Sizing of pressure relief valves mounted on storage tanks or vessels rated for low pressure (<15 psig) need to account for the losses in the inlet piping system. This is because the flow is almost always sub-critical, and sizing is governed by the difference in pressure between the valve inlet and outlet.

PSV Sizing for Hexane Storage Tank

You need to design an emergency pressure relief valve for a 20,000 gallon storage tank. Since the tank will be storing hexane, a flammable vapor, a flame arrestor will be installed beneath the valve to protect against flashback. The tank is rated for a maximum allowable working pressure (MAWP) of 2.5 psig, and will be protected by a remote pilot operated valve with a setpoint of 2.25 psig. The external pool fire case requires a relief rate (standard air basis) of 160,000 SCFH.

If inlet losses are ignored, the sizing of the PSV would be based upon an inlet pressure of 2.25 psig plus the 10% overpressure to achieve full lift, or 2.475 psig. Using a short discharge pipe, the capacity of a 6-inch valve is 256,800 SCFH. By contrast, if the inlet friction losses and pressure drop across the flame arrestor are taken into account, the situation changes dramatically. The true PSV inlet pressure under flowing conditions is only 0.72 psig. This change in sizing approach lowers the valve capacity to only 129,900 SCFH, which is not sufficient to relieve the vapors during a fire.

To remedy this situation, a larger 8-inch flame arrestor is specified to reduce upstream pressure drop. Recalculating shows that the capacity of the 6-inch PSV increases to 172,100 SCFH, which is adequate. Correctly accounting for PSV inlet pressure prevented the installation of an undersized relief valve.

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