Dear All,

If I may can I ask your expert advise on a PSV sizing problem we have come across? We are trying to resolve sizing of a PSV for the MR Surge Vessel, with calculations completed with the PSV sizing program indicating that the valid FIRE GAS EXPANSION sizing case the vessel wall temperature will exceed the limit of the material before the design pressure is reached inside the vessel. The vessel contains mixed refrigerant with the only apparent valid relief case the fire gas expansion (as BLOCKED OUTLET is protected elsewhere). This conclusion has me concerned as according to this calculation there appears to be no valid case where the vessel can reach the design condition which I find hard to believe as this would indicate that no PSV is required! I am also concerned that it may be concluded from this that the materials of construction are not suitable if the vessel wall will fail before the vessel reaches the design pressure under a fire scenario but I guess the vessel material will fail in any case at this temperature regardless of whether a PSV is installed and is relieving. What is not taken into account by the sizing program I guess is also the impact of internal pressure within the pressure at higher wall temperatures and whether this leads to premature failure of the vessel?

WE ARE USING FARRIS SIZE MASTER SOFTWARE

THE MESSAGE:
Sizing Warnings:
SSMCalc.3014:Warning: The calculated relief temperature is greater than the wall temperature, therefore relief will not occur.
User may elect to disregard this overpressure scenario.