Reliefs for Thermal Expansion of Process Fluids

The required vent area is determined from Equation 9-35 Qm0V(g 

An alternative way to look at the problem might be to ask the question, What initial fill fraction should be specified in the tank to avoid two-phase flow during a fire exposure incident No tested correlations are presently available to compute the height of a foam layer above the boiling liquid. 

For fire reliefs with single-phase vapor flow the equations provided in sections 9-2 and 9-4 are used to determine the size of the relief. 

As mentioned previously, two-phase flow discharges for fire scenarios are possible but not likely. To size the relief for fire and a single-vapor phase, use the heat input determined from Equations 9-36 to 9-38, and determine the vapor mass flow rate through the relief by dividing the heat input by the heat of vaporization of the liquid. This assumes that all the heat input from the fire is used to vaporize the liquid. The relief area is then determined using Equations 9-3 to 9-12. 

Liquids contained within process vessels and piping will normally expand when heated. The expansion will damage pipes and vessels if the pipe or vessel is filled completely with fluid and the liquid is blocked in. 

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