Effect of Pulsation Bottle Design on the Performance of a Modern Low-Speed Gas Transmission Compressor
Dynamic pressure drop refers to the calculation of instantaneous pressure drop at each point through the crank rotation. The horsepower cost is computed by integrating the instantaneous pressure drop over a cylinder cycle. If the instantaneous velocity across a square law restriction is much higher than the mean velocity, then the cycle integrated horsepower cost will be much higher than the horsepower cost estimated by mean flow estimates. Field analyses have documented instances where dynamic pressure drop effects have incurred hundreds of excess horsepower and, in some cases, were so high as to inhibit full load operations. In one such case, strong cylinder interactions combined with a significant pressure drop restriction downstream of the compressor cylinder valves resulted in excessive horsepower losses. This paper presents a case study of a low-speed reciprocating compressor unit installed in a natural gas transmission service. On-site testing following start-up indicated total losses of approximately 55 percent of the indicated horsepower while operating on the bottom load step at rated speed. Interim modifications to the existing primary suction and discharge pulsation bottles were moode, which allowed the units to operate much more efficiently through the winter. Redesign of the primary pulsation bottles were necessary to further minimize cylinder interaction and the associated horsepower cost. Full documentation of the design process is presented in this paper, along with supporting field data.