7 replies to this topic

[bios16]

Hi,

I wanted to know how to calculate the maximum design pressure of a pressure vessel given that:

Tank thickness is unkown.

This is required to set the relief device nozzle on the tank.
Thank you.

[JoeWong]

bios16,

Your question is rather confuse.

i) what is the coincident maximum working pressure ?
ii) would you like to set the relief device ? what the relief device ?

Design pressure is just maximum working pressure plus design margin...

[bios16]

bios16,

Your question is rather confuse.

i) what is the coincident maximum working pressure ?
ii) would you like to set the relief device ? what the relief device ?

Design pressure is just maximum working pressure plus design margin...

The maximum working pressure is very less about 1-2 PSI
But we want to keep the size of the PSV/PRD not more than 2 in and therefore we want to set the tank design pressure higher.. thats what I want to figure how to do given that there is no tank thickness info therefore no MAWP.

[Qalander (Chem)]

bios16,

Your question is rather confuse.

i) what is the coincident maximum working pressure ?
ii) would you like to set the relief device ? what the relief device ?

Design pressure is just maximum working pressure plus design margin...

The maximum working pressure is very less about 1-2 PSI
But we want to keep the size of the PSV/PRD not more than 2 in and therefore we want to set the tank design pressure higher.. thats what I want to figure how to do given that there is no tank thickness info therefore no MAWP.

Dear bios16, It is definetly true that you may keep puzzling yourself and others;without related info on size, shape, thickness& service of the concerned vessel even if the matter is that of 1or 2 psi as we all must not forget incidents like 'Bhopal','Buncefield' being responsible professionals.
Pardon my harsh words.
Regards
Qalander

[JoeWong]

I just don't quite understand the approach...

Normally you set the design pressure with maximum operating pressure plus design margin. Then you calculate the relief load with design pressure and other information. Once you have the relief load, you calculate the required PSV size.

Design pressure ==> relief load ==> PSV size

I don't see the logic you keep the PSV and back calculate the design pressure.

Anyway, you can trial and error and follow above calculation until you get a PSV size equal to 2".

After you have done all these, please make sure the tank MAWP is higher than design pressure and "2" PSV" is large enough for the relief load.

GOOD LUCK.

[djack77494]

If I can read a bit "between the lines", I'm guessing that you're discussing an existing tank that (maybe) has an available 2" nozzle that you'd like to use for a PSV. (Reaching) you've decided that if the tank's design pressure was sufficiently high, then you could get by with using the 2" nozzle for the PSV. What you propose is a full fledged re-rating of your tank. That can be accomplished by a competent professional experienced in this area. If you go this route, the vessel should be restamped with the re-rated MAWP. I would caution you that this approach may or may not work. If you seek a small increase in rated pressure, you may have a good chance of success. If you want a larger increase, the chances are high that the tank cannot withstand your desired pressure. My best advice to you is that if you don't know how to rerate a tank RIGHT NOW, then you should be seeking professional help. This is not a task for the uninitiated.

[GMP]

I agree with emembers here that please do not BHopal so fast. Get your Process safety information organized first.
Know the vapour load and design presure of the tank (get the tank spec sheet), and then calculate the PRV size.
Dont try going other way around, because Ur PRV size shoud be for the venting the amount of vapor that the pressure does not increase no more than 6% of your tank design pressure.

[Andrei]

bios16,

It is the first time I am hearing the term "maximum design pressure". Till now I was used to the terms like Maximum Allowable Operating Pressure (MAOP) and simply Design Pressure.
You want to calculate a Design Pressure of an existing vessel? And you don't even know the wall thickness? I think this is the most dangerous thing you can do. What about the material? What about the operating temperature? Design Pressure and Design Temperature are related.
I don't know where are you located but normally there should be a code that specifically forbids this kind of operation.
The Design Pressure of a vessel it's set once and only once, by it's original designer. According to codes and regulations this is recorded in the documents accompaning the vessel, it may be stamped or attached sometimes to the vessel.
Maximum Operating Pressure? That's another story, that can be set later, but only downwards.
You have an existing vessel and don't know anything about it? Assume atmospheric pressure as maximum alowable operating pressure. That is the safest thing to do. Do you have proof that the vessel was designed for a higer operating pressure? No. So don't go above.

Safety first,
Andrei