Mawimum tolerated build up back pressure for a Rupture Disk

[Matchess]

Hi everybody

I put a Non-fragmentable rupture disk device fitted in series with a conventional atmospheric pressure relief valve.
I red in API 520 that place a pressure switch alarm between the rupture disk and the PRV is recommended.("rupture disk will not burst in tolerance if backpressure builds up in a non-vented space between the disk and the PRV, which will occur should leakage develop in rupture disk due to corrosion or other cause")
(NB: I can't have a free vent because of the fluid characteristics)

but I don't fund any information about the set pressure of this alarm (0.1% of the rupture disk or PRV set pressure ???)

Thanks you in advance for your help

 

[zdas04]

Bottom line is that it mostly doesn't matter (as long as the set point is below the minimum expected operating pressure). Any leak into a closed space will fairly quickly equalize the pressure across the disk. With a tiny hole, it might take a couple of hours to reach 10% of normal operating pressure. A slightly bigger hole might reach 100% of operating pressure in a few minutes. Either case will tell you that you have a problem pretty quickly.

Set it wherever you want as long as the set point is well below the minimum normal operating pressure. The set point on this alarm has nothing to do with overpressure protection on the vessel, it is only to tell you that your corrosion protection has been compromised.

David

 

[Latexman]

The guideline a company I used to work for is the pressure setting between the RD and PSV should be the greater of 3 psig or 5% of RD/PSV set pressure.

Good luck,
Latexman

 

[don1980]

You want to set it high enough such that you avoid nuisance alarms (false alarms) and low enough such that you're notified relatively soon after a disk failure occurs. The advice provided by Latexman accomplishes this.

A significant crack or hole in the disk doesn't pose much of a safety problem. Pressure on both sides of the disk are continuously equilized. Even though the disk is cracked, and pressured up on the downstream side, the PSV will still lift when the system get to set pressure. Once the PSV vents off a puff, depressuring the cavity between the disk and the PSV, the disk will pop and the relief system (disk & valve) will function as designed.

The worst situation is one in which you have a tiny pinhole in the disk. Say you have a vessel that operates at 50 psig, and the vessel's MAWP is 150 psig. The pinhole causes the cavity bewteen the disk and PSV to be at 50 psig. Now, if a rapid pressurization occurs in the vessel, the disk won't burst until the system reaches 200 psig (the point at which the disk differential is 150 psi).

 

[Matchess]

Thank you for your quick and interesting responses

Latexman, I am not quite sure, I correctly understood your answer.

In my case: The PSV and Rupture disk set pressure are the same (equal to 25 Barg)

So, If I follow the guideline from your former company. The alarm set pressure will be 25 Barg + 3 psig = 25,2 Barg
Right ?

 

[Latexman]

No.

What is 5% of the set pressure?

Pick the greater of 3 psig or "5% of the set pressure".

Spoiler

The answer is 1.25 barg.

Good luck,
Latexman

 

[Matchess]

All righ
Thank you Latexman