Natural gas accounts for nearly one-fourth of the energy consumed in the United States, and 33 states have produced it in commercial quantities. It has over 68 million residential and five million business customers in the U.S., which receive natural gas through 2.6 million miles of pipelines. Gas pipelines are categorized in various ways, depending on their capacity, destination and jurisdiction. For example, these pipelines can be classified as gathering, transmission and distribution lines, which determine not only how they’re used but how they’re regulated. Anyone involved with gas pipelines must understand the occasionally subtle distinctions between these pipeline classifications.
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Gathering Lines
Gathering lines transport gas from a production facility such as a wellhead to a transmission line, also known as a main. The diameter of these pipes ranges from two to eight inches, which is relatively small. Gathering lines can be this narrow because they typically use field compressors to create pressure that moves the gas through the pipeline. These devices use a turbine or internal combustion engine that’s typically powered by a small portion of the gas they transport.
Some gathering systems include a processing facility that performs additional functions like removing impurities. Substances such as water, carbon dioxide and sulfur can corrode pipes, while inert gases such as helium reduce the energy value of natural gas. These impurities are often used in applications such as chemical feedstocks.
Transmission Lines
Natural gas moves from a gathering system to a transmission system, which transports gas long distances. The diameter of these pipes typically ranges from 6 to 48 inches and is pressurized to between 200 and 1,500 pounds per square inch (psi), depending on the production method. These high pressures are needed to move the gas from producing regions to local distribution companies (LDCs), which can be a distance of thousands of miles.
Transmission pipelines are typically designed to handle much greater pressure than will ever be required as a safety measure. For example, the lines in populated areas generally don’t operate at more than half their designed pressure limit. Furthermore, many of these pipelines are looped, meaning that is more than one line pipe running between the same origin and destination. This redundancy increases a transmission pipeline’s maximum capacity, which may be needed during periods of peak demand.
Distribution Lines
The gas in a transmission pipeline typically passes through a gate station when it reaches a local gas utility. The gate station reduces the pressure in the line to a distribution level, which is between 0.25 and 200 psi. This facility also introduces an odorant to the natural gas, which is normally odorless. The odorant gives gas a sour smell that consumers can detect in small quantities as a safety measure. A gate station also measures the gas’s flow rate to determine the quantity received by the gas utility.
The gas then moves from the gate station to a distribution line, which typically has a diameter ranging from 2 to 24 inches. A distribution lines usually has sections that operate at different pressures, which are controlled by regulators. Pipe size and pressure generally decrease as the distribution line approaches the customer.
The operators in the gas utility’s control center continuously monitor the gas’s flow rate and pressure at various points to ensure that gas reaches customers with enough flow rate and pressure to run equipment. They must also ensure that pressures remain below specified limits for safety purposes. The proximity of distribution lines to customers generally limits their pressure to 20 percent of their design maximum.
Regulators control the flow of gas through a distribution system. They will open to increase gas flow when the pressure in a section has dropped below a specified set point and close when the pressure rises above another set point. Distribution pipelines also have relief valves that can vent the gas into the atmosphere as an additional safety measure to prevent the pipes from bursting.
Modern gas distribution systems use software to evaluate their capacity and ensure that consumers receive gas above the minimum pressure needed to upgrade appliances. These lines are also interconnected in a grid pattern with a series of shutoff valves that minimize service disruptions during maintenance and emergencies.
Construction
Safety is a critical consideration in the construction of gas pipelines, due to the pressures they must withstand and the consequences of a ruptured pipeline. Distribution lines have the highest construction standards due to their proximity to people. Pipes must be inspected to ensure they meet both government and industry safety standards. Gathering and transmission pipelines are specifically designed for their intended role in a gas pipeline, although they’re both generally made of rolled high-carbon steel. The length of each pipe segment is generally between 40 and 80 feet. Diameter and thickness are highly dependent on factors such as prevailing soil conditions, geography and population density.
Distribution pipelines were originally made of cast iron, which becomes brittle with age. Steel is still a common material for older pipelines, although new ones are increasingly likely to be made from high-strength plastic or composite materials. Older distribution pipes may be made of Aldyl-A plastic, which is particularly susceptible to becoming brittle. The National Transportation Safety Board has recommended the replacement of distribution pipelines made of this type of plastic.
Pipelines are subject to constant stresses that can cause them to fracture. Ground movement due to freeze/thaw cycles is the primary cause of these stresses, which is commonly known as frost heaving. Some states require pipelines to be inspected during the winter, allowing them to be repaired before they fracture.
Installation
Pipelines have historically been installed with open trenches, which is still the most common method for gathering and transmission lines. Distribution lines are more likely to be installed with trenchless methods such as boring and horizontal directional drilling (HDD), since they involve less environmental disturbance. Boring is particularly common for distribution pipelines in urban environments due to its usefulness in crossing roads.
Trenchless methods pose a greater risk of damaging existing utilities, since they involve boring and drilling rather than open digging. Metal lines are relatively easy to detect with metal detection equipment, but sewer lines made of clay and plastic require must be detected with less reliable ultrasound technology. Furthermore, damaged sewer lines may go undetected until a home owner notices a clogged sewer. The greatest risk of a cross bore is that plumbers often use a powered auger to clear a clogged sewer line, which can break a gas line.
Federal regulations generally require all transmission lines and some gathering lines to be buried at least 30 inches underground in rural areas and at least 36 inches in populated areas. Roads and railway crossings also require these lines to be buried at least 36 inches deep. The minimum depth for water crossings can vary from 18 to 48 inches, depending on the composition of the soil or rock. Distribution lines generally must be buried at least 24 inches deep, although the minimum depth is reduced to 18 inches along roads and 12 inches on private property. These minimum depths only apply at installation and aren’t required to be maintained over time.
Underground Services, Inc. is one of the oldest, full-service Subsurface Utility Engineering (SUE) companies in the United States. Contact us today at (610) 738-8762 or request a quote online to find out how we can help you with your construction project.
