The transmission system is responsible for moving large quantities of gas over long distances (often from supply basins to consuming regions). Transmission systems typically operate at pressures between 600 and 1,200 psi (pounds per square inch). Key components of the transmission system include the pipe, compressors, valves, overpressure protection, monitoring equipment, and metering equipment. The volume of gas that a pipeline can transport is determined by the diameter of the pipe, the maximum allowable operating pressure (MAOP) rating of the pipe, the location of compressor stations, the amount of compression at each station, and ambient conditions such as temperature and elevation.
Transmission line pipe is typically 24 to 48 inches in diameter and constructed of .25" to .75" thick steel. Laterals off the main pipe may be constructed of smaller 6-to-16 inch diameter pipe to provide service to LDC systems or directly to large end-use customers. The pipe is coated with a specialized fusion bond epoxy coating to prevent corrosion. Transmission pipelines must be buried at least 30 inches below grade and often are as deep as 6 feet to protect them from potential damage.
As gas enters a transmission system it must be pressurized to match the higher pressure of the system. This is achieved through the use of compressors, which are contained within a compressor station. A typical compressor station includes one or more centrifugal compressors that use a fan to squeeze or compress the gas. As the gas is compressed, its pressure rises, thus forcing the gas into the pipe at the outlet and driving the gas down the pipeline. Centrifugal compressors are driven by the drive shaft of a prime mover which is either a gas turbine or an electric motor. If a gas turbine is used, gas from the stream flowing in the pipeline will be used to drive the turbine.
An alternate compressor technology is a reciprocating engine. This engine, similar to the engine used in your car, is also powered by natural gas and drives the compressor drive shaft. In addition to the compressor technology, compressor stations also generally include scrubbers and filters (to capture any liquids or solid particles that have condensed out of the gas stream during transport), monitoring probes for the Supervisory Control and Data Acquisition (SCADA) system, bypass piping and valves that allow the gas to be routed around the station if compression is not required or if maintenance is being performed at the station, gas coolers, and an emergency shut-down system. Compressor stations are generally located 50 to 100 miles apart on the pipeline.
Because gas moves from areas of high pressure to areas of low pressure, the gas is pushed away from the high pressure of the compressor station to downstream areas where the pressure is lower. As gas flows through the pipe, pressure drops due to friction with the pipe walls. When the pressure gets too low to maintain an effective rate of flow, more compression is used to force the gas molecules together, again propelling them through the transmission line. This process is repeated until the gas reaches a distribution system or end user. Because gas that is under greater pressure moves more quickly and takes up less space, pipeline companies are often able to increase the capacity of their systems by adding compression rather than actual pipe.
For obvious reasons, it is important for the pipeline company to always know how much gas is in its system as well as how much gas is delivered to downstream pipelines. To ensure this, metering stations are installed at various locations, typically wherever gas enters or leaves the system. Since gas meters measure volume of flow (Mcf) and gas is often traded based on energy content (Btu), calorimeters must be used at various locations on the system to determine heating value (the amount of Btus per Mcf). A metering station may also contain pressure regulation equipment to ensure that gas leaving or entering the system does so within a specific pressure range. This is important for the safe operation of the transmission system as well as the gathering or distribution systems connected to it.
It is also important for a pipeline company to monitor the many miles of pipe that comprise its transmission system. SCADA systems automatically monitor the operations of the system. Information such as flow volumes, pressures and temperature is transmitted via a variety of communication devices to the pipeline’s Gas Control room, many hundreds or even thousands of miles away. From this room, Gas Control personnel can monitor and control many of the pipeline’s operations.
One way that Gas Control regulates the flow of gas on the pipeline is through use of mainline valves. These valves, which are installed anywhere from five to 20 miles apart, enable the pipeline to isolate an area of pipe in the event of an emergency or the need for pipeline maintenance, or to restrict flows to reduce volumes based on operational or market needs. If, for instance, the pipeline were to seriously rupture, the SCADA equipment would quickly report the situation to Gas Control. Gas Control would then shut the closest valves on either side of the rupture to isolate the leakage of gas. This can be done remotely if automated valves have been installed. Otherwise it must be done manually by a crew in the field. Once the trapped gas has been vented, maintenance personnel can then safely repair the pipe. Unfortunately, SCADA equipment is not sensitive enough to detect all problems with the pipeline, so operators must also rely on visual inspection of the system.
Gas moves relatively slowly on a transmission line, typically from 15 to 30 miles per hour. In fact, it can take days or even weeks for a molecule of gas to travel from wellhead to burnertip (compared with electricity which travels at the speed of light). When the gas arrives at the citygate (the intersection between the mainline transmission and the distribution system, operated by the local distribution company), the pressure is reduced and the gas enters smaller distribution pipes for ultimate delivery to end users.
Since gas pipelines operate at high pressures, leaks can result in serious accidents. Thus, pipeline companies have rigorous safety and maintenance procedures that include SCADA monitoring, aerial patrols, periodic pipe inspection, pipeline markers, and emergency response teams. Pipeline safety is regulated by the U.S. Department of Transportation.