Matching centrifugal compressors with gas turbines

Multiple-shaft gas turbines as drivers of centrifugal compressors are widely used in the oil and gas industry. While gas turbines are of standard design, centrifugal compressors are precisely designed and manufactured to meet customer specific process requirements.

Complete package efficiency, turbine fuel consumption, time between overhauls, and the package flexibility in meeting alternate process requirements are to a large extent influenced by proper integration and matching the centrifugal compressor to its gas turbine driver.

Although there may be very few applications in which a centrifugal compressor may be required to operate in a single operating point, the majority of applications require operation within a specified operating envelope comprising of several operating points at different speeds and various power levels. The gas turbine should be able to provide adequate power for any and all compressor operating scenarios that may be anticipated during project life.Moreover, the power range and speed range of the gas turbine, which is of standard design, should be well matched with the specified required power and speed range of the centrifugal compressor for all the operating points in the operating envelope.

A clear definition of all the possible operating conditions for the centrifugal compressor, therefore, as well as specifying the required performance curve characteristics, will assist the vendor to comprehend specific application requirements.

Duty cycles

Gas turbine-driven centrifugal compressors are broadly employed in the oil and gas industry for semi-constant or variable compression duty applications as depicted in Figure 1. Semi-constant duty services tend to fall between two operational extremes of either a narrow speed range or which operate over a narrow flow range and power. Examples may be found in downstream applications such as refrigeration compressors in LNG plants.

Variable compression duty services, on the other hand, include those for which the gas composition, throughput, and process conditions like temperatures and pressures are variable and create a broad range of operating conditions that must be accommodated during the compressor’s lifetime. These variations may be temporary and may follow repeatable patterns due to seasonal demands in gas transmission, or may be permanent change from one operating point to another.Gas gathering and gas lift applications are common examples of variable compression duty services.

(More in September/October, 2012 issue of Turbomachinery International magazine)