Effects of Elevated Temperature in Pressure Vessels

June 23, 2016

Design engineers are tasked with multiple chores when pressure vessels are first conceived. The fluid contained within the sealed chamber is stored at some specified pressure, obviously, but there are other parameters to account for in the design. The chemical could be caustic, for example, capable of catastrophically reacting with a certain alloy, so material attributes have to be assessed. Similarly, temperature extremes occupy a crucial "maximum allowable" parameter as dictated by engineering specifications, but what happens if these carefully incorporated temperature boundaries are exceeded? Well, as with any causal loop, a damaged mechanical profile is the likely effect of this baking-hot cause.

Considering the Pressure-Temperature Relationship

Chemical facilities and heat exchangers employ highly energetic power sources to change the state of a fluid. Catalyzing agents and additives react to produce heat as a byproduct. Liquids flash and change form, transforming with some energy until the running liquid achieves its gaseous form. Now, in accordance with the laws of fluid dynamics, such energetic activities will cause particles to clash, which, in turn, causes a proportional rise in pressure. Weld seams and overall structural integrity must account for this relationship.

Thermal Fatigue Cracking

As cyclic changes occur due to expansion and contraction events, the design accommodates such mechanical fluctuations, but thermal fatigue is an entirely different matter. Pressure vessels are inherently rigid but a finite amount of elasticity is part of the structural makeup. Conversely, thermal stress works at a localized level, with the temperature gradient exposing different parts of the vessel surface to large temperature differentials. Microscopic flaws develop and become fine cracks, at which point the cracks propagate and vessel failure is possible.

Pressure Vessels Experience Cyclic Creep

Again, we're looking at a cycling action, the partnership of heat and pressure. In tandem, these two wearing forces cause the structural integrity of the vessel to deteriorate. The action, as the label suggests, is almost imperceptible. Long-term thermal exposure actually deforms the physical profile, thus mechanically crippling the fundamental strength of the container until its weakest spot, a weld seam or flange, gives way.

Elevated temperatures are a hazard, but they're usually unavoidable. Transient thermal spikes appear and vanish as quickly as they arrive, and we just design around such events. Special alloys offset thermal fatigue while allowable deformation properties account for creep. Alloys that incorporate nickel and chromium, for example, can deal with oxidation as well as all of the above thermal issues.

Contact Details

Fusion - Weld Engineering Pty Ltd
ABN 98 068 987619

1865 Frankston Flinders Road,
Hastings, VIC 3915

Ph: (03) 5909 8218

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