Centrifugal compressor systems include many different subsystems that facilitate performance monitoring, operational controls, vibration monitoring, and surge protection, etc., that are integral to the safe and proper operation of the compressor station. Centrifugal compressor stations can be operated in different configurations and for different services that include units in series, parallel, or mixed with reciprocating units. Due to changing market demand or changing operating conditions, there are higher required compression ratios on the steady operating state of compressor units in order to achieve higher flow demands. This forces the compressor to operate in a higher energy state and taxes the anti-surge control system during normal operational variances and during the sudden transient emergency shutdown (ESD) events. The anti-surge system maintains forward flow through the compressor by reducing the compressor pressure ratio and increases suction flow by diverting the compressor discharge flow path to the compressor suction piping through a recirculation loop.
When designing the piping system, designers may be conservative in adding a hot gas bypass valve to assist the anti-surge valve (ASV) in protecting the compressor unit from surge during a high-energy fast stop. The advantage of the additional valve is the increased capacity of the recycle system, while the disadvantages are increased system complexity, additional capital cost, possible unit overheating, and space considerations. Refined system design and detailed engineering analysis considering various aspects, including but not limited to actual piping volumes, piping distances, ASV selection, hydraulic resistances, control systems, and compressor inertial coast down data that will increase the accuracy of hydraulic flow modeling and assist in determining if a hot gas bypass valve is required.
This paper presents a general approach for conducting centrifugal compressor dynamic modeling and the considerations that should be taken when designing a recycle loop with or without a hot gas bypass. In addition, cases studies are presented to illustrate how to possibly avoid hot gas bypasses while increasing the effectiveness of the anti-surge system.