Why 21% over pressure is considered in fire case PSV sizing scenario My response: Fire under a vessel is a low probability event - unlike say a blocked discharge or upstream PCV/ LCV failure. So code allows relief valve sizing to be based on 121% pressure. This does not mean the vessel will undergo that pressure. All it allows is to select a smaller relief valve. That is all. If you get a PSV area of A sq.inch for normal case with 10% overpressure, then in fire case you are allowed to buy a smaller PSV of A*1.1/1.21. But you are going to select a standard size PSV. If its area is more than what you calculated, then this bigger than required standard size PSV will pass the required fire flow at a pressure lower than 121%! Got it. No real gain. *Extra 1* As I mentioned in another posting the fire case PSV sizing ignores the higher metal temerature and the resultant decreased ability of the metal wall even to withstand its design pressure. In all probability the vessel will rupture under fire before the PSV lifts. Thst is why we provide blowdown valve to reduce the internal pressure *Extra 2*. BDV provision is a common practice in the upstream Oil & Gas units. May not be that common in the downstream refinery and petrochemical plants. There is no universal rule.
Posted on: Thu, 27 Jun 2013 11:39:20 +0000