September 13, 2021
Pressure vessels seem static, at least from a distance. They stand tall and resolute, but they also remain idle. That's not exactly true, though. Pressure vessels swell as they endure hoop stress. They expand and contract slightly as fluid stresses change. Keeping them ductile, alloy elasticity and shape plasticity counteract the outward-facing energies. The metal bulges. Unfortunately, there's such a thing as a brittle fracture hazard.
What Is a Brittle Fracture?
With a modulus of elasticity in evidence, a durable alloy bulges until the fluid stresses are held in balance. What if that modulus somehow subsides? Sure, the metal is still hard. It's rigid and stiff, but it doesn't deform plastically. It doesn't bulge, nor does the metal rebound after the pressure is removed. Without energy-dampening material ductility, a stiff vessel fractures. The crack starts small, perhaps as a hairline fracture, but this weak spot can now leak kinetic energy, and fluid. Either the fractures will propagate as a network of fine cracks or it will split open, at which point a leak and/or a combustive reaction will take place. For that last scenario, the properties of the fluid determine the outcome.
Brittle Fracture and How to Avoid Them
Obey pressure vessel design specs, that's always an essential first response. Transient energies and chemical reactions also impair rigid sheep metal sections and weld integrity. Keep those reactions within manufacturer specs. Certain chemical reagents can also damage alloy microstructures, too. The aggressive compound makes the metal brittle, frangible, so advance knowledge of the stored chemical medium should always be included on the original design docket. Next, as the first line of defence, a stress management strategy is essential. Of course, microcrystalline fractures and/or unpredictable environmental conditions are, by their very nature, hard to divine. As one engineering-centric option, a reassessment of a vessel's sheet metal thickness should be implemented. Even if the sheet is thin and rigid, it'll deform slightly. By opting for thicker, more rigid sections, a hazard fracture becomes more likely.
In closing, we can say this: an "uncertainty factor" cannot be allowed to exist. Heat treatment facilities have to do their job. By this, we mean a suitable modulus of elasticity must be embedded in the various vessel workpieces. Furthermore, the source metal must be free of microcrystalline defects. Quality assurance codes and top-tier inspection procedures guarantee this particular proviso. Sheet metal thickness, pressure relief valves and fittings, and internal support mechanisms, all of these engineering mechanisms must also be in-situ. Finally, harsh environmental elements and aggressive chemical mediums must be known when designing a properly ductile pressure vessel.
Fusion - Weld Engineering Pty Ltd
ABN 98 068 987619
1865 Frankston Flinders Road,
Hastings, VIC 3915
Ph: (03) 5909 8218
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