Depressuring valves are generally designed to de-inventory a system within a specified amount of time in the event of a process excursion or other emergency. Depressuring may occur as a direct result of any of the common contingencies
described in “Simultaneous Releases Caused by a Common Contingency” on page 1200-77 and can have a significant impact on the operation of the disposal system.
Depressuring valve discharges are time dependent. In the absence of a detailed analysis of the sequence of events, the initial flow, based on the maximum expected pressure upstream of the depressuring valve, should be used as the input to the disposal system.
The impact of the built-up back pressure on the depressuring valve capacity should be evaluated upon completion of the hydraulic modeling of the disposal system. An iterative calculation may be necessary to determine the expected initial flow from the depressuring valve.