(546f) Design of Relief and Flare System for Liquefied Natural Gas Plant Using Dynamic Simulation

Process system equipment should be protected from pressure increases of internal fluid pressure in vessels, pumps, compressors, or columns. Overpressure is caused by abnormal operating conditions for examples, exposure to fire, operator error, and heating/cooling medium failure. Relief and flare system (RFS) including pressure safety valve, piping network, knock-out drum, and flare stack provides a protection from overpressure in such upset conditions by discharge of toxic or flammable vapor. Practically, the design of RFS complies industrial code, standard or internal guidelines of company. Those methods consider a number of possible scenarios causing overpressure of equipment and calculate maximum relief load of each scenarios for every unit in the plantwide system. The largest relief load of the various scenarios and units is selected for sizing RFS. However, the result of relief load from these conventional approaches are too conservative and lead to excessive equipment size because they are usually based on the unbalanced heat method or empirical value. Oversizing of RFS not only increases unnecessary expanse but also causes flooding and lifting internals when overpressure occurs in column units. The key point of selection of proper pressure-relieving system is how many possible credits reflected to the examined scenarios. Dynamic simulation takes several credits such as system instrumentation and transient responses of the process, which reduces relief capacity under an overpressure situation. Dynamic simulation is recognized for the calculation method of relief load from American Petroleum Institute code and also utilized in industrial field. However, taking credit by dynamic simulation and reduction of RFS size should be carefully because it can be harmful to process safety. Therefore, a general methodology or procedure of relief load calculation using dynamic simulation is needed for process reliability. In this study, a column operation of natural gas liquefaction process is selected to examine the sizing of relieving devices and a formalized design procedure of RFS is presented from analysis of the dynamic simulation results.