PTS –Petronas Technical Standard- Sizing of PRV at Vaporised Liquid - Fire Case with Hysys

PTS 80.45.10.11. has recommended following procedure to select a suitable PRV in vaporised liquid - fire case, by aid of UNISIM/HYSYS as per different applications;
1) If a vessel contains two separate liquid phases,e.g. hydrocarbon and water, all the fire heat input shall be applied to one phase and then to the other phase. The case that results in the highest orifice area shall be used. (it means, orifice area is estimated based on that phase with lower latent heat, and relief temperature is reported based on that phase with higher bubble point.)
2) If a vessel contains a mixture of fluids, the relieving fluid properties shall be calculated by assuming that 5 % by weight of the original mixture has already flashed. This shall be done as follows:
a) Evaluate all the following properties at the maximum allowable accumulated pressure.
b) Flash off the lightest 5% wt.
c) Use the remaining 95% wt to determine the average properties.
d) Determine the latent heat of vaporization of the remaining 95% wt by:
- Finding the bubble point specific enthalpy of the liquid in J/kg (Btu/lb).
- Finding the dew point enthalpy of the vapour in J/kg (Btu/lb).
- Subtracting the liquid enthalpy from the vapour enthalpy to find the latent heat of vaporisation in J/kg (Btu/lb).
e) Find the average molecular weight, compressibility, and temperatures associated with the vapour at Maximum Allowable Accumulated Pressure and use these properties for the PRV vapour sizing.
NOTE: The latent heat, temperature, and molecular weight of the liquid that is initially vaporized will be lower than the average values. A rigorous (dynamic) analysis of one mixture has illustrated that the simplified approach of using average values is conservative.
3) For a column's fire relief, the PRV shall be sized assuming two compositions as follows:
a) First composition based on the tray below the top tray.
NOTE: For modelling purposes, this is the third theoretical stage composition or reflux composition if the third theoretical stage data is not readily available.
b) Second composition based on the column bottom.
c) The column's PRV shall be sized for both compositions and the larger size of the two shall be used. Do not apply the 5 % by weight rule since the removal of light materials is already taken into account by using the composition in the tray below the top tray.