Basic Depressuring - Why 15 minutes ?

In previous posting, we have discussed that PSV wouldn’t provide adequate protection for vessel of fire case. Therefore, depressuring can be applied for another safety layer.

Commonly the plant area is divided into the ESD Zone. Each ESD zone may contain one or more equipments. ESDV or SDV valves are provided in each ESD Zone to isolate the system zone. In case of fire, a system will be isolated by those SDV valves. Then the inventory fluid (commonly gas phase only) in the system will be released to flare through BDV valve. Commonly one BDV is provided for one system zone, but in some cases, it is possible to provide more than one BDVs in one system zone.


See the picture.

EDP (Emergency Depressuring) is generally initiated by manual push button. In case of fire, the operator will push the EDP push button in the control room. That will initiate SDV valves closing 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 time (recommendation from API STD 521, decrease the pressure to 100 psig or 50 % of the system design pressure within 15 minutes)

For example, If a pool fire exposes the un-wetted carbon steel vessel, it will take about 15 minutes to heat the vessel wall to around 1 200 °F (very close to material’s allowable stress condition). If the vessel is depressurized within the 15 min to 50 % of the initial pressure, then the time to rupture would increase to about 2 - 3 hour "

Hope this picture will give better explanation

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.

Consideration of limiting flare capacity, the depressuring time longer than 15 minutes may be applied. It will result in lower depressuring load. Considering of the maximum reduction of the vessel stress, vessel with thickness less than 1 inch, generally requires faster depressuring rate. The faster the depressuring time, the higher the depressuring load. And for the vessel with stainless steel material, the depressuring rate may be longer than 15 minutes for 1 inch thickness or more.

Based on my experience, many companies have their own manual for conducting depressuring study. Generally, the maximum depressuring time of 15 minutes is applied. But each company has difference consideration of thickness vessel and depressurized pressure. Some companies apply that for thickness greater than 24.5 mm, the pressure is depressurized to 50% of design pressure, but other companies apply that for thickness greater than 60 mm the pressure can be depressurized to 50% of design pressure.

I have ever read discussion in Cheresources ( check here and here) about the depressuring time, one of the participants says that his company applies the depressuring time which depends on the vessel thickness. For thickness greater than 25.4 mm, 15 minutes depressuring time is applied. The depressuring time will be decreased 3 minutes for each 5 mm decrease in thickness.

Yesterday, I checked to API STD 521, (Fifth Edition, Jan 2007), figure 1 (section 2.15.1.2.2), the graphic show “ Plate Temperature vs Time After Fire “ for carbon steel 3.2 mm, 12.7 mm and 25.4 mm thickness. It is very logic that the required depressuring time for those vessel are different each other. In my opinion, it is better to state in manual ‘the depressuring time will be decreased, say 2 or 3 or xx minutes, for each 5mm decrease in thickness”.

Oh,,I miss something important. Even though the depressuring time of 15 minutes is used, the depressurization will not stop after 15 minutes and that the pressure will continue to decline.

Ha ha ha,,I guess you already know that :D

Let’s imagine, a PLANT is shutdown for annual maintenance purpose, the fire does not exist, then the system is to be 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 the adiabatic depressuring case

My friend, that’s all I can share today. Hopefully it is useful.

Have a great day, my friend...

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