A study of the gas flow through a LNG safety valve

A safety valve functions to control an upper limit of pressure inside the LNG line of transportation.If the pressure inside the safety valve nozzle exceeds a pre-determined value on the valve sheet which plugs the nozzle,an ex- cess of LNG discharges through the gap between the nozzle exit and valve sheet.In this situation,the forces act- ing on the valve sheet are gasdynamic forces generated by the discharge of LNG and mechanical forces supported by the spring behind the valve sheet. The flow through the gap is very complicated,involving vortices,flow separation,and shock waves.These affect adversely on the system accompanying with noise and vibration.The present study aims at understanding the flow physics of safety valve.A computational work using the two- dimensional,axisymmetric,compressible Navier-Stokes equations is carried out to simulate the gas flow between the nozzle exit and valve sheet,and compared with the theoretical results. It has been found that there exists a distance between nozzle exit and valve sheet in which the thrust coefficient at the valve sheet increases abruptly.