Simulation of natural gas pipeline systems requires accurate representation of all components in the pipeline system model. Optimization, in order to minimize pipeline fuel consumption to the maximum extent possible, requires detailed compressor information for each of the individual compressor components. Presented here are the fundamental thermodynamic equations used to model reciprocating and centrifugal compressors. These equations are then developed into the more practical form used in performance modeling. The types of prime movers used to drive natural gas compressors are discussed along with the characteristics of each. The compressor and engine performance characteristics are then coupled to show how they can be used to maximize compressor throughout per volume of fuel consumed.
There are many types of compressors in use today. The Chemical Process Industries probably use more different types of compressors than any other industry. It is common to see reciprocating piston compressors, centrifugal compressors, axial compressors, screw compressors, rotary compressors and numerous other variations of the types mentioned here all in use in the same plant. Each compressor has its own operating characteristics and is matched to the particular duty. In the natural gas transmission industry the reciprocating piston and centrifugal compressors dominate. Dominance by these two types occurs primarily due to their operating characteristics and excellent fit to the pressure maintained in the pipeline system and the volumetric capacity requirements. According to the American Gas Association's Engine and Turbine Database"' (see Fig. 4), approximately 7,388 reciprocating compressors capable of delivering 10,700,518 horsepower to the gas stream, and 916 centrifugal compressors capable of delivering 5,753,898 horsepower to the gas stream are installed today. Usage of the other types of compressors by the gas transmission industry is negligible. Compressors add energy to the fluid stream to cause the gas to flow in the 1 L pipeline. The example given in Fig. 5 highlights the important aspects of energy transfer in a pipeline system. The diagram shows a segment of a pipeline system encompassing one compressor station and the associated suction pipeline. The pipeline is a 26" line looped with 36" and 26" segments. The gas enters the pipeline at Point A and exits at Point 8. The gas both enters and exits at 800 PSIG. The flowing volume is at a rate of 1,051.2 MMSCF/D. Energy is lost from the gas stream to the surroundings, and this loss appears as a pressure drop from Point A to Point B.
