Effect of Check Valve Dynamics on the Sizing of Recycle Systems for Centrifugal Compressors

A recycle system for centrifugal compressors is designed to minimize the potential for surge, as can occur during shut down of the compressor. The recycle system volume capacitance (V) and the recycle valve capacity (Cg) are sized to give the required system dynamic response for surge prevention. Thusfar the check valve dynamic characteristic was not taken into account in the sizing of the recycle system. A numerical investigation was carried out on a scaled-down model of a compressor station to study the dynamic behavior of the check valve in relation to the recycle valve capacity (Cg), its dynamic characteristics, and the recycle system volume capacitance (V) during an emergency shutdown (ESD) of the compressor unit. A model describing the dynamic characteristics of an undamped check valve was introduced into the dynamic simulation of the entire system during ESD. The results show that the sizing of the recycle system can be optimized by also taking into account the check valve dynamic characteristics. Furthermore, it was found that Cg is proportional to the logarithm of the slope of the check valve dynamic characteristics, demonstrating that a small increase or decrease in Cg could have a dramatic effect on the maximum allowable reverse flow through the check valve without surging the compressor during ESD. The recycle piping volume (V), however, was found to be linearly proportional to the slope of the check valve dynamic characteristics resulting in a less dramatic effect. This paper presents numerical results and a discussion of these relationships.