The characteristics method was employed to elucidate the effects of fluid-phase transition on the dynamic behavior of emergency shutdown valves following full bore rupture (FBR) of long pipelines containing high-pressure hydrocarbons. The responses of both check valves and ball valves were simulated. The application of the model to the FBR of the Piper-Alpha MCP-OI gas riser reveals that condensation of the gaseous inventory results in a delay, in valve activation times, as well as an increase in the amount of inventory released prior to pipeline isolation. However, the magnitude of the pressure surge brought about from having to bring the high-pressure escaping fluid to rest following emergency isolation was significantly reduced.