PSV or a Rupture disc 1

[PCM9]

Dear All,

This is just a basic question.

I am designing a relief device for a Tank handling a flammable fluid. What are the points to be kept in mind to determine whether we need to choose a PSV or we should choose a Rupture disk for this new tank. The relief device will be routed to atmosphere.
Thanks in advance!

 

[Latexman]

Why don't you give a side-by-side comparison and we'll point out any issues we see. Otherwise, we're doing your job.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[PCM9]

Sure. I am noting down the main differences I know:

Aspects-----------PSV------Rupture disc
Chance of Leak-----High-----Leak tight
Cost---------------High-----Low
Reusable-----------Yes------No
Adjustable---------Yes------No- fixed setting
Maintenance--------High ----Low
Inventory loss
once opened ------Low------High
Response time------High-----Low

 

[mk3223]

It's a good list. The next step is to set the preference tank design based on the list and involve the right peoples to review for your system and select the correct equipment as needed.

For application, the rupture disc may be added in the dirty fluid if the concern of the slide particle might be tripped on the PSV seating area.

 

[Latexman]

You have the components, but not the essence of "Long Term Cost of Ownership".
What is the fluid? Is it a human or environmental threat, nuisance, etc.?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[RVAmeche]

Another consideration is device size (and associated cost). Depending on your relief requirements you may need a very large relief valve while a smaller rupture disc may be acceptable. How close to the relief set pressure is the operating pressure? If this product is particularly nasty, you could consider a rupture disc upstream of the PSV (more cost). But this would allow you to ensure there's been no leaks, protect the PSV from the process, and re-close if that's an important criteria.

 

[moltenmetal]

A couple more points to consider:

Is there a risk of an extended fire which may boil the vessel dry? Or is the vessel gas-filled? If so, a disk is also inherently a thermal relief device because it doesn't reseal.

Is release of the material in the vessel itself a hazard or risk of environmental harm? If so, the relief valve's benefit is resealing, reducing inventory loss as you've noted, and with that, reducing the environmental harm.

How often are pressure upsets expected? If almost never, or a calculated risk only, the disk is cheaper and its downsides disappear.

Is the receiving device a flare or scrubber? A PSV will likely (greatly) reduce the instantaneous flare/scrubber load and reduce its cost, if it's a new install.

If leakage risk is an over-riding concern, but a PSV's inventory control is desired, the solution is disk plus PSV plus disk leakage detection device- and preferrably, a means to automatically vent the spool between the disk and valve rather than merely detecting the leak across the disc. Otherwise you end up with an unsafe situation until someone notices and does something about it- the exact opposite of what you want a passive layer of protection to provide to a safety design.

 

[don1980]

My view is that a re-closing device is almost always the better choice for any major piece of process equipment. That decision is very clear-cut in favor of a re-closing device when the system contains a hazardous material and/or when there's a high cost for an unplanned outage (unexpected disk failure). Also, one can't categorically say that disks have the advantage from the perspective costs. Replacement disks are relatively expensive. There are very few cases in which I'd recommend installing a disk as a stand-alone device on a major piece of process equipment. I view disks as special-purpose devices, which should be used in applications that specifically need a disk. The default choice, for me, is always a re-closing valve.

 

[PCM9]

Thank you all for the valuable reply.

Yes, the fluid is a siloxane which is flammable and slightly toxic.

 

[georgeverghese]

Check if this siloxane decomposes to some solid product on exposure to air or humid air. If it does, then in a spring loaded PSV, this may bung up the seat when there is a slight leak. An RD upstream of the PSV may be the solution in this case. A PSH alarm will be required in the interspace between the RD and the PSV.
Obviously, dispersion calcs will be mandated to show that, with the remote vent fron this relief device, the ground level concentrations for all chemical compounds with HSE concerns resulting from full flow will be acceptable under all expected wind speeds and wind stability classes. The same check should be done wrt to all elevated operator access platforms in the vicinity. Flame radiation exposure calcs should also be done. Have all these simulation runs and results properly documented and cross checked - you never know when and what the safety auditors will be sniffing around for when you least expect it.

 

[Compositepro]

No one has mentioned that you can use a rupture disc in series with a relief valve to get the benefits of both. The relief valve can reclose while the rupture disc protects the relief valve from corrosion and possibility of slight leakage. In corrosive service this allows a much less expensive relief valve to be used. Where I have seen this done, there is a pressure gauge and a manually controlled, normally closed vent valve installed between the two devices. The gauge indicates that the rupture disc is intact and not blown or leaking.

 

[moltenmetal]

I did mention that, actually. I also mentioned that the pressure gauge and manual valve between the disk and valve is an accident waiting to happen. An excess flow or adjustable crack pressure check valve replacing the hand vent valve, and a pressure switch or transmitter with alarm is required any time a disk ahead of a valve has a reasonable likelihood of leakage, i.e. where the disk is being used to reduce the risk of corrosion.

Disks also have higher availability, i.e. lower risk of failure on demand, in services where the product solidifies below room temperature and in some other services where plugging in the upstream or downstream lines is a risk. Flush mounted rupture disks are the go-to tool for many polymer services for instance.