# Cubical Pressure Vessel - Maximum Pressure Computation

• TomCo
In summary, a pressure vessel with a diameter of 540mm, a thickness of 25mm, and a maximum pressure of 20 bars would be suitable for a pressure application.

#### TomCo

TL;DR Summary
Pressure vessel
Aluminium
Maximum pressure computation
Hello everyone - this is my first post on the forum and this would be for a real world application.
I have no mechanical engineering experience.

We plan to build a cubical aluminium (6082 grade) pressure vessel.
The total external dimensions are 540mm x 400mm x 75mm.
The aluminium wall thickness would be 25mm.
The inner chamber would therefore be 490mm x 350mm x 25mm.

One 540mm x 400mm x 25mm plate (part of the total external dimensions) would be fixed to the rest of the structure with 38 x M6 bolts, each going 20mm into the other structure. These bolts could be changed to M12 if needed.
The rest of the structure would be made in a single machined part.

What is the maximum pressure that the pressure vessel could withstand without having any risk of failure for the bolts or the aluminium itself?
What is the safety margin ratio usually used for such machine, ie what should be the maximum operating pressure?

Is there any free FEA software that could be used for such computations and where we could import a STEP file into?

Any help to understand how this should be computed would be greatly appreciated!

Thank you,

TomThank you,

Last edited:
Welcome to PF.

Please keep in mind that this thread may get closed for safety reasons. We don't allow dangerous discussions here, especially with inexperienced folks.

That certainly is an odd shape for a pressure vessel. Can you say what the application is? What kind of pressures are you wanting to pump this chamber up to? Why not use an off-the-shelf pressure vessel of a more standard construction? What-all could happen if this pressure vessel bursts? Any people nearby during operation?

russ_watters, BvU and Lnewqban
Hi Berkerman,

Thank you for your answer - if there is any better forum for such question and application, I am happy to repost there.

20 bars maximum. We would put thermoset composites to shape and harden them. These parts have specific geometries. Most composites autoclaves are way over our budget and often do not go to that pressure.

We would use a hydraulic press as an additional safety, and yes people would be close to the pressure vessel.
The press dimensions explains the 540 x 400mm.

No serious pressure vessel would ever be permitted to have flat sides.
Each time the pressure was established, then released, the flat sides would bulge out and then in. The seam would split after a few cycles.

A 20 bar pressure vessel would be a sphere, or a cylinder with hemispherical ends.

Grelbr42, russ_watters, Lnewqban and 1 other person
TomCo said:
20 bars maximum. We would put thermoset composites to shape and harden them. These parts have specific geometries. Most composites autoclaves are way over our budget and often do not go to that pressure.
Wait, this is for autoclaving composites? What temperature will it be heated to?

https://en.wikipedia.org/wiki/Autoclave
Autoclaves are also widely used to cure composites, especially for melding multiple layers without any voids that would decrease material strength, and in the vulcanization of rubber.[9] The high heat and pressure that autoclaves generate help to ensure that the best possible physical properties are repeatable. Manufacturers of spars for sailboats have autoclaves well over 50 feet (15 m) long and 10 feet (3 m) wide, and some autoclaves in the aerospace industry are large enough to hold whole airplane fuselages made of layered composites.[10]

russ_watters
Get a copy of the ASME Boiler and Pressure Vessel Code and skim through it. Do not expect to understand it because a good mechanical engineer needs years to to do that.

TomCo said:
Most composites autoclaves are way over our budget and often do not go to that pressure.
After looking through the ASME Code, you will know why those autoclaves cost so much and why higher pressure costs even more. They are worth every dollar (or euro, or yuan, or ...).

We can help you understand the specifications of commercial autoclaves, and how those specifications apply to your needs. But we cannot help you design a heated pressure vessel of that size. I suggest that you tell your management that building this is completely unrealistic, and that they need to either find the money to buy a proper autoclave, or contract that work to somebody who has such a unit. You may want to contact your corporate insurance company if you need extra help making your case.

This thread is closed, but feel free to start another thread asking for help selecting a commercial autoclave.

russ_watters, Tom.G, berkeman and 1 other person