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FluxIsRad
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Join Date: Feb 2014
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ASME Section VIII Div. 1 - Reducing Head Thickness Due to Pressure on Convex

02/21/2014 2:33 PM

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 around to check me or ask questions of.

I am designing an Aluminum Vacuum Tank with two compartments, both which are to be code and pressure tested.

I am worried about the required thickness of the head (torispherical 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 pressure of 2.13 psi (59" OD Head) applied to the convex side by the front compartment.

While the rest of the tank components (shell, front & rear heads) 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 receive.

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 how to do this stuff using the code books and common sense.

I do have the Section VIII Div. 1 Code Book available here so any code references to point me in the right direction would also be very helpful.

Thanks!
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Pathfinder Tags: ASME section VIII Internal Head Thickness Vacuum Tank Design
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lyn
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#1

Re: ASME Section VIII Div. 1 - Reducing Head Thickness due to Pressure on Convex

02/21/2014 3:40 PM

A single separator/head?

This is not my field, but you need two identical compartments, end to end, not a single bulkhead in the middle.

Maybe that's why it's so thick?

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FluxIsRad
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#2
In reply to #1

Re: ASME Section VIII Div. 1 - Reducing Head Thickness due to Pressure on Convex

02/21/2014 4:14 PM

The design is a single shell with two compartments, separated by a single bulkhead if that is your question. like this : (__(___)

While separating the two compartments (still with one shell) by two opposing bulkheads, like this: (__)(__) would be ideal for managing the forces, this would require additional tank length or diameter to get the same capacity.

This may screw up the weight balance or conflict with height requirements, which I need to look further into, but keeping the same, single bulkhead design is preferable.
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SolarEagle
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#4
In reply to #2

Re: ASME Section VIII Div. 1 - Reducing Head Thickness due to Pressure on Convex

02/22/2014 3:26 AM

Something like this?

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Tornado
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#5
In reply to #4

Re: ASME Section VIII Div. 1 - Reducing Head Thickness due to Pressure on Convex

02/22/2014 3:43 AM

Nah, (__(__) doesn't look like that at all.

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SolarEagle
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#6
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Re: ASME Section VIII Div. 1 - Reducing Head Thickness due to Pressure on Convex

02/22/2014 1:59 PM

Oh sorry....

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Tornado
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#3

Re: ASME Section VIII Div. 1 - Reducing Head Thickness due to Pressure on Convex

02/22/2014 12:31 AM

Your mode of thinking, and your questions, are excellent.

I'm not sure of all possible combinations of pressure on one side vs vacuum on the other that might be encountered, but it seems as though ~40 psid might be possible against a convex surface. That would be the worst case to check.

Another head shape might give a slightly less required thickness. If that can push it below 0.375", you might find it useful.

I don't have a code copy, so I can't readily confirm your calcs.

There might be some ways for relief devices to limit the maximum psid, without affecting any process requirements. Those too might be worth a look.

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FluxIsRad
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#8
In reply to #3

Re: ASME Section VIII Div. 1 - Reducing Head Thickness due to Pressure on Convex

03/05/2014 3:55 PM

What scenario are you considering that could give a 40psi difference?

My initial post was looking at 25psig design + 2.13psi Static Head on the Convex side of the shared head with perhaps 0psig on the concave side which I think would give the maximum differential at 27.13psig.

Can anyone confirm this?

I'm new to this industry so bear with me, but is this the worst case scenario?

I know we usually consider 15psig external pressure for full vacuum, so should I instead compute this scenario with the 27.13psig on the convex and 15psi on the concave sides, giving a maximum differential pressure of 12.13psi?
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Anonymous Poster #1
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Re: ASME Section VIII Div. 1 - Reducing Head Thickness Due to Pressure on Convex

02/24/2014 6:35 AM
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