Simulation of Centrifugal Compressor Trips For Surge Avoidance System Design

Centrifugal compressors are required to rapidly shutdown or trip in the event of a high vibration, a loss of lube oil, or a similar alarm condition and to avoid energetic surge events during these rapid shutdown transients. The recycle valve and its controller and actuator as well as the suction and discharge piping volumes, aftercoolers, operating conditions, other parameters, and various control response times are all characteristics that govern whether a compressor will reach its surge limit at a detrimental high head condition or at a more satisfactory low head condition. Although there are rules of thumb and other general guidelines for designing recycle valves and surge control systems, the only way to assure a proper design such that a sudden shutdown of a compressor from a full load or high head operating condition will not result in a damaging surge event is to simulate the transient events that take place immediately after a trip. In the past, SwRI conducted these transient simulations with large special purpose computer codes written in FORTRAN, which required extensive specific input data and considerable computer execution time. Recently, SwRI has been able to implement use of the Stoner Pipeline Simulation software to model rapid trips of centrifugal compressors in order to avoid energetic, potentially damaging surge events. These simulations account for the actions of recycle valves, their actuators, vent valves, compressor inertia, and control system responses while accounting for upstream and downstream piping, scrubbers, aftercoolers, parallel compressors, and other features of the compressor installation. The simulations require modeling of small pipe sections, very small time steps, and full details in terms of compressor performance curves, recycle, and other valve characteristics. The factors that determine if a damaging surge occurs when a compressor trips includes recycle valve sizes, response and opening times, piping size and arrangements, location of the compressor on its operating map, and many other factors. Detailed simulations that account for all of these factors are necessary to design surge control systems that avoid high energy damaging surge. Centrifugal compressors experience surge whenever they are suddenly tripped, however, controlling the head and total energy of the surge event by reaching a sufficient recycle valve opening and a low enough head before the surge occurs is the objective of transient simulations. It will be shown that this type of transient simulation is a valuable tool for designing surge control systems for centrifugal compressors.