Natural gas is delivered from the transmission system to end-use customers by the distribution system. Unlike the transmission system, which carries large volumes of natural gas at high pressures, the distribution system winds through cities and other areas of gas demand at much lower pressures and through much smaller line pipe — typically from two to 24 inches in diameter. Pressures typically range from 60 psi (nearer the transmission line) to 1 psi as it reaches a home or small business. This pressure is important because the appliances used in your home or business are not designed to accommodate high gas pressure. Thus, as a rule, the closer the pipe gets to the end user, the smaller it is and the lower the pressure gets. Most commonly distribution mains operate at pressures of 10 psi or more, and the last pressure reduction to 1/4 psi occurs just prior to the customer meter.
While most residential and small commercial customers accept gas service at 1/4 psi, larger industrial and commercial customers may operate machinery that requires a higher pressure. Regardless of the ultimate delivery pressure, regulators are used to drop the pressures on the system to acceptable levels for the various end-use customers who take service from the distribution system.
Gas distribution systems are joined to transmission pipelines at an interconnect. At the interconnect are meters, regulators to control the gas pressure, and scrubbers and filters to ensure the gas is clean and free of water vapor. If it has not already been injected further upstream, the distribution company will inject mercaptan into the gas. Mercaptan is a harmless odorant that has the familiar smell of rotten eggs we all associate with natural gas. Because natural gas has no natural odor, this odorant is added before the gas enters the distribution system so that gas can be detected in the event of a leak.
Distribution systems consist of pipe (also called mains and lines – see below), small compressors that are used to boost pressure, regulators that are used to reduce pressure, valves that are used to control flow, metering used to measure flow at each customer location, and a SCADA system that provides the capability to monitor and sometimes remotely control components of the distribution system. In some cases, distribution systems also include local gas storage.
In many areas, plastic or PVC is now used for the construction of some distribution lines. Unlike the steel pipe, PVC is flexible, corrosion-resistant and costs less to install. But in some cases, steel is still used in areas with heavy external loading or high potential for third-party damage. In early times cast iron pipe was used for distribution and these pipes still exist in some areas.
The distribution system comprises five types of piping:
- Supply main — This is the pipe that runs between the interconnection with the transmission system and the feeder mains. Supply mains can also be used to provide direct connection to a large industrial customer or power plant. Typical pressures range from 150 to 400 psi.
- Feeder main — This is the pipe that connects the supply main to the distribution main. Feeder mains are connected to supply mains at a regulator station that reduces the relatively high pressure of supply mains. Typical pressures for feeder mains range from 26 to 60 psi.
- Distribution main — This is the pipe that snakes throughout the service territory bringing gas to areas of mass consumption. Typical pressures range from 1 to 25 psi.
- Service line — This is the much smaller line that connects a home or business with the distribution main that may be running underneath your street or sidewalk. Typical pressures are ¼ to 1 psi, but may be higher for larger customers.
- Fuel line — The final connection to a customer’s appliances, the fuel line is anything beyond the LDC meter that runs into a home or office. This is owned and maintained by the property owner and typically operates at a pressure of ¼ psi or less.