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Temperature for PSV orifice sizing.

Temperature for PSV orifice sizing.

Temperature for PSV orifice sizing.

When sizing an orifice for a pressure relief valve based on external fire and liquid storage, what temperature should be used? Is there a standard figure for all external fire cases? Should it be the boiling point of the liquid or can this be superheated by the fire?

The temperature at the relief valve is very important as it relates to gas density.

RE: Temperature for PSV orifice sizing.

Use the temperature of the boiling liquid at the relieving pressure (set pressure plus the overpressure, usually 21% for a fire).  The vapor is not superheated by the fire since heat transfer from the fire, to the unwetted wall and to the vapor is minor (frankly, if the fire is heating the unwetted wall that much, the vessel is going to fail making the relief valve sizing issue a moot point).

If you have a broad range boiling point liquid, a common practice is to do a 5% flash on the liquid and use the resulting temperature and associated properties (MW, compressibility).

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