Bursting Disc Cold set pressure: Temperature compensation or not:

[jamesbanda]

I've been doing some background reading on pressure compensation for bursting discs.. and would like to poll what other do..

in can see advantages in doing it such as.
Case 1: Corrosive vessel, relief case 10 barg at 300 C,operating temp 200C. vessel design pressure 10 barg
if you dont temperature compensate the bursting disc: Cold set at 22 would be 10 barg, so at 200C, burst pressure could be say 8 barg.
..so you cold burst the disc, and not know about it in normal operational cases..and endup having an fairly big issue due to corrision of the relief valve..

Case 2.but if you do consider case1 but temperature compensate the valve.. you could have an operational case COLD(say thermal expansion.. ).. and exceed design pressure in cold case..

so..in my view you need to consider the cold set pressure against the credible relief cases and not just in isolation against one case.. so in some cases you will temperature compensate so the burst pressure cold exceed design pressure and in other cases you will not and the burst pressure hot will "go early"..

i've read the codes api etc.. and they tend to suggest to myself to use guidance..

 

[Duwe6]

Rupture Disks are simple for the user, tough for the R/D Mfr. You are to specify the R/D material-of-construction [316 s/s, Inconel, Titanium, etc.] burst pressure at the relieving temperature. Burst pressure should be about 80% of the PRV setting, thus your Case #1 is a 'correct' example -- 8 BARG R/D under a 10 BARG PRV. Case #2 is immaterial; incorrect assumptions.

The R/D Mfr will make a batch {Lot} of R/D's to satisfy your specified material, and burst pressure at temperature. Mfr. will then test-burst at least 2 R/D's from that batch, and certify the burst pressure at room temperature and burst pressure at Relieving Temp. You have really paid for the whole batch, not the 1ea. you requested. Thus, the smart decision is to purchase however many R/D's from that certified batch as you think you will use in the next 10-20 years. The additional cost will be small -- they had to make a batch larger than 4 R/D's [probably made 10(+) disks to your spec].

 

[Latexman]

You are correct. The devil is in the details! A preliminary design carried forward without considering these details can turn into a most unsatisfactory final design.

Good luck,
Latexman

 

[Latexman]

Case 2 is possible. It depends on your numbers, but you may be able to use a small PSV or RD to handle the low temperature scenarios, like thermal expansion. Then, a temperature compensated RD (with a burst pressure that is too high at low temperatures) would be fine for the high temperature cases. No credible scenario set pressure can exceed the Code rules.

Good luck,
Latexman

 

[don1980]

Temperature sensitivity is one of the main reasons why you should use Inconel as a default disk material, except for those rare cases in which Inconel isn't chemically compatible with the process fluid.

Inconel has far less temperature sensitivity (the burst pressure is less affected by temperature) than all other standard alloys, including SS.

The cost inpact of specifying Inconel is usually trivial - there's very little mass in most disks.

 

[Latexman]

20% change over 280 C - I think he may be looking at Inconel.

Good luck,
Latexman

 

[jamesbanda]

thanks for the comments,

say you only have one case.. fire on the vessel no other cases.. design pressure 10 barg/150psig.. so would you specify a disc at 10 barg at ambient or at burst pressure.. and why not a burst pressure.. and whats wrong with that ?

 

[Latexman]

Assuming the RD is close to the vessel (i.e. right on top), 10 barg at the temperature coincident to the burst pressure. Assuming the RD is far from the vessel, probably ambient.

Good luck,
Latexman

 

[moltenmetal]

If the fire is vapourizing liquid, you will have vapour condensing on your rupture disk unless you've pocketed it.

If the fire is heating gas and the disk is far away from the vessel, specifying ambient burst pressure is safer.

If this is a new design situation, what you have to contend with here is not only how to render operation safe against excessive pressure, but also how much extra design pressure margin you want to put in the vessel to accomodate your anticipated operating conditions without unnecessary rupturing of the disk. If the vessel exists already, temperature derating and MAWP determine how tight a mfg range you need on the disk to permit safe operating at your required max operating pressure. This is especially true if the disk is the sole means of protecting the vessel, i.e. loss of the disk equals total loss of contents- but also matters if there's a relief valve on there as back-up, since presumably the disk is there for a reason beyond merely reducing the risk of leakage through the relief valve. The manufacturing range of the disk alone, never mind the temperature de-rating as well, typically requires you to add quite a bit of extra metal to the vessel to avoid challenging the disk during normal operation.

Duwe6: we normally have the disk burst pressure equal to the relief valve set pressure, which in turn is equal to the MAWP. The burst pressure is determined at the lowest temperature expected at the disk during a credible relief case. The manufacturing range takes care of ensuring that the disk bursts at a safe pressure, and determines how close to the MAWP your maximum operating pressure can be- or if the maximum operating pressure is already specified, this is how the minimum required MAWP is determined. The trick in this process is to ensure that you don't arbitrarily trip across a natural breakpoint for cost such as flange class in that process.

As long as you do a proper job of both monitoring (preferrably with an alarmed pressure instrument) and VENTING the space between the disk and the relief valve inlet, this approach for selecting the required disk burst pressure and temperature is adequately safe. Merely putting a pressure gauge on there that people may not look at frequently enough to detect a pinhole is inadequate if the disk is exposed to any credible risk of fatigue or corrosion. Your point about ordering more disks up front is definitely a good one- mfgs usually offer very significant discounts for quantities purchased for the reasons you've mentioned.

 

[Duwe6]

NBIC 'National Board' in the USA prefers to see a R/D bursting pressure of about 80% of the PRV set-pressure.

 

[Latexman]

Duwe6,

What NBIC document contains this preference? Thank you.

Good luck,
Latexman

 

[psafety]

There is also a "manufacturing tolerance" to consider in the burst pressure with the temperature compensation.