(46w) Advantages of Constant Flowrate Depressuring

Depressuring system design is a very important part of overpressure protection of plants.  Depressuring systems consist of multiple equipment, usually vessels, depressuring valves, flare headers and flare stacks.  These systems are normally designed for one relieving event and depressuring certain area of the plant with limited number of depressuring valves.  What would be the consequence of a scenario where all the depressuring valves are simultaneously opening?  This was the daunting situation that engineers at Bechtel faced when, as part of a risk analysis, it was identified as a possible situation.  In this presentation we will show how advanced tools such as robust dynamic simulations were used to analyze the situation and design mitigation.    

The depressuring system is dynamic in nature:  flows are higher in the beginning and then decrease as the system depressures.  Complex systems with multiple vessels are particularly difficult to analyze.   They may have large volumes and varying depressuring times, connected to each other or the header by different size lines, each vessel may be at different temperature, pressure and compositions.  A rigorous model should be able to size the depressuring value using ISA formulas, consider two phase flow through the depressuring valves, handle a network of piping, and the possibility of reverse flow between equipment.   Further, the program should be able to calculate the pressure if the flow in the piping chokes. This may occur where a small lateral tees into a large header.  If the flow chokes, the backpressure in the lateral is based on the choking pressure at the tee, and an incorrect calculation of the choking pressure may have a profound effect on the backpressure.  

This situation of simultaneously release was analyzed using dynamic simulation of all the relieving systems and relief header.  An integrated system of all the release sources and complete relief header system was modeled.  Simulation of different sections and components of the system was done to analyze the pressure in the header, backpressure at the depressuring valves, acoustically induced vibration and temperature.  Overall risk to the system was mitigated using active detection of simultaneous release and depressuring in a staggered manner if simultaneous release was detected.  

The presentation will graphically illustrate the pressure profiles, the critical areas, and where and how mitigation was implemented.