Know the Natural Gas Lines Before Digging

Flowlines, Gathering Lines, Transmission, Distribution, and Service Lines

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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. 

The 5 Types

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.

1. Flowlines

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. Flowlines are relatively narrow pipes that carry unodorized raw gas at a pressure of approximately 250 psi (pounds per square inch). Typically, they are buried four feet underground and can corrode, especially if they are carrying wet gas. They 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

Gathering lines collect gas from multiple flowlines and move it to centralized points, such as processing facilities, tanks, or marine docks.

 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 and carry corrosive content that can affect pipeline integrity within a few years.

3. Transmission Pipelines

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. 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. 

Transmission pipelines can fail due to:

  • seam failures
  • corrosion
  • materials failure
  • defective welding

4. Distribution Pipelines

Distribution pipelines, also known as "mains," are the middle step between high pressure transmission lines and low pressure service lines. Distribution pipelines operate at an intermediate pressure. This type of pipeline uses small to medium sized pipes (2" to 24" in diameter) that are federally regulated and carry odorized gas at varying pressure levels, from as little as 0.3 up to 200 psi. Distribution pipelines typically operate below their carrying capacity and are made from a variety of materials, including steel, cast iron, plastic, and occasionally copper.

5. Service Pipelines

Service pipelines connect to a meter that delivers natural gas to individual customers. Service pipelines are narrow pipes (usually less than 2" diameter) that carry odorized gas at low pressures, such as 6 psi. Service pipelines are typically made from plastic, steel, or copper.

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