Rupture Disc Bursting Pressure Settings (Flare Back-pressure inlfuence)

[dewed12]

Dear forum members,

recently I have encountered the following issue when specify rupture discs - seems to be trivial, but the longer
I think the more questions are coming up:

Rupture disc is installed at the low pressure side of heat exchanger. Rupture disc outlet is connected to the flare system. Design pressure (MAWP not known so far)of heater low pressure side is 6 bar[g], constant flare back-pressure is established to 0.2 bar[g].

That's my first rupture disc specification. In my mind rupture disc are always mentally connected with differential pressure, which implies the following (regardless bursting tolerances, manufacturing ranges etc.):
Bursting pressure shall be established to 5.8 bar, what guarantees heat exchanger protection for conditions described.

Thus what about stamped bursting pressure of rupture disc - are they stamped for required pressure difference across the disc?
Based on which pressure value(s) vendor is specifying rupture discs? I have seen many offers in internet and mostly gauge pressure is given, what is only straightforward for non back-pressure affected installations.

How bursting pressure of rupture disc should be shown on P&IDs to do not confuse anybody - bar, somehow bar[g]?

I have searched API 520 and many internet threads, but I did not find full approach explained.
Would be appreciated if anybody is able to advise what is a the best way to to that.

Regards, dewed12

 

[EmmanuelTop]

Dewed12, you are correct as to the burst pressure calculation. See the articles below, which also explain how to select burst pressure based on downstream conditions and Code requirements:

http://www.cheresources.com/content/articles/safety/rupture-disks-for-process-engineers-part-4?pg=2

http://www.cheresources.com/content/articles/safety/rupture-disks-for-process-engineers-part-3?pg=1

You can navigate through different sections of the article through the drop-down menu below the article contents.

Dejan IVANOVIC
Process Engineer, MSChE

 

[dewed12]

@EmmanuelTop

Thanks for sharing. However, I have read that comprehensive summary before placing my question here.

Assuming design pressure of the vessel protected is equal to MAWP equal to 100 psig, what should be burst pressure of rupture disc - 95 psi or 95 psi[g]?

Refer to Figure 1B here:

http://www.cheresources.com/content/articles/safety/rupture-disks-for-process-engineers-part-4?pg=2

If burst pressure is obtained from differential pressure what is a source of that gauge pressure value - this I do not fully get to be honest?

 

[EmmanuelTop]

Gauge pressure relates to atmospheric pressure, not to flare backpressure - if that is the question from the last paragraph. A vessel designed for 100 psig will have the design pressure 100 psig regardless if the flare backpressure is 0 psig or 50 psig. Burst pressure is set in gauge units, not differential units.

Marked Burst pressure of the relief device must be less or equal to the vessel MAWP according to the Code, and the maximum allowable accumulation at relief conditions must not exceed 110% of the MAWP for single rupture disk installation (other than for fire exposure). In your particular case, if burst pressure is set at 6 barg, the relieving pressure will be 6.2 barg and thereby both requirements from the Code are met because the maximum allowable accumulated pressure for this case is 6.6 barg. The disc achieves full capacity at its bursting point (6.2 barg inside the vessel) and pressure will not rise above this figure.


Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

Is this RD in liquid service on the LP side of this HX?

 

[dewed12]

Thanks for feedback!

@EmmanuelTop

Burst pressure is set in gauge units, not differential units. - that was exactly my question - how to mark RD burst pressure correctly - thanks!

@georgeverghese

RD is installed on low pressure side of HX (shell). This is gas-gas service. Is there any difference in the approach in case liquid service is needed?

 

[georgeverghese]

The risk of accidental rupture of an RD in liquid service is more that that when in gas service, so wouldnt recommend these for liquid service.
Also presume you are aware that RDs' can rupture at pressures as low as 90% of marked set pressure. And also check what the rupture pressure tolerance is with the vendor. Vaguely recall there are temperature corrections also required for set pressure.

 

[EmmanuelTop]

There is no difference in determination of the set pressure in liquid service.

However, rupture disc may not be the best device for this case - George has already mentioned some of the reasons. An alternative is to use a PSV but in order to evaluate that option you need to calculate pressurization rate and see if 110% of MAWP would be reached before the PSV is fully open. For gas service, the rule of thumb says this is not usually the case but you can't confirm it without calculations. Dynamic simulation and use of actual flows and volumes can give you the answer. See attached article.

Dejan IVANOVIC
Process Engineer, MSChE

 

[TD2K]

< are they stamped for required pressure difference across the disc >

They are not stamped as a differential but they essentially are. If you have a rupture disk stamped as 10 bar, it will nominally burst at 10 bar with 0 bar backpressure. Put 1 bar backpressure on the rupture disk and it now nominally bursts at 11 bar upstream pressure (10 bar differential). It's similar to a conventional relief valve.

< Also presume you are aware that RDs' can rupture at pressures as low as 90% of marked set pressure. And also check what the rupture pressure tolerance is with the vendor >

You can specify the tolerances but it costs you more money.

< Vaguely recall there are temperature corrections also required for set pressure. >

Very true. Some metals are less senstive to temperature than others but the temperature sensitivity is much greater than say what you are used to seeing in the piping codes. SS and CS piping under say B31.3 have no or little temperature de-rating for strength for say up to 400F. Rupture disks are very different and it's the temperature that the rupture disk sees when it bursts that affects it, not the relieving temperature unless the two are the same (obviously, the relieving temperature sets the capacity). This temperature can be quite a bit different than the relieving temperature depending on the piping configuration, ambient temperatures, is the burst disk and piping heat traced, etc.

 

[georgeverghese]

Though this is not the subject of your posting, you could tell us why an RD is selected for the LP side of this HX? All in all, the actual rupture pressure may be some what less than 90% of marked set pressure by the time you've tallied up all the numbers - you've got head room to spare for pressure excursions on LP side?

 

[dewed12]

@EmmanuelTop
@TD2K

Thanks for feedback again. There are valuable information provided.

@georgeverghese
RD has been chosen as Client standard requires for that type of service. I have roughly checked unavoidable tolerances with maximum expected operating pressure and in my eyes sufficient operational room is present so far (0.4 bar), because goodly margin for design pressure. I would assume that will be revised in the future, when vendor information will be provided.

Regards, dewed12

 

[georgeverghese]

So your estimate is that max operating pressure is approx (0.9*5.8)-0.4=4.8barg.

Given that an RD is selected for overpressure protection in unusual circumstances only, it may be a good idea to bring this up to the plant owner's attention and get their specific reasons for this application, given the many disadvantages of an RD in comparison to less risky ways of overpressure protection - the sooner this is done, the better. They may hold your company accountable for this unusual selection regardless of what is in their standards!