. Detailed Subpart W reporting requires site-specific measurements.

Quick Reference — EPA Fugitive EFs

Valves (gas): 0.0268 kg/hr/component
Connectors: 0.0006 kg/hr/component
Open-Ended Lines: 0.0165 kg/hr/component
PRVs (gas): 0.0447 kg/hr/component
Pump Seals: 0.0160 kg/hr/component
Compressor Seals: 0.2360 kg/hr/component

GHG Emissions Estimator

Emission Source

Combustion Parameters

Venting Parameters

Flaring Parameters

Fugitive Emission Components

Pneumatic Devices

Dehydrator Vent Parameters

Understanding GHG Emissions from Midstream Operations

Learn the Theory

Results

Key Emission Factors

Standards & References

Engineering Notes

Quick Reference — EPA Fugitive EFs

Related Calculators

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Glycol Dehydration

Compressor HP

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