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Flat Head Design Pressure 1

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chuckoehler

Mechanical
Sep 23, 2004
8
When designing a Flat Head vessel that will be atmospheric and ambient, what should I use as the design pressure? Technically speaking atmospheric will be 14.7psi, but if I use that in lieu of 0psi, the vessel will require a 1.25" plate. This vessel will have a 1/4" shell, and be supported on < legs. It will be 108" overall length and made of 304SS.
 
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Generally, I would select the minimum thickness material and limit the deflection to half the thickness considering the plate weight. This would be done by using stiffeners as needed. However, there may be some loads on the head. How about equipment or someone walking around to service valves?
 
Your 1.25" thick 304S.S. plate is not practical. Since this is a non-code vessel, you don't need to have a design pressure. It is acceptable to specify ATM as a design pressure.

You didn't say what is the shell diameter, however, per Bobfromoh's recommendation just make the head out of .25" thk 304L plate and weld some stiffeners over it. You should ask the process engineer if there is any other design conditions you should consider such as condensing, exothermic reaction, etc. that could create a vacuum or pressure. The other thing is, would anyone be walking on the cover plate? If so, you may want to use a 50 lb/ft2 load as a design load for the cover and design it for deflection, for example.

 
Chuck,
Be careful with the way you specify your design parameters. If the vessel is at atmospheric pressure, then there is ZERO differential pressure (Inside = Outside = 14.7 psia). The walls and heads are designed to withstand DIFFERENTIAL pressure, not absolute pressure, so I worried when you seem to indicate a design to withstand 14 "psi" when at atmospheric pressure. I like to always think in terms of absolute, gauge, and differential pressures and to label them appropriately (psia, psig, & psid in the FPS system). Avoid psi without a sufix. It will help avoid a lot of heartache (i.e. confusion, misinterpretation) later.
Doug
 
The diameter of the vessel will be 60"OD. After checking this through, my pressure is 0PSI. I have checked the deflection on the top head and found that a thickness of 1/4" will be acceptable. However, when figuring a water weight of approx. 10,000lbs, I find that the deflection on the bottom plate will be approx. 1/4". I don't want to increase the thickness (1/4" thick), so I will use stiffners. When using stiffners I was planning on using (2) C3x5 channels in a cross formation welded to the bottom head and shell on the inside of the tank. Does anyone have any other ideas? I was thinking about welding the channels to the outside, but I would have to lengthen the shell to have it fully braced and I want to avoid that. Thanks.
 
Consider draining the tank and maintenance if you are going to have all those stiffeners on the inside. If the tank is to be supported by legs and is only 60"OD why not just use standard ASME F&D heads or does the process require flat covers. I am sure you could do away with all that bracing and save yourself a lot of money and issues later on to eliminate the bracing. Channel in 304 is costly most likely hard to work with giving its inertia values.
 
Did I miss something? The tank is 9ft tall, and filled with water.

Then there surely is pressure in the tank. The equivalent of a 9ft head is (9 x 0.4335) =3.9 psi at the bottom and varying linearly to zero at the top.

Paul Ostand
 
Paul,
the vessel has a flat cover in the top (it could be welded to the shell, but that's still irrelevat) and the internal pressure is equal to the external pressure (atmospheric) or zero gauge pressure at the top, as you indicated.
cheers
 

This vessel seems to be an atmospheric tank, doesn´t it?

API 650 has lots of interesting points on the issue, specially about safety. I see that you should be conserned with venting, over filling, weak joint between cover and sidewall, and the like. STD 650 will lead you to the thickness of side walls, if needed - but 1/4" will fit.

About the bottom of the vessel, I´d design a stand with a top grid, small enough to allow me to use 1/4"

Carlos Filgueiras
 
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