3 replies to this topic

[Ghasem.Bashiri]

Dear Process Experts

I want to reintroduce an old problem within this topic: Design Temperature of Flare Network. As I checked there are some old topic within the site for this issue without clear conclusion.
Problem: During Relief Load Calculation based on API 521, high temperature up to 300-350 C have been reported! Therefore such high temperature may be used for design temperature of Flare Network. It is clear that carbon steel can work with such high temperature without problem. But expansion loop should be designed for flare header with corresponding capital cost!
Major shortcome for such high temperature prediction is boiling point of heavy component of liquid within a given liquid filled or partially liquid filled vessel. In real world for such high temperature long time (I think more than 20-30 min) is required to heat up vessel content during fire scenario.
Therefore we can ask that if such high temperature is real (e.g. 300 C) we should report it for vessel also. But such high temperature design due to fire case is not available in process design criteria of different projects!!
At same time we should note that when a vessel work in 40-50 C and then got high temperature around 300-350 C (after 20-30 min expose to fire) vessel expected to mechanically fail before PSV action.
I got some recommendation from well known companies to consider fire case design temperature as 180-220 C without any technical backup (Just Experince).
I shared all issues related to this problem here. Please complete it with valuable points. Some designer proposed to design only PSV tail pipe for fire case (e.g. 300 C) and then flare header only for 80-100 C.
I prefer reply with some technical and clear backup such that new user can use it for their design.

Ghasem Bashiri

[paulhorth]

Ghasem,

I hope that I do not contradict what has been said in the past on this forum. However, my view on this problem has always been that it is not logical to design the flare system for a fire case design temperature, when the vessels themselves are not designed for fire. I have used as design temperature the maximum non-fire relief temperature entering the flare.
As you say, a PSV does not protect a vessel against rupture in a fire. Even if it opens, the internal pressure is not reduced, while the vessel walls become hotter. And in some cases, as you say, the relief pessure is not reached before the vessel ruptures. This is particularly true for gas-only vessels.

Vessels should be protected against rupture in a fire by (1) automatic depressurisation (blowdown) (2) deluge, or possibly fire monitors at a safe distance, (3) free drainage under the vessel to minimise a pool fire (4) passive fire protection .

Paul

[Ghasem.Bashiri]

Dear Process Expert
My main issue is design temperature of Flare Network that I should submit some data for it.
Is there any general practice for this issue?
Ghasem Bashiri

[fallah]

Because in most criteria the vessel subjected to fire hasn't been designed based on relieving temperature in fire case,it is logical that one of its downstream piping systems (flare network) wouldn't be designed as per mentioned temperature.

Indeed,with considering BDVs activated on maximum operating pressure in fire case (mostly with low temperature affects during relief on downstream piping) relevant PSV action wouldn't be occured after depressurization and just the vessel itself fails before PSV action.

Edited by fallah, 15 February 2011 - 02:48 PM.