Natural gas travels from the wellhead to end consumers through a series of pipelines. These pipelines -- including flowlines, gathering lines, transmission lines, distribution lines, and service lines -- carry gas at varying rates of pressure.
The higher the pressure of gas in a pipeline, the more potentially dangerous an accident with that pipeline could be.
Pipelines are usually buried underground. Pipeline markers, such as those shown at right, do not always sit directly above the pipelines. For safety before digging, dial 811 to learn whether any pipelines are buried nearby.
Purpose: Flowlines connect to a single wellhead in a producing field. Flowlines move natural gas from the wellhead to nearby storage tanks, transmission compressor stations, or processing plant booster stations.
Description: Flowlines are relatively narrow pipes that carry unodorized raw gas at a pressure of approximately 250 psi (pounds per square inch). Typically, flowlines are buried four feet underground.
Special considerations: Flowlines can corrode, especially if they are carrying wet gas. Flowlines are also prone to methane leakage. According to the EPA, "Methane leakage from flowlines is one of the largest sources of emissions in the gas industry."
2) Gathering Lines
Purpose: Gathering lines collect gas from multiple flowlines and move it to centralized points, such as processing facilities, tanks, or marine docks.
Description: Gathering lines are medium size steel pipes (usually under 18" diameter) that carry unodorized, raw gas at a pressure of approximately 715 psi. Typically, gathering lines are buried four feet underground.
Special considerations: Gathering lines carry corrosive content that can affect pipeline integrity within a few years.
3) Transmission Pipelines
Purpose: Transmission pipelines carry natural gas across long distances and occasionally across interstate boundaries, usually to and from compressors or to a distribution center or storage facility.
Description: Transmission lines are large steel pipes (usually 2" to 42" in diameter; most often more than 10" diameter) that are federally regulated. They carry unodorized gas at a pressure of approximately 200 to 1,200 psi.
Special considerations: Transmission pipelines can fail due to: seam failures, corrosion, materials failure, or defective welding.