3 replies to this topic

[zulu]

hi all!!!

I just wanted to clear my concept regarding the Blowdown and de-pressuring. Is it necessary to depressurize the system (in fire case) from its design pressure to 100Psig or half of the design pressure??



I mean if system design pressure is 4000 KPag, for blowdown purpose we need to depressurize it from 4000 to the 690KPag???.



I checked API 521 and it doesn’t talk about the design/initial pressure, it only talks about the final pressure of the system which must be 100Psig ~690 KPag.



My concern is considering the design pressure we may get the maximum peak load( from hysys dynamic utility) but practically how that system will reach to its design pressure which we are considering for de-pressuring and blowdown???

thanks

[fallah]

hi all!!!

I just wanted to clear my concept regarding the Blowdown and de-pressuring. Is it necessary to depressurize the system (in fire case) from its design pressure to 100Psig or half of the design pressure??



I mean if system design pressure is 4000 KPag, for blowdown purpose we need to depressurize it from 4000 to the 690KPag???.



I checked API 521 and it doesn’t talk about the design/initial pressure, it only talks about the final pressure of the system which must be 100Psig ~690 KPag.



My concern is considering the design pressure we may get the maximum peak load( from hysys dynamic utility) but practically how that system will reach to its design pressure which we are considering for de-pressuring and blowdown???

thanks

Design pressure for initial pressure would be conservative and maximum operating pressure or PSHH could be given as initial pressure of depressurizing.

About final pressure API 521 FIFTH ED. says for vessel with thickness of almost 25.4 mm the final pressure of depressurizing could be 50% of vessel design pressure and final pressure of 100 psi is considered in the event of a vessel leak.

[daryon]

I agree with Fallah, there is no need to take the design pressure as the initial pressure for blowdown calculations. Blowdown upon fire detection will normally be initiated by fusible loop system, heat or flame detectors, push-button pressed by operators etc. Depending on the process it is normally safe to assume that blowdown will be initiated prior to the process vessels/equipment being heated sufficiently to generate enough vapour (or thermal expansion for vapour filled systems) to raise the pressure to the design pressure. Being conservative I would take the high pressure trip for the vessel in question, but I have seen the high pressure alarm (maximum normal operating pressure) value used in the past. It should be possible to estimate how long the vessel will take to reach its design pressure from being heated by a external fire, and you can use this time frame to access if blowdown is likely to be initiated prior to the conditions being reached. I think it will.

API 521 does suggest depressurisation to half the vessel design pressure for wall thickness > 1" is acceptable, but my philosophy is that it is always best to depressurise as fast as your flare/ vent disposal system will allow. If your flare system can handle blowing down to 6.9 barg in 15 minutes then I'd do this as it helps reduce the risk of fire escalation and large sustained jet fires.

[fallah]

If your flare system can handle blowing down to 6.9 barg in 15 minutes then I'd do this as it helps reduce the risk of fire escalation and large sustained jet fires.

It may be too much conservative to perform for all cases.API 521 limits the application of above for depressuring system would be designed to reduce the consequences from a vessel leak.