Dynamics of the Recycle Systems of Two Centrifugal Compressors Operating in Series in a Natural Gas Compressor Station

Abstract

Compression systems employed in gas pipeline transmission systems and gas processing plants provide a vital function to the overall operation of both systems, and therefore must be vigilantly monitored in order to ensure a high level of operational reliability. The majority of these compression systems employ centrifugal compressors, driven by either gas turbines or electric motors with/without gearboxes. These systems are designed and operated in a manner to eliminate or minimize the potential for surge, which is a dynamic instability detrimental to the integrity of the unit. Surge can occur when compressors are subjected to rapid transients such as one following an emergency shutdown (ESD) or power failure. To prevent this, compressor stations are designed with recycle systems and special types of recycle valves required to open upon ESD. This paper describes the dynamics surrounding an ESD operation of a compression system composed of two units in series and two independent recycle systems, one for each unit. The conventional sizing of the recycle system applied to a single unit operation needs to be evaluated carefully when there are two units operating in series, and in particular sizing of the recycle system surrounding the higher pressure unit, i.e. the downstream unit. This paper provides an insight into the salient parameters that impact the operation of such system via several dynamic simulations of an ESD event. Different ‘what-if’ design scenarios are analyzed and results presented. The data was obtained from an actual station operation on TransCanada’s Alberta gas transmission system in Canada. This station also employs two large gas volume capacitance aerial coolers which are also involved in the dynamics of the system during ESD.