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ratings of vessels which may collapse under vacuum
QUESTION: I have a common sense question. When a vessel is rated for some positive
pressure (say 150psi) it means that it holds up again that pressure from
inside. However the same vessel may collapse under vacuum as little as 20in
Hg? My question is why the difference? Why don't pressure ratings of vessels
mean the same when the pressure is from outside as opposed to inside.
Thank you for your help.
ANSWER: The shell is in compression if the pressure is greater outside.
Although in two dimensions, that is still pushing a rope.
It depends on the construction of the pressure vessel. if it is a metal
like a DOT or ASME, it would withstand the internal vacuum. However, if
the pressure vessel is like a rubber truck tire, which can easily
withstand 150 psi internal pressure, it will be sucked flat under vacuum
because the walls are designed to be "rigid" only when the walls are in
tension (internal pressure) vs compression (internal vacuum).
It depends on the shape and mechanical properties of the container.
Look at a balloon, as an example. Any positive external pressure
differential causes it to collapse. But it will stand a far higher
positive internal pressure differential.
Part of the reason is, for external pressures, the force can cause folding
or bending of the container. For internal pressures, the stresses are mostly
shear forces.
In general, shear/tearing forces of a structure are higher than the bending
forces that it can stand.
It is easy to fabricate a balloon or hose or ball or thin wall metal
pressure vessel, but to contain a vacuum the walls have to be rigid
enough not to collapse. A tennis ball will probably hold 150 psi, but it
would get sucked flat as a pancake under internal vacuum.
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