In the supply pipelines, the natural gas pressure is about 1.7 MPa (approximately 250 Psi) and before entering distribution pipelines, this pressure should be reduced to a lower level of 0.4 MPa (approximately 60Psi). This press ure reduction is performed in Town Border Stations (TBSs) in which the sizeable amount of pressure energy is wasted by employing throttling valves. One way to recover pressure energy during pressure reduction process is to employ an expansion machine such as the reciprocating expansion engines. The purpose of this study is thermodynamic simulation of one-sided medium pressure reciprocating expansion engine. The simulation is based on energy and mass conservation laws and the AGA8 equation of state. The results show that, with using expansion engine in TBS stations the efficiency of pressure energy recovery is about 91%. Also, with the suction of 18 g gas per cycle, the engine generates 46.2 kW power. The results also show that the engine could generate highest power at specific suction port to bore diameter.

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