(104ao) Using a Process Simulator to Improve Relief System Analysis

Incorporating a process simulator in the design and/or analysis of pressure relief systems can improve the quality of the effort in a cost effective manner.

Often some variation of a fire case is the basis for a relief system design.  These cases usually involve vessels or piping exposed to large heat fluxes caused by flame impingement from an uncontrolled fire.  Before process simulators were incorporated in relief systems design and analysis, designers focused on selecting the appropriate size relief device and the adequacy of the inlet and outlet piping from the relief device.  Typically a worst case vapor flow was calculated based on empirical relationships developed by ASME, API and others.  The main criterion was avoiding exceeding the maximum allowable working pressure of the vessel.  This approach could be described as a single point design.  Designers often had no awareness of events occurring at other times even though these events potentially could compromise the safety of the system in question.

Initially process simulators were developed to model steady state processes because everything else was much more difficult.  As process simulators became more sophisticated and computational processing power exploded many non-steady state features were incorporated into these simulators.  Today several quality process simulators are commercially available.  Each one may have various capabilities to model different aspects of pressure relief systems.  This paper describes how EEC used WinSim Design II in a pressure relief system analysis and design of vessels and piping in a gasoline/diesel terminal.

The simulator allowed EEC to model thermal expansion in liquid full vessels with respect to how PSV’s are affected, the relief discharge into piping with and without liquid flowing in the downstream piping, the frequency of PSV reseating, the change in temperature of vessel walls with time and to characterize the consequences of the time required to extinguish the fire.  The availability of this information allows for a much more meaningful relief system design or analysis, often in less time.  This approach allows identification and prioritization of potential safety issues that would otherwise be unknown and facilitates a cost effective mitigation strategy for these threats.