Actual External Pressure When Subjected to Vacuum

"I work for a company which sells pumps and often have trouble explaining to people that you can pump at several bar of pressure with no problem, but you can only 'suck' at 1 bar"

I have this problem explaining vacuum to people. We use pumps to extract all of the air from a vacuum chamber then use cryo pumps to remove the remaining molecules. People often ask how much the vacuum increases when we do this & have trouble understanding that 1 atmosphere is all you get.

And soda straws don't work well on the moon!

You can collapse a straw by blocking one end sucking on it.

You can't blow it up, no matter how hard you blow.

In addition to the difference between internal and external pressure, a normal person can suck about 5 psid, but blow only about 2 psid.

"You can't blow it up"

Oh yeah?, I have access to some good explosives.

29 inches of vacuum equals 14.2psi but I doubt applying that little pressure on the exterior walls of a vessel is enough to cause it to collapse, so there must be more to it.

You just did you first mistake.

Remember that surface area, how many sq/in * 14.2.

The strength of a vessel is its tensional strength. Look at a thin wall aluminium soda can.

Pressure alone does not describe the problem. Shape, size and material composition and thickness are other necessary considerations.

So, let's say I connect a 2" pipe to the vessel vented to atmospheric pressure and place the vessel inside a larger sealed container with just the 2" pipe sticking out and pressurize the container to 14.2psi, would the force created be enough to collapse the internal vessel?

The force created inside the sealed container would equal the force inside the vessel.

Zero differential pressure. No collapse.

I meant 14.2 above atmospheric. The interior of the vessel would be at atmospheric pressure, the container and outside walls of the vessel at 14.2 above atmospheric pressure.

Just want to have an idea on how much force is created by full vacuum as 14 psi seems very little.

Atmospheric pressure is the pressure created at sea level by a 1 square inch column of air that extends into space, pushing down on the surface of any object at sea level.

Is the pressure inside the sealed vessel absolute, or gauge pressure?

If I understand the question you would,in effect, have the equivalent of asmospheric pressure on the outside of the vented vessel, inside the sealed vessel.

Don't overthink this.

I like these practical examples. Now I have another in-the-kitchen science experiment to show my grandkids and see if we can understand what we just saw!

Easy: 14.2 psi differential pressure between outside and inside of vessel. But I think you already knew what I meant.

Here's a dramatic little video of a rail tank car collapsing:
http://www.pipingguide.net/2009/04/rail-tank-car-collapse.html
You can Google 'vacuum collapse' for more examples. (Ignore the quantum physics references.)

I saw that video, that's why I'm surprised 14psi could cause an implosion of such magnitude.

Mexican user safety standard for pressure vessels just changed and there are no provisions for vacuum tanks other than they are considered a type of pressure vessel so I'm documenting myself as best I can before taking my case to the labor agency.

A tank will handle a greater internal pressure than it will an internal vacuum. Under the vacuum, the wall can buckle under compression before the material yields, under pressure it is in tension and good until the material reaches the yield point.

Think of your pipe, now use it as a form for a pipe made from aluminum foil. The aluminum foil pipe will handle a reasonable internal pressure but next to zero vacuum.

Oops, I should have payed more attention to Del's previous post.

I had an encyclopedia that showed a man bent at the waist with a plank across his back and on the plank, an elephant. It said that the weight was the equal of the total atmospheric pressure over his back.

Thanks everybody for helping me picture the effects of vacuum. Just wanted to be sure of what I'm taklking about when I go to the gov. agency. I have a 15" and a 28" of vacuum tank and it is in fact a lot of pressure.

Mexican standard divides vessels into 3 categories depending on relief valve set pressure but has no specs for vacuum tanks.

Good luck!

It looks like you've got the message now, but from your original post, it seems you couldn't picture how a vessel would collapse under vacuum, when it could stand 1 barg internal pressure easily. It's because the walls can buckle under compressive stress (vacuum case), but not under tensile (pressure case). For a typical pressure vessel, design for full vacuum leads to a shell thickness good for perhaps 8 - 10 barg internal. It's not unusual to add reinforcing rigs, internal or external, to increase vacuum capability at lower shell thickness.

14.7 psi ≈ 2119 psf. I think it then becomes more intuitive.
Also, in compression is like pushing a rope; it doesn't take much force to bend it.

OK now?