As to whether more savings can be made on much stronger vessel compared to savings on the relief valve is another issue altogether. It appears the designer has adopted a clever strategy to reduce the relief valve size. Therefore, I suspect the Emerson/Leser Valvestar software are both calculating the allowable overpressure wrongly. A similar case is presented in there where the PSV set pressure was below the MAWP of the vessel. I am attaching the allowable overpressure calculation from Table 4 API 520 as reference. 'this is the set pressure plus the allowable overpressure plus atmospheric pressure'Īllowable overpressure = Maximum Accumulated pressure (1.21 x MAWP) - PSV set pressure Indeed, API 520 defines the upstream relieving pressure as: It may also be set below the MAWP in accordance with API 520. Also, the vessel operates between 0.3 barg to 0.6 barg. RE: Sizing of PSV - Calculation of the Upstream Relieving Pressure The Obturator (Mechanical) 24 Nov 19 Obturator (Mechanical).thank you very much as ever. Please share your thoughts - did I approach the hand calculation the wrong way? It appears the designer wanted to obtain a smaller relief device - by setting the vessel MAWP so high. I reckon the problem emanates from the calculation of the relieving pressure. When I enter these same values into Lesser's Valvestar and Emerson's PRV^2Size (screenshot attached).they calculate a Q orifice. 764.88 mm^2 requiring a J orifice.īut then this is where it gets interesting: Relieving Pressure: 3.4 barg + 15.96 bar + 1.01325 bar = 20.37 bar Ĭalculating required area for critical flow yields an orifice area of approx. Accumulation Pressure: 1.21 x 16 barg = 19.36 bargĪllowable Overpressure: 19.36 barg - 3.4 barg = 15.96 bar

I calculate relieving pressure as follows: ID = 2.5 m, Ls/s = 10.9 m, Liquid depth = 2.63 m, elevation = 1.2 m and so I am calculating wetted area to be around 75.65 m^2.Ĭalculated relieving load is approx. Maximum allowable Backpressure: 0.29 barg I am reviewing the sizing of a PSV for a pentanes product storage tank.

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