COMPUTER-SIMULATION OF EQUILIBRIUM CONDITIONS FOLLOWING A PLANT TRIP

When a process or part of a process experiences an emergency 'trip', t he contained fluids redistribute themselves based upon the prevailing pressure gradients and depending upon the positions of valves at the t ime of the trip. Reverse flow through rotating compression machinery m ay occur, depending upon the locations of non-return valves. Reduction in pressure and mixing of cryogenic fluids of different compositions and/or temperatures can both lead to generation of significant volumes of vapour. This equilibration process is usually largely over in a ma tter of seconds rather than minutes. Key questions facing process and mechanical designers are: what is the settle-out pressure, and can we ensure relief valves do not lift following a trip? To answer these rel ated questions it is necessary to analyse the state of the system prio r to the trip, and then, based upon valve positions, etc., construct a model of the worst case probable scenarios for the qualitative redist ribution of fluid inventory. At this point the simulation program may be employed to help calculate rigorously the final settle out conditio ns for each of the possible scenarios. This technique is particularly appropriate for cryogenic processes including refrigeration cycles. It is illustrated here with the help of a multistage hydrocarbon compres sor example. Other related non-standard applications of the steady sta te process simulation program are identified.