2 replies to this topic

[rohan]

Dear All,

Please do give your valuable suggestion for the following queries.

Background:

An LPG storage tank (130 ft X 13 ft) has multiple rupture disks (7 in
numbers). No set pressure given, MAWP of tank = 200 psig, size of each RD=4
inches.


Q.1 What is the reason for having so many rupture disk installed?

Q.2 Set pressures of all rupture disks should be same or different? Why?

Regards
Rohan Sule

[pleckner]

1. One large rupture disk is probably impractical (mechanically) for the required relieving rate for the controlling relief scenario. A single relief line will be huge and problamatic to design properly.

2. For this service, I would venture to say they all have the same bursting pressure. I can't come up with a reason to stagger the bursting pressures for this application.

NOTE: All these rupture disks must have tags attached with all the pertinent information. If these tanks are in the U.S. and they do not have these tags, the process owner is in violation of the law.

I invite you to read my series on rupture disks, which can be found on this site. Go to the main menu and click on Free Articles, then click on Pressure Relief Design.

[Art Montemayor]

Rohan:

For my professional interest - and perhaps in the interest of safety - could you give us some additional background and details of this application? I find it difficult to believe that an owner would equip a tank full of LPG (Propane + Butane mixture) with a rupture disk. I also find it difficult to justify an LPG tank (especially of this size) with an MAWP of only 200 psig. I would expect an MAWP of approx. 350 psig for an LPG tank.

I find great difficulty accepting a scenario where the rupture disk(s) ruptures and the entire contents of the tank are released without any prospect of controlling the release. The release of such a large quantity of LPG would be catastrophic if the tank were located in an inhabited area or close by. The heavier-than-air LPG vapors would "hug" the ground as they spread out across the countryside and any ignition point - a flame, a spark, or even static electricty would set off a horrendous fire ball that would literally roast any and everything that it would be close to. Every LPG tank that I have designed, installed, worked with, and modified has depended on conventional pressure safety valves. The more sophisticated ones have a three-way valve to select one of two installed PSVs. This enables maintenance or replacement to be done on a leaky PSV while still maintaining safety integrity of the tank.

Another potential difficulty with a rupture disk on a liquefied compressed gas (such as LPG) is that the vaporization of the liquid will cause a reduced temperature within the tank - possibly causing the carbon steel to undergo embrittlement.

I would be interested in knowing where and how this tank is being operated. Is it an ASME Section VIII based tank?

I'll look forward to your reply.