Chamber Construction/Strength

[Rich Feldman]

Hi David.

Good luck, and please tell us what you learn about your tank head thickness requirement.
I did a little more investigation of design calculations.
Much of the following may be redundant with your own searches.

The external pressure case is, of course, more complicated than that for internal pressure.

There are a handful of industry standard head shapes, such as 2:1 semi-ellipsoidal.

I think most "vacuum tanks" are designed for only partial vacuum, such as we might find in a honey wagon application.

Fabricators eagerly provide data on volumes and weights, but seem to assiduously leave all questions of strength to the customer's engineer.

Most leads I have found have the goal of "building to code". In most of the world, the code is called ASME VIII, more specifically ASME VIII-1 UG-28,
which is probably hard to find for free on the Internet. You can buy it at https://www.asme.org/shop/standards/new ... -code-2013
One handbook with formulas is Pressure Vessel Handbook by Eugene Megyesy.
There must be some fusor forum readers with easy access to resources like those.

On other forums I saw suggestions to use pressure vessel design software with demo versions or trial periods.

Here are a couple of my favorite instructional links.
http://www.pveng.com/ASME/ASMEComment/E ... essure.php
http://www.eng-tips.com/viewthread.cfm?qid=84873
http://www.eng-tips.com/viewthread.cfm?qid=196371

[David Kunkle]

Thanks for the links, Rich. The first one was very helpful. At first I was worried, but after getting to this part, I think I am in good shape.

"A typical vessel 48" diameter with a straight shell 96" long needs a shell 0.056" thick for an internal pressure of 30 psi, but needs to be 0.225" thick for a 15 psig external pressure (full vacuum) per VIII-1 UG-28. The F&D head on the left end needs to be 0.082" thick for the interior pressure but 0.142" for the vacuum. A SE head needs to be 0.046" thick for the interior pressure but 0.127" for the vacuum. The F&D and SE heads are both calculated as if they are part of a sphere, but the two heads are given different equivalent radiuses resulting in different required thicknesses."

Before that, he stated how increasing diameter and length, and decreasing thickness weaken a vessel.

For a comparative whopper of a tank at 4' diameter and 8 ' long, it only need be .225" thick and .142" thick on the heads. What I already ordered is 3/16" or .1875" for my 20" chamber. The heads are F&D- about 2.2:1. Only about .04" thinner on the cylinder, but .05" thicker on the head. The author goes on to say this large tank amounts to 4x the collapse strength, and 3x is the usual safety factor.

Even if he is using some metal stronger than SS304 on this 4' X 8' tank, my less than 2' X 2' cylinder with .188" thickness should be way overkill based on my smaller size- at least I would think. Cost is $625 for the cylinder and 2 tank heads, and weight is about 90 lbs. Might be somewhat cheaper and lighter to go thinner, but I'd rather not worry if it's strong enough. I found one manufacturer of SS vacuum hemispheres that goes from 1/8" to 3/16" at 18 to 20 inches in diameter.

Thanks for the input.