Depressuring Basic

• In case of fire, the operator will push the EDP push button in the control room. That will initiate SDV valves closing at both outlet and inlet of an equipment and BDV valves opening. The hydrocarbon fluid will be released to flare so that the pressure of the system will be depressurized to lower pressure at certain. (The pic is stolen from another blog, its too good not to steal..the link to that blog is http://process-eng.blogspot.ca/2012/05/basic-depressuring-why-15-minutes.html)

• For thickness of vessel less than 1 inch, the system is depressurized to 100 psig, and for more than 1 inch thickness can be depressurized to 50% of design pressure. The depressuring time can be longer and less than 15 minutes. The depressuring time of 15 minutes is only an example in API STD 521 which is applicable for carbon steel vessel with has thickness greater than 1 inch.

• Depressuring time depends on the vessel thickness
• Refer to section 5.15.1.2 in API 521 for graphs for steel plates and carbon steel for various thicknesses 3.2 mm, 12.7 mm and 25.4 mm thickness.

• Adiatbatic depressuring case
• Normally for most plants are shutdown for annual maintenance purpose. Such a system is depressurized to atmosphere condition. In this case, the system is depressurized in adiabatic condition, which means no heat input to the system. During depressurization, the pressure decreases, and the temperature decreases as well. The final temperature of adiabatic depressuring could be very low. As Process Engineer, we have responsibility to determine the Minimum Metal Design Temperature (MDMT) for each system zone based on this case.

• Fire case is use only for define max capacity of BDV and flare system.
• In Hysys we can calculate depressurization by: