Gas compressors and their application

Gas compressors in natural gas production and transmission are fundamentally of two types: 1) positive displacement and 2) centrifugal or turbo-compressors. Displacement compressors are reciprocating and rotary screw machines. They are designed to move relatively small quantities of gas at high pressure differentials, and turbo-compressors convey gas in large quantities with a relatively low pressure rise. Smaller and medium-sized reciprocating compressors are applied in gas production and gathering where high-pressure differentials are required. For smaller gas flows, rotary screw compressors are often used.

Larger-size turbo-compressors, or “pipeliners,” have been mostly utilized in long-distance transmission service. While there is an ongoing debate about whether to use large reciprocating compressors in gas pipeline compressor stations as opposed to turbo-compressors, one should keep in mind that the function of these two types of machines is based on completely different principles. Also, the efficiency of reciprocating compressors can be significantly lower than that of modern centrifugal pipeline compressors.

The main parameters to be considered when matching a driver to a given compressor are size (bhp or kW) and speed (rpm). There are other secondary, albeit important, considerations such as speed-torque behavior, the ability to couple the driver directly to the driven machine, speed variability and others.

 Reciprocating compressors in gas production and gathering range in size from below 100 bhp (75 KW) to around 6700 bhp (5000 kW) with  a  median size from about 1300 bhp (1000 kW) to approximately 2600 bhp (2000 kW). Screw compressors used in oil and gas exhibit approximately the same size ranges. Reciprocating compressor speeds range between 200 and 1500 rpm, with screw compressors at median speeds of about 1500 rpm.

There are three types of compressor drivers available: Reciprocating gas engines, mechanical drive gas turbines, and electric motors. A fourth choice has been steam turbine drivers, which are used extensively in hydrocarbon processing plants and other industries with ample steam supplies. Traditionally, direct-coupled reciprocating gas engines have been the driver of choice for reciprocating and rotary compressors where remote access and absence of electrical utilities dictates the use of self-contained compression units.

Gas turbines have seldom or never been used as drivers for reciprocating and rotary compressors. The reason must be seen in the high specific investment costs ($/kW) of smaller gas turbines; their low efficiencies; and their requirement for speed-decreasing gearboxes as variable speed mechanical drive gas turbines, which normally operate at speeds in excess of 4000 rpm. Some reciprocating compressors could possibly be driven by microturbines.

In the past, microturbine suppliers had been targeting the reciprocating engine market. They soon realized that, for their costs, reciprocating engines were the lowest-cost drive solution. Microturbines require 24/7 operation and a low maintenance load to be able to compete with reciprocating engines. An electric motor drive, however, might well be considered in oil and gas producing locations and pipeline transmission stations where electrical power is available.

Given a favorable constellation of the above criteria, an electrical motor drive for reciprocating and rotary compressors could be more cost-effective than owning and operating a reciprocating engine driver.   

 Gas turbines come into their own as compressor power requirements increase. Approximately above 10,000 bhp (7500 kW) gas turbines become less expensive in terms of lower $/kW and increased thermal efficiency. It makes good sense to apply them to drive centrifugal pipeline compressors where speed requirements are much higher than those for reciprocating and rotary compressors. In most cases, a gearbox would not be required since gas turbine speed is matched to that of the centrifugal compressor.