The shutdown of charge gas compressor in large-scale ethylene-cracking plant always involves emergency pressure relief of charge gas through multiple safety valves. The emergency relief capacity plays an important role on the safety of the overall plant. In this paper, by studying the difference between the configuration of the pressure relief system of two 1000 KTA ethylene-cracking plants (the inner diameters of the charge gas pipeline in both plants are 2m, while the number of same-sized relief valves are 28 and 19, respectively), the relief capacity of multiple relief valves is studied and compared with empirical calculation and numerical analysis. It is found that, due to the interruption of fluid flow when compressor is emergency shutdown, the upstream pressure of each relief valve increase steadily with the continuously make-up of the charge gas, but the difference between the inlet pressure of all relief valves can be neglected. With the increase of the upstream pressure, the opening of relief valves is determined mainly by the set pressure. In multiple valves pressure relief scenario, normally the downstream valves have greater relief capacity than those upstream valves if both relief valves have the same back pressure. Also, by analysis it is noted that the pressure relief capacities of multiple relief valves in both plants are sufficient. The minimum number of relief valves required for process safety is obtained. The maximum achievable Safety Integrity Level (SIL) of pressure relief system is determined by calculation of the reliability of the redundant relief valves. The analysis is used for determination of the SIL of the pressure relief system. The finding is also significant for determination of the required capacity of multiple relief valves.
