High pressure rating = vacuum rating..?

[jamesbanda]

Dear sirs,

I'd would appreicate some expert comments on the below statement..

I've read the comment " a vessel with a 50
psig (or higher) ASME pressure rating is frequently capable of withstanding a full vacuum". How valid is this general comment?

Our Engineering contactor has ordered boilers that were not requested to have a vacuum rating. But they are designed to withstand at least 20 barg / 300 psig pressure. ( I am a process guy).,. what would be required to estimate if these vessels would implode if we shutdown below 100 C?

We had all kinds of conserns about o2 issues if we install a vacuum breaker on the boilers.. the conserns are o2 cuases corrision.

The current spec for the boilers is a rating of 0.8 bar or 10 psia (-5 psig).

 

[psafety]

It is NOT valid. Shell length plays a large role in vacuum.

 

[pastyl]

ASME VIII UG-28 "shell under external pressure" will help you check if the vessel can withstand -5psig.
But as it has been designed for 20barg internal pressure I am quite sure that she could withstand -5psig i.e -0.34barg.
However the comment you have read is no valid. In any case you must advise the Code.

 

[jte]

James-

An internal pressure of 50 psi frequently will not result in a shell capable of full vacuum without stiffening rings.

You should run some numbers based on the original code of construction. If you are using ASME VIII then, making a few assumptions, my numbers show that your vessel will be ok for full vacuum as long as the tangent to tangent length is not more than 7 times the diameter. Consider this a rough guide, and please consult a qualified vessel engineer for the final answer. Remember, this answer is worth every penny you paid for it.

jt

 

[jamesbanda]

thanks for the information.

 

[psafety]

I worked on a large 150-psig vessel, best I remember it was 7-8 ft in diameter and 20+ ft long. It required a couple of stiffeners to achieve a FV rating.

 

[vesselguy]

Psafety nailed it spot on. It's got to do with the stiffen length of the shell and shell diameter. You can design a vessel 40 ft diameter x 120 ft tall for 50 psig and it won't even be good enough for a little bit of vaccum if it is not stiffen along the shell length. On the other extreme, you can have 8ft x 20 ft drum designed to 50 psig and it will likely withstand full vac without stiffening.

 

[JohnGP]

Follow jte's advice. It is a fairly easy process to use the relevant vessel code to check what vacuum the present unstiffened length of shell can withstand.

There are a few more hoops to jump through if you want to rerate the drum though. If the boiler fabrication is not started, then it may be possible to revise the design (with some financial pain and suffering).