2 replies to this topic

[Zack Adam]

Hi Guys,

Really appreciate your experties feedback.

My consultant has done a PSV sizing based on blocked discharge case (Pset of 260 psig - HEX design pressure, Prelief = Pset*1.1 + atm). But the PSV set pressure was set based on fire case condition at 80 psig - reason being to limit the metal temperature to < 1100degF.

My query:

I am not so sure wether this approach is correct. Please could you help me on this matters.


Regards

Zack

[pleckner]

I'm not sure I understand this. Is the PSV set for 260 psig or 80 Psig?

If this is a check of an actual PSV, then we start with the stamped set pressure and work from there. If this is a new design, then you can set it for anything you want up to design (or MAWP).

Now, what to set it for is where the fun comes in. Yes, you can set it at a lower pressure than design (or MAWP) to keep the coincident relieving temperature where you feel you need it. The coincident relief temperature will be dependent on the liquid being boiled off, not the metal. A relief valve is used for fire for only one purpose, to control the release of gases that will be generated from the contained liquid. Once the liquid is gone, all bets are off. The equipmnet will eventually fail; gaskets will melt, etc. and you can't stop this.

So, I would at least question the basis for the set pressure. It is atypical to worry about metal temperature as the basis for setting a relieving pressure.

[Zack Adam]

Mr. Pleckner,

Thanks so much of your fruitful explaination...

If I were to set this PSV set pressure, I will set it at 260 psig and calculate the PSV size based on the governing case (blocked discharge), and no worry on the fire case.....

For your info, This PSV is located at the tube inlet of HEX (design P of the tube is 260 psig). The PSV is set at 80 psig. But for the PSV sizing, for relieving Pressure calculation, they have used Pset of 260 psig, so their Pressure relieving = 260*1.10 + atm.



Regards
Zack