4 replies to this topic

[sms3499]

Hello Ladies and Gentlemen,

I have been visiting this forum for quite some time, and find it very useful. I am a recent Chemical Engineering graduate and working at an OEM.
The issue I am having at the moment is, I have two heat exchangers on my project sized ~10" OD & ~14" OD. Per the datasheet provided to me by my customer, the Heat exchanger vessel are to have Rupture Pin Technology, Model C, of the size 8". I have two issue with that:
1) ASME code does not allow you to have a nozzle greater than 10% of the vessel size. Therefore, by code, I am not allowed to have that big a relief valve.,
2) Secondly, why do they need such a big size anyways? Looking at the data provided, it does not really make much sense, even if you take the worst possible case scenario into consideration. Somewhere I read that oversizing the relief valve is also a technical mistake, as it will oscillate.

I have attached the data for one of the vessels. Could anyone of you kindly look into it, and advise. Your help will be much appreciated.

Attached Files

•  HBG__4350_RUPTURE_DISC.pdf   199.03KB   125 downloads
•  RP_8_MODEL_C_BAL__2_.pdf   160.26KB   81 downloads

[pleckner]

Of course I have to assume we are talking about a relief device on the shell of this exhanger. Not being a mechanical guy I can't comment on the nozzle size for the relief device without doing some homework. I'll take your word that you can't have a nozzle greater than 10% of the vessel size. And if this is indeed the case then end-of-story and you need to go back to your customer. I always expect my vendor to become my second (or third) set of eyes and make sure I'm not specifying something totally out in left field.

Saying this, you can't make judgements as to the required size of the relieving device without knowing the sizing basis, and you didn't specify what it is. Because of the difference in design pressures between the shell and tube side, tube rupture appears to be a credible relief scenario. I'll bet fire is another one and this may even invoke a two-phase initial relief; and this goes a long way into making the relieving device big.

Note that the rupture pin is NOT a relief valve but a non-reclosable relief device, similar to a rupture disk. There is no "oscillation" if you will. The pin breaks and the valve opens, and stays opened. The pin would then need to be replaced to put the beast back into operation.

My suggestion is to go back to your customer with your concerns about the nozzle size first and see what they say. You can question their relief sizing basis but unless you have their calculations, I would assume they are correct. This may make them investigate other types of relief such as a fusible plug.

[JoeWong]

sms3499,
The relief device seem excessive...you may go back to your client to looks for more information...

If the relief device can't be avoided (assuming it is on the SHELL) side and you would like to design to ASME, you may provide extended head (like boot but facing top and located rupture pin on the extended head...


Since your OD is range from 10-14" (rather small)
...you may consider to design according to PIPING instead of VESSEL...
...you may use a Tee and locate the rupture pin on the branch...


Hope this help...

JoeWong

[djack77494]

sms3499,
Would you kindly quote the section of code you believe prohibits you from using a relief device > 10% of your vessel size. This is a very significant restriction, and I am totally unaware of any requirement of this sort. For a S&T heat exchanger, tube rupture scenario, you'd want the PSV on the exchanger shell, and it could easily be a pretty good size. If I were forced to respect a "10% rule", I'd be limited to 1" & 1-1/4" PSV's for your two cases. I don't believe that is the case.
Thanks,
Doug

[sms3499]

I apologize. I meant 70%. Will post supporting document in a while.