A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. This is accomplished by confining a volume of gas in a given space, and then using mechanical energy to reduce the space. This squeezes the gas molecules together, raising the pressure of the gas.
Compressors are typically found on high-pressure gas transmission systems housed in compressor stations. 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. Smaller compressors are also sometimes used in gathering systems and in gas distribution systems.
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.
Compressor installations consist of two key components – a prime mover and the compressor itself. Multiple technologies may be used for each component. The prime mover is used to spin the drive shaft of the compressor. Typical prime movers include:
- Gas turbines: The most common prime movers, gas turbines are low-cost to operate, require less maintenance than reciprocating engines, and can run with natural gas from the pipeline stream.
- Reciprocating engines: Reciprocating engines are cheaper up-front, but are more costly to operate and maintain than gas turbines. They are most commonly used with reciprocating compressors and can run with natural gas from the pipeline stream.
- Electric motors: Electric motors are often more costly than gas prime movers but are often the best choice in areas where there are concerns over air emissions or noise and for applications where quick start/stop or large variations in compressor speed is required. Electric motors require an external source of power.
Types of compressors include:
- Centrifugal compressors that utilize rotating wheels with vanes
- Reciprocating compressors that utilize positive displacement piston
The choice of prime mover and compressor type depends on the design needs associated with a specific installation.
Centrifugal compressors are well suited for high flow rates and for applications where pressure differentials between inlet and outlet are relatively small. To achieve higher pressure differentials, centrifugal compressors are configured in series. They are commonly coupled with a gas turbine prime mover, but they may also be coupled with a reciprocating engine or an electric motor.
Reciprocating compressors can achieve higher pressure differentials than is possible with a single centrifugal unit and work well over a wide range of flow rates and pressure differentials. They do require more maintenance and thus are more expensive over the long term. They are most often coupled with a reciprocating engine as a prime mover but can also be used with an electric motor.