Potential Stresses on Pressure Vessel Walls to Consider During the Design Process

October 21, 2019

Foreign terms are used when describing pressure vessel wall stresses. Well, let's say that they're foreign to the average engineer but very familiar to every fluid containment specialist. Hoop stress, fluid state change stress, temperature transient expansion and contraction stress, all of these metal-fatiguing pressures push outwards. Of course, since these containment units are designed from highly durable alloys, they're intended to be thin-walled vessels. And therein lies the problem.

Dynamically Specced Pressure Vessel Alloys

Here's a misconception that can be dismissed once and for all. Pressure vessel walls are not static. They don't remain rigid and fixed. In point of fact, the walls, because they're thin, swell and contract as conditions change inside the container. Most forces push externally outwards, including pressure and temperature highs. The metal therefore expands. There are exceptions to this rule. For example, when the pressure inverts, a vacuum is created and the walls pull inwards to compensate for the effect. Low-temperature extremes also cause vessel walls to contract. To create products that can defeat these dynamic loading factors, engineers source alloys that have a predetermined modulus of elasticity. High in tensile strength, thanks to a special heat treatment process, the alloys are still slightly malleable. In this way, potential fluid stressors are neutralized.

Considering and Addressing Design Stressors

Is this an active chemical environment or a storage vessel? If a fluid is stored, it's not going to undergo too many energy states. The outside temperature and the pressure at which the fluid is held at are the two most wall-influencing factors to consider here. But what if the containment unit stores a heated liquid? A gaseous phase gathers above the liquid, the substances change state. A catalyst is added, and the chemical reagents change state again. Such energy-heavy stressors must be accounted for during a pressure vessel's design phase so that extra performance overhead is provided during the system's normal operations. Finally, hoop and longitudinal stress factors are common. Unless the fabricated walls are completely curved, which is possible if the vessel is a sphere, then additional internal energies will press against the cylindrical outlines of the tank. The longer the vessel, the more worrisome these forces become, especially at the centreline, where the walls are furthest from the vessel end caps.

Fortunately, there are a number of engineering equations that exist solely to handle longitudinal and hoop stress. The thickness of the metal is plugged into those formulas, as is the length of the cylinder and its diameter. If those stresses do threaten to cancel out a blueprinted pressure vessels' energy-containing overhead, then internally installed ribs and stiffeners can be added to the design to offset the shell strain.

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