By Jeffrey A. Groves, P.E., EDT Forensic Engineering & Consulting
You may be familiar with the consequences of applying too much air pressure to an item such as a balloon or a tire. Most people intuitively understand that there is only so much air that can be added to your car tires before they burst. Items that are designed to contain pressurized gases or liquids are often made from shapes that are considered strong – such as cylinders, spheres, or some combination thereof (think of a propane tank or the water pipes in your home).
However, have you ever considered what would happen if, instead of increasing the internal pressure of a pipe or 55-gallon drum, you decreased the internal pressure such that a vacuum (i.e., negative pressure) existed? Perhaps you have seen videos on YouTube of metal 55-gallon drums or – even more dramatic – railroad tank cars that are crushed in the blink of an eye after being steam cleaned. In this post, we will explore the catastrophic effect that a small amount of internal vacuum can have on objects that otherwise might be able to withstand hundreds or even thousands of pounds per square inch (psi) of internal pressure.
Source of Vacuum
A common cause of vacuum in equipment such as tanks, ducts, and pipes is steam. When water is boiled it expands in volume (i.e., steam) by approximately 1,700 times. Given steam displaces air, equipment containing steam tends to have little to no atmospheric air. If this steam is cooled – either intentionally or inadvertently – to an extent where it condenses back into a liquid, the change from a large volume of steam to a very small volume of water will produce a vacuum within the equipment.
Imagine someone sealed the openings on a vessel right after steam cleaning its internals. If the vessel was not designed to resist the effects of vacuum, the condensing steam would likely result in a catastrophic collapse (i.e., crushing) of the vessel. Such collapses have been known to happen in steam ducts in power and process plants when the effects of vacuum were not considered during the design phase.
Equipment that is intended to, or may experience vacuum is often provided with stiffening rings and/or bracing that resist the ejects of the vacuum. The American Society of Mechanical Engineers (ASME) publishes the Boiler and Pressure Vessel Code that provides design and construction criteria for the design of equipment under vacuum.
Pipes, tubes, and many pressure vessels are capable of containing very high internal pressures. However, the same vessel may experience collapse when exposed to a relatively small amount of vacuum. As such, designs for equipment and ducting that are intended to contain steam, or that may be steam cleaned, should undergo careful review given their susceptibility of collapse resulting from vacuum.
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