# How can a railcar withstand high pressure but fail under a vacuum? [duplicate]

A 25,000 gallon (95,000 litre) bulk chemical storage railcar can store products with vapor pressures in excess of 200 psia (1.38 Mpa). The same railcar can not withstand a vacuum when being unloaded. I want to understand why.

A bulk chemical storage car in this example (assume refrigerant on a warm day) is unique in it's design in the sense that it is a pressure vessel contains well over 200 psig (1.38 MPa) internal pressure whereas many metal tanks are rated for far lower pressures thus the pressure differential between atmosphere and the inside of the metal tank may be typically be close to the difference found in this example, 14.7 psi (101 kPa) or less.

The definition of a metal tanks can be open to interpretation not only with regard to pressure ratings but also wall thicknesses. One of the answers posed refers to plastic or aluminum soda containers. The material properties are far different than those typically found in the types of rail cars I have presented here.

• Related: Imploding train car on Mythbusters. – Qmechanic Jan 14 '17 at 5:31
• I fixed the units but I'm not sure if psig and psia are typos or different units than psi. I'm also not sure if you mean US gallon or UK gallon, but that's not very relevant for the question. For the conversion I assumed US gallons. – gerrit Jan 14 '17 at 22:20
• Vapor pressure is psia and pressure rating of vessel or railcar typically given in psig. The differential of course is neither gauge or absolute. – ARinLA Jan 14 '17 at 22:36

A pressure vessel consists mainly of a thin metal plate. If you experiment with a thin sheet of material (for example a piece of paper) you will find that it is very easy to bend it, but much harder to stretch it.

If there is internal pressure in a spherical or cylindrical vessel, the only way the pressure can do mechanical work (i.e. force $\times$ distance) is to increase the internal volume of the vessel, and the only way to do that is to stretch the material, which is difficult.

However, external pressure can do work by reducing the internal volume, and that is easy to do without stretching the vessel wall, by breaking the original cylindrical or spherical symmetry of the vessel and "crumpling" the wall. Since the structure will never be a perfectly uniform shape, there will always be a "weak point" somewhere from which the crumpling can start. This page http://publish.ucc.ie/boolean/2010/00/dePaor/11/en has some nice pictures showing what happens.

The basic principle works exactly the same way with simpler geometry, for example Euler buckling of a column. If you apply a tension load to a column, the only possibility is to stretch the material and make the column longer. But if you apply a compressive load, you can make the column shorter without changing the length of the material, when it buckles into a curved shape.

• Or replace the column with a length of string, and suddenly nobody is surprised that it can bear more tension than it can compression, because we're used to that ;-) – Steve Jessop Jan 14 '17 at 20:04
• Is this the same phenomenon that makes spokes placed under countervailing tension a good design? – bright-star Jan 15 '17 at 2:32
• @bright-star Different, but yes, very similar principle. A spoke will bend pretty easily under compression, but it's strong under tension. So you pre-tension the spokes of a wheel enough that when the wheel is loaded, all of them are still under tension to some degree, which makes the wheel stronger and the spokes wear out less quickly because they flex less. – hobbs Jan 15 '17 at 7:56

Expanding pressures have a very different effect than crushing pressures. Expanding pressure tends to be stable. If there is a defect in the shape of the railcar, the pressures tend to push it outward into the proper shape. However crushing pressures are unstable. If there is a defect in the rail car in your vacuum example, it tends to pull the defect further away from the correct shape. This makes the defect worse, and starts causing a runaway effect.

We can see this effect on a human scale with a 2 liter bottle of soda. It can obviously withstand a great deal of pressure from the inside as the soda is under pressure when delieverd. If you empty the bottle and try to blow it up with your own breath, you'll quickly find that the bottle is way stronger than you are in this direction. However, if you evacuate the bottle by sucking air out of it, it takes almost no effort at all.

• Soda containers are typically at about 3 1/2 atmospheres (50 psi) before being opened. – Jasper Jan 14 '17 at 6:56
• For an even more dramatic example, consider something like a mylar foil balloon. Inflated, such a balloon can resist a significant pressure without stretching or popping, but without the internal pressure to support it, it cannot even hold up its own weight. – Ilmari Karonen Jan 14 '17 at 7:23

The sealing of a port in a pressure vessel might use the pressure in the vessel to compress a seal (push an O-ring firmly against a door); such a seal will not be properly compressed if the vessel is under vacuum.

Many pressure vessels, with thin walls, use pressure-induced tension to hold the shape (like a balloon); under negative pressure, like a balloon, that kind of vessel will collapse.

The railcar has additional failure modes under compression - instability, or buckling (https://en.wikipedia.org/wiki/Buckling)

• I'd suggest adding a bit more on the subject, most notably the fact that in most pressurized vessels are constructed so that positive pressure will tend to push any localized defects in the vessel back to their proper shape, while negative pressure will push them further away from their proper shape. – supercat Jan 14 '17 at 22:26