Excess Flow Valve for Rupture Disk Installation 3

[carol2005]

Excess flow valve is regarded as a accessory for Rupture Disk installation. Not quite sure the purpose of it. Please advise.

 

[Latexman]

In 27 years, I've never seen an excess flow valve in series with a rupture disk. Please describe how thet are used together.

Most excess flow valves I've seen are used on railcars and tank trucks to limit the spill rate if the outlet valve gets sheared off in a wreck.

Good luck,
Latexman

 

[djack77494]

I've seen an excess flow valve sometimes used in combination with a rupture disk/PSV. Unfortunately, I lack the ability to send a sketch that would make the arrangement clear. Instead let me try to describe the configuration.

1) Starting at the vessel being protected,
2) Fluid flows to rupture disk, then
3) Through a tee fitting.
4) In the branch of the tee, locate the excess flow valve.
5) One side of the excess flow valve is connected to the tee.
6) The other side of the excess flow valve is open to atmosphere or other suitable destination.
7) Install the excess flow valve so that high flowrates from the tee to atmosphere will cause the valve to close.
8) Install a pressure gauge upstream or downstream of the tee.
9) Install the PSV downstream of the pressure gauge connection.

The idea is that the rupture disk could develop a leak. You want to know about the leak so you can replace the rupture disk. Presumably, it is undesirable to have the PSV pop, else why even bother with the rupture disk. Thus if the rupture disk actually pops due to overpressure, the excess flow valve will close and the full pressure will be experienced at the PSV inlet. The pressure gauge will indicate this high pressure, so it will be apparent that the rupture disk has burst.

Instead, if the rupture disk just develops a small leak, it will escape through the excess flow valve.

That's what I've seen. As I think about it, I believe the situations where you would want this type of performance are quite limited.
Doug

 

[Ashereng]

Presumably, it is undesirable to have the PSV pop, else why even bother with the rupture disk.

A rupture disc ahead of a PSV is common in situations such as corrosive service.

It is expensive to have a PSV with the metallurgy to match the process fluid corrosion. It is much cheaper to use a rupture disc, with the correct metallurgy.

In relieving case, on over pressure, the rupture disc ruptures, the pressure reaches the PSV, and the PSV relieves.

In normal case, the process fluid only contacts the rupture disc. Hence, the PSV can be non-exotic metallurgy.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[sshep]

Carol,

My own experience of the simplest installation of excess flow valves as a small tubing size device (1/4" or so diameter), mounted into a 3/8" or 1/2" threaded well on the rupture disk holder (i.e. the two steel rings that clamp onto the disk and hold it). Typically a small gage is also mounted at that point via branch on a tubing tee.

If the rupture disk leaks, or more likely if the disk has burst during an upset which was not reported, the pressure on the gage can be used to determine this. If the disk is good the gage is reading atmospheric, if bad it is reading process.

These excess flow valves often leak just a little, and any condensible fluid will slowly drip out forming a small oil spot below- another sure sign the disk is bad.

On the otherhand you can get as complicated as you want- many times I have seen pressure transmitters broadcasting the rupture disk status to the panel. This can be nice as gages eventually go bad, but it is obviously some extra cost. As an added note, the gage needs to be able to read process pressure. I have seen a few times improperly specified low pressure gages on failed rupture disks completely "wrapped" back to near zero giving a false reading that everything is ok.

best wishes,
sshep