Equipment Leaks

Equipment leaks, commonly referred to as fugitive emissions, are characterized as unintentional emissions. Leaks occur from equipment components such as connectors, valves, open-ended lines, pressure relief valves, and storage tank thief hatches. There are millions of such components that are prone to unintended leaks throughout the natural gas supply chain. Leaks can be repaired when found, but some components are prone to eventually leak again. 

Equipment leaks, also known as fugitive emissions, are characterized as unintentional emissions. Leaks may occur from equipment components such as connectors, valves, open-ended lines, pressure relief valves, and storage tank thief hatches. Component leaks have many different causes, including: 

• Improper or loose fittings and connections: bolted flanges as well as threaded, compression, and bell-and-spigot connections may not have proper fit or become loose.  
• Damaged valves: fouled or damaged gates or plug seats in valves prevent a tight closure (i.e., through-valve leaks). 
• Worn valve stem seals: the packing around the shaft connecting the external valve hand wheel or actuator and the internal valve plug (stem) must be loose enough to allow stem movement, but tight enough to prevent significant leakage. Stem packing wears with frequent valve usage. 
• Corrosion/damage: deterioration of metal causes equipment connections and other components to degrade. 
• Worn/damaged gaskets: thief hatches on storage tanks may not properly seal if the gasket is worn or damaged.  
• Human error: components such as valves or thief hatches are inadvertently left open. 

Different types of components have different leak potentials and wear out at different rates. In general, equipment components develop leaks caused by wear and tear from use as well as vibration and temperature variations in the operating environment. Components on or near compressors are particularly prone to develop leaks because of the vibration and temperature/pressure variations caused by compressor starts and stops. Leaking components, even after successful repair, may eventually leak again without regular inspection and maintenance. 

Emissions that are designed to occur, such as compressor seals and pneumatic pumps and control vents, or are caused by intentional operations, such as opening vents on equipment to expel combustible gas for maintenance, are not considered “fugitive” emissions. These are categorized as intentional “vent” emissions. Generally, sweet (i.e., sulfur free) natural gas is odorless and colorless so leaks can easily go unnoticed. However, leaks will be immediately obvious to an operator at a gas production site if the gas is “sour” (i.e., contains hydrogen sulfide [H2S], a poisonous and odorous gas). Additionally, natural gas in the distribution segment, and in certain areas in the transmission segment, is odorized to make leaks more apparent through olfactory (smell) identification. Therefore, components in sour or odorized gas service tend to be better maintained and have lower emissions than those in sweet or unodorized service. 

Factors that differentiate equipment leaks in each industry segment are primarily related to the size and number of components, as well as system pressure, including the following: 

• Production segment: Production sites typically include components such as wellhead(s), compressors, generators, dehydrators, storage vessels, engines, boilers, heaters, flares, separation and processing equipment. A single well pad may contain one or more wellheads with associated equipment and fugitive components, which are generally small (e.g., flanges, valves, meters, hatches) and operate under low to moderate operating pressures ranging from 100 to 500 pounds per square inch gauge (psig)).  
• Gathering and boosting and processing segments: Gathering pipelines and other equipment collect gas from onshore production wells, and compress/transport the gas to a natural gas processing facility or a natural gas transmission pipeline. Gathering and boosting equipment includes, but is not limited to, gathering pipelines, separators, compressors, acid gas removal units, dehydrators, pneumatic devices/pumps, storage vessels, engines, boilers, heaters, and flares. Facilities in this segment typically have thousands of fugitive components, from small pneumatic gas tubing connectors to large pipe connectors and valves, operating at pressures up to 600 psig.  
• Transmission segment: Fugitive leaks from transmission stations are associated with pipeline equipment (i.e., meters, dehydrators) and compressor-related components operating under high pressure up to 1,000 psig. Transmission compression stations typically have hundreds of potential leak sources (e.g., valves, seals), while transmission pipelines have isolation valve stations which are typically underground.  
• Distribution segment: Fugitive components in this segment are associated with pipelines and meters that operate at low to moderate pressures, ranging from 0.25 psig to 600 psig, with small, mostly below-grade (i.e., underground) components.  
• Liquefied natural gas (LNG) import/export facilities: LNG facilities operate at very low, near atmospheric pressures and store liquefied methane at cryogenic temperatures. More of the piping connections are welded in LNG facilities compared to all other sectors, so fugitive sources are generally limited to valves and flanges around compression equipment which operates between 100 and 1,000 psig pressure.

New Mexico Methane Advisory Panel. (2019, Fall). New Mexico Methane Advisory Panel draft technical report. https://www.env.nm.gov/wp-content/uploads/sites/15/2019/08/MAP-Technical-Report-December-19-2019-FINAL.pdf (6 MB)

U.S. Environmental Protection Agency. (2018, September). Oil and natural gas sector: Emission standards for new, reconstructed, and modified sources – Background technical support document for the proposed reconsideration of the New Source Performance Standards 40 CFR Part 60, subpart OOOOa.https://www.regulations.gov/document/EPA-HQ-OAR-2017-0483-0040

U.S. Environmental Protection Agency. (2020, February). Natural Gas STAR Methane Challenge Program Continuous Improvement Update: Finalizing an equipment leaks/fugitive emissions commitment option for compressor isolation and blowdown valve leakage. https://www.epa.gov/sites/default/files/2020-07/documents/mc_ci_equipleaks-compvalves_techdoc_final.pdf (468 KB)