Components and Characteristics of Transmission Lines and Pipelines
Descriptions of major components of electric transmission and natural gas and oil pipeline facilities.
What Are the Components of Transmission Lines?
Transmission towers (support structures) keep the high-voltage conductors (power lines) separated from their surroundings and from each other. They can be open lattice metal structures or one or more wood or metal poles. Height can be over 150 feet with crossarms as much as 100 feet wide.
Conductors are metal wires, cables, and bus-bars used for carrying electric current. Conductors may be solid or stranded, that is, built up by an assembly of smaller solid conductors.
What Are the Components and Characteristics of Natural Gas Pipelines?
Natural gas mainline pipes usually vary from 16 to 48 inches in diameter, but most interstate pipelines are between 24 and 36 inches in diameter. The pipe consists of strong carbon steel material that meets the standards of the American Petroleum Institute. The pipe is also covered with a specialized coating (e.g., fusion bond epoxy) to ensure that it does not corrode. In addition, cathodic protection is often used to ward off corrosion and rusting. Natural gas travels at high pressures of 200 to 1,500 pounds per square inch (psi) which reduces the volume of natural gas being transported by up to 600 times and provides the propellant force to move it through the pipeline. General information on interstate natural gas pipelines, including numerous reports, can be found on the Web site of the Interstate Natural Gas Association of America.
Compressor stations generally occupy 15 to 22 acres of land and are located at 40- to 100-mile intervals along a pipeline. These facilities compress the natural gas by use of a turbine, motor, or engine to maintain the flow of the gas along the pipeline. Compressor stations also usually have a liquid separator to capture any liquids or other undesirable particles that may have condensed out of the gas stream while in transit.
Pipeline Inspection Gauge Launch/Recovery (Pigging) Facilities provide access to a pipeline for the purposes of cleaning and maintenance (pipelines are cleaned and monitored internally by devices known as "pigs" that travel through the pipeline). These facilities are usually smaller than pump stations or compressor stations and typically consist of one or more short sections of above-ground pipeline, valves, and other control equipment, and may include buildings (typically made of sheet metal), generators, storage areas, and a helipad. Pigging facilities are normally fenced and surfaced with gravel. Metering and regulating stations are placed periodically along the pipeline to allow pipeline companies to monitor and manage the natural gas in their pipes. They measure and track the flow of gas along the pipeline without impeding its movement.
Valves are placed every 5 to 20 miles along the pipeline. These devices work like gateways to allow gas to flow freely through the pipe when open, or they can be closed to stop gas flow along a section of pipe. They may be enclosed by a railing and are typically about waist high.
What Are the Components and Characteristics of Oil Pipelines?
Oil mainline pipes can be up to 48 inches in diameter. Oil moves through a pipeline at speeds between 2.5 to 5.0 mph.
Pump stations are generally located every 50 to 200 miles along the pipeline. They contain centrifugal pumps used to maintain movement of oil within the pipeline. Pump station sites, which may exceed 25 acres, may contain large liquid storage tanks. Pigging facilities for oil pipelines are usually co-located at pump stations.
What Are the Ancillary Facilities Associated with a Transmission Line and/or Pipeline?
Access roads are gravel or dirt roads (rarely paved) that provide overland access to transmission line or pipeline rights-of-way (ROWs) for construction, inspection, maintenance, and decommissioning. Access roads have an average distance of 5 miles or less and have a nominal width of 15 feet, and exist within the center of a nominal 25-foot-wide ROW. Access roads of the type minimally necessary to support operation and maintenance would be maintained along the mainline ROW throughout the operating period. Access roads to critical support facilities located along the mainline ROW, such as pump stations, compressor stations, and electric substations, would be maintained in gravel throughout the operating life of the transport system; vegetation at these facilities would be continuously managed for security, operational expediency, and fire safety purposes.
A centralized control station monitors and controls the natural gas and oil flow through the pipeline by collecting, assimilating, and managing data received from metering and regulating stations and compressor or pump stations located along the pipeline. The control center also monitors all leak detection systems, isolation valves, and other fire and building monitoring systems of remote facilities, such as pump stations.
A city gate station is a small facility that is located where gas from a transport pipeline would be distributed to small-diameter gas mains for eventual end use. City gate stations are normally gravel surfaced, fenced facilities that have short segments of above ground pipes and valves and one or more control buildings.
A ROW is the land on which transmission lines and/or pipelines are located. The ROW is usually acquired in widths that vary with the kilovolt (kV) size of the transmission line, diameter of the pipeline, or number of collocated transmission lines and/or pipelines (e.g., they may range from as little as 25 ft to hundreds of feet wide). They provide a safety margin between the transmission line or pipeline and surrounding structures or vegetation. Some vegetation clearing is usually required within the ROW for safety and/or access. Vegetation favored within the ROW normally consists of species that are slow growing or that have low mature heights. Access roads may also be contained within portions of a ROW to provide convenient access for repair, maintenance, and inspection vehicles.
Staging areas are areas located along the route where materials are received, stored, and shipped to the ROW. They are located adjacent to established roads with easy vehicle access. Disturbed areas that require minimal grading are usually selected.
A substation is a facility that switches, changes, or regulates electric voltage. Substations vary in size and configuration but are often several acres in size. The area within a substation is normally unvegetated and surfaced with gravel. They are normally fenced and reached by a permanent access road. A substation generally contains a variety of structures, conductors, fencing, lighting, and other features that result in an industrial appearance. Transmission lines start from and end at a substation.