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Re: Horizontal Pressure Vessel

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>I am checking an existing  horizontal pressure vessel for a Client and have
>been using finite elements to model the tank.
First off--If I were you I wouldn't touch a job like this. You're on soft 
legal and professional ground. The consequences of a rupture throwing 
that volume of whatever it is at 300F would be life threatening. I've 
been in the pressure vessel biz for about 30 years, and this sort of job 
is a technical minefield.

This vessel needs to satisfy the requirements of the ASME Unfired 
Pressure Vessel Code as a minimum. Never mind what you may have been told 
about the scope of the Code--the issue is prudent design practice, not 
the weasel words. From the nominal original design pressure I'd guess the 
tank probably isn't a Code vessel. Check for a Code plate (metal tag 
somewhere on the shell with the allowable pressure and temperature and 
manufacturer's name alone with a cloverleaf ASME stamp). Unless you are 
in one of two states that have no pressure vessel laws either the state 
or your client's insurors or both won't let your client use the vessel 
legally without a Code plate. If the contents are flammable, the NFPA 
also has something to say about it. Even if it has a Code plate the 
National Board Repair Code forbids upgrading in this way beyond the 
original design pressure. Your one chance may be to get the vessel passed 
as a special design by your local jurisdiction and the Client's insuror, 
but get ready for some serious work. 

The nominal shell stress at 10 psi is only about 3200 psi, which is 
probably why your client thinks he can put 10 psi into vessel, but that 
decision is a lot more involved. 
First, to give you some idea of the scope, you have a total load on one 
end of the vessel of just over 113000 lb, around 50 times what it had 
with 6 InH2O. You're already encountering problems with higher pressure, 
I suspect because the knuckles at the head OD have too small a radius. 
There's nothing to be done about this--those knuckles are there to carry 
the head membrane load into the shell. Too small a radius and they buckle 
beside subjecting the attachment weld to very high bending stress. That's 
a statically determinate load, so you can't count on shakedown to relieve 
it. Even if you could, fatigue effects would take over. If there's a 
manway into the vessel, the manway and the closure both need to meet Code 
requirements. (If it doesn't have a manway, drop everything and run 
away--the vessel is absolutely unsuitable for pressure, since the welds 
were made from one side only.) You may also need stiffening rings for 
whatever new internal vacuum requirements your client has in mind.

Next is the matter of QA. If the vessel wasn't made to satisfy the ASME 
Code, chances are the construction isn't up to the standards for pressure 
vessels. There are some specific requirements for shell and head 
attachment welds in terms of the details and the required QA, not to 
mention hydrotesting and welder qualifications. The low membrane stress 
is meaningless if the weld quality isn't appropriate.

Apparently the stuff inside isn't gas, so you don't quite have a bomb, 
although 1300 cu ft of pressurized liquid at 300F is the equivalent over 
a shorter range. But your next job is figuring the stress in the wall due 
to the dead weight of the vessel and its contents. This isn't a tough 
job, but you've added a lot of stress by increasing the pressure. There 
are closed form analysis methods to check this. 

In your shoes I'd do some quick numbers to justify _not_ doing this job 
and get myself off the hook. I've seen enough bad things of this sort to 
last me 30 more years. If you need anything more, you can e-mail me. 
Despite the negative tone of this, I really do like pressure vessel work, 
and I'll be glad to do what I can.

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw