Safe Depressurization of Large Thickness Vessels Under Pool Fire

Vessels fail at pressures lower than MAWP during fire due high temperatures. Analytical solution of the vessel wall temperature profiles as a function of fire exposure time available in API 521 Annex A for thicknesses of 0.125”. 0.5” and 1” was extended for vessel thicknesses up to 6”, to cover high pressure/large diameter vessels encountered in gas processing. For fire depressuring, plots of ratio of vessel failure pressures to MAWP were developed for various wall thicknesses and compared to a plot of the ratio of internal pressure to MAWP, both as a function of time. If internal pressures throughout fire depressuring were less than the lowered vessel failure pressures, then the vessel rupture is avoided.  The methodology is illustrated for the case of a high pressure vessel of wall thickness 3” in natural gas service