2 replies to this topic

[Hafiz Zakaria]

Greetings,

I have a simple question here:

One of global scenario that have multiple relief for flare header analysis is fire. That means during fire case, we can assume multiple psv in a fire zone is being relieved at the same time. Usually those psv's is designed for fire case. My question is, there are also other psv's that are not designed for fire (for example they may be designed for blocked outlet) but they also popped during fire case. Should these psv's also considered for fire relief scenario in flare header analysis?

Let's say that on a plant, i have 50 psv that are designed for fire, but during fire, 100 psv will pop(other 50 psv are not designed for fire). So for flare header analysis, should i consider all 100 psv or only 50?

thanks,

[fallah]

kelabukelala,

When we size a PSV for,let say blocked outlet,it usually means the blocked outlet is governing case among all applicable scenarios for that PSV.Fire case may be included in those scenarios or may not.

Thus,if fire case would be applicable for a PSV,it can handle relevant relief load regardless of fire case being governing case (fire case as basis of PSV sizing) or not.

Is all 100 PSVs located in one fire zone? If so and if fire case already considered in their sizing precedure as applicable scenario you may have to consider relief loads of all (100) PSVs in flare header analysis.Of course,for better assistance one should have complete information about all scenarios,installed BDV (if any),......

Edited by fallah, 25 February 2011 - 11:40 AM.

[paulhorth]

Kelabukelala,

PSVs are not intelligent. They will open whenever their set pressure is reached, whatever causes that to happen. So if you have a fire, and the equipment reaches its set pressure, the PSV will open, even if it is not sized based on the fire case. The equipment might not reach its set pressure, if it is not blocked in. But the fire might cause blocking in by causing ESDVs to close.
When considering the flare header analysis in a fire, it would be conservative (and thus correct) to assume that the PSVs of all equipment in the fire zone will open. If you have a blowdown system, then the PSVs should not open because the set pressures will not be reached, but you should still do the analysis assuming that the blowdown system has failed to operate.
Doing the flare header analysis for the blowdown cases as well as the relief cases is also necessary, of course, and the total blowdown flow will probably be higher than the relief flow.

Paul