Section VIII Div. 1 Reducing Head Thickness due to External Pressure

[FluxIsRad]

Hello Everyone,

I've recently inherited the responsibility of performing Section VIII Div. 1 calculations for Code Vacuum Tanks, and I don't particularly have anyone else to check me or ask questions of.

I am designing an aluminum vacuum tank with two compartments, both of which are to be code and pressure tested.
I am worried about the required thickness of the head (ASME F&D) that separates these two compartments, if my logic in the design is indeed correct.

I figure the worst case that must be tested for is the design pressure (25psi) plus static head of 2.13psi (59" O.D. Head) applied to the convex side by the front compartment.
While the rest of the tank components (shell, heads, baffles) only need to be 5/16" thick, I'm getting a minimum thickness of .3953" with SB-209 5454 0-Temp (CEI DesignCalcs) for this intermediate head, which will be a lot more costly and take longer to get.

So my questions are:
1. Is my reasoning sound in my method of determining the required thickness for this head? (My assumption being that the head must be evaluated under full internal design pressure on both convex and concave sides)
2. Do these numbers sound like they're in the ballpark for an application of this magnitude? Might I be doing something else incorrectly?
3. Are there any methods of reinforcement that I can use to reduce this thickness? If so, are they documented in code or would I need additional calculations to prove their effectiveness?

Any help is greatly appreciated, as I'm pretty much a noob trying to teach myself using the code books and common sense.
I do have the Section VIII Div. 1 Code Book available so any code references to point me in the right direction would also be helpful.

 

[SnTMan]

FluxIsRad, hard to reproduce exactly your case, but:

1) I get about 0.39 for the external pressure, and yes, unless diff pressure design can be justified, see UG-21
2) Yeah, for "large, thin" parts, not unusual for ext pressure to govern
3) Don't know of any convenient calculations for stiffened heads. Likely would fall under U-2(g) [look it up ]

Some advice for you as you enter this field, often a little extra "iron" is the cheapest insurance you can get

Regards,

Mike

 

[SnTMan]

Oh, and welcome both to the forum, and to the fulfilling world of PV design

 

[intaim13]

The intermediate head is thicker because of both pressures on the two compartments. If the first compartment on the concave side is not under any pressure, the convex side of the head will receive full external pressure, which is the pressure on the second compartment. In my opinion, the vessel when operates or hydrotests, pressures on both compartment should not be greater than delta of the design/hydrotest pressure.

 

[intaim13]

I meant
...pressure on the second compartment should not be greater than delta of the two compartments.