Editor’s Note: This blog post is the final installment in a three-part series. Previously we discussed gas gathering pipelines and gas transmission pipelines.
The three main types of natural gas pipelines are gas gathering, gas distribution, and gas transmission.
This week, we’ll look at gas distribution lines.
What are Gas Distribution Pipelines?
Distribution Pipelines are the smaller lines that move mainly gas within communities and to our individual homes and businesses. Distribution pipelines consist of a system of mains and service lines and operate at a relatively low pressure
Currently, about 2,300,000 miles of natural gas distribution mains and service pipelines are in operation.
Processed natural gas is odorless, so all distribution pipelines are required to be odorized so leaking gas is readily detectable by a person with a normal sense of smell.
At the “city gate” a transmission system feeds into a lower pressure distribution system that brings natural gas directly to homes and businesses. This is where the pressure of the gas is reduced, and it is normally the location where the odorant (typically mercaptan) is also added to the gas.
Distribution systems operate at much lower pressures. Some gas mains (two to 24 inches in diameter) in a distribution system may operate up to 200 pounds per square inch (psi), but the small service lines that deliver gas to individual homes are typically well under 10 psi.
Distribution pipelines may also be made of steel, but now, increasingly high strength plastic or composites are used. Older distribution pipelines were frequently made of cast iron. Cast iron gets brittle with age, and can be susceptible to fractures when subjected to ground movement from freeze/thaw cycles or other causes.
Distribution mains must be buried at least 24 inches deep with a few exceptions. Service lines on distribution systems must be 12 inches deep on private property, and 18 inches deep along roads and streets. The depth of burial is just for installation, and there is nothing in the federal regulations that requires this depth be maintained over time. These depth requirements went into effect in 1970, and pipelines that were installed before that time did not have to meet these requirements.
In 2006, Congress passed a statute requiring excess flow valves (EFVs) to be installed on every new home built and when service lines are replaced. EFVs are small inexpensive valves that go on each service line. These EFVs shut off the gas to a home automatically if the line is broken by things such as excavation. In 2016, Department of Transportation’s Pipeline and Hazardous Materials Safety Administration (PHMSA) expanded the requirements to include installation of EFVs on multifamily dwellings and some commercial buildings.