June 19, 2019
Potentially seen as a pressure vessel weak point, nozzle problems have been known to take precedence over other design features. As a matter of course, vessel geometries, wall thicknesses and weld seams account for the majority of this expert team's focus. All the same, from a safety standpoint, welded vessel nozzles are treated with equal respect, for they have been known to develop significant flaws.
Undermining Pressure Vessel Integrity
A thin-walled shell, with its walls made of some durable alloy, is built to withstand torturous energies. It'll shrug off the effects of high pressures and higher temperatures. Chemical attacks make no difference, for the welded seams and surfaces are designed to be chemically inert. However, the stresses produced by those forces can accumulate elsewhere. Ironically, it's the same situation with non-destructive tests; the NDT forces test the tank-like construct. It passes with flying colours, but the stresses accumulate somewhere else. At a welded nozzle, crack patterns begin to propagate.
Vessels Incorporate Multiple Nozzles
The porting points are there to facilitate several functions. The nozzles facilitate a process, aid flow rates, reduce vessel stress, provide fittings access or inspection points, or they perform as system interconnectors. Whatever their role, they're there, perhaps as a row of a dozen or more shell-penetrating apertures. This can be something of a problem. The curved geometry of the tank walls and end caps are fabricated so that they handle hoop stress and other outward-pressing forces. However, not all of these energies are countered. And there are transient or "hammering" forces to offset, too. Some of this excess energy is going to escape. Like iron filings to a magnetic pole, the stress accumulates around the nozzle openings.
Interpreting Nozzle Crack Patterns
Anchored by deep toed fillet welds, the aperture joints hold firm. Only, after using a dye penetrant, cracks are observed by a sharp-eyed welding inspector. There are cracks, as caused by chemical attacks, which are circumferential at the toe of the nozzle's weld. Metal fatigue also occurs around the weld zone. On another pressure vessel, nozzle cracking is caused by a penetration defect. A welder hasn't deposited enough filler, so radial crack patterns have spread between the nozzle wall and the root of the joint. As expected, loading problems also create very clear crack patterns, which resemble metallic stretch marks. If this issue is ongoing, the processing equipment is delivering too much pressure to the nozzles.
Welding experts and pressure vessel designers can work together. They cooperate when identifying weld defects on system nozzles. Oftentimes, a weld defect repeats, so the issue is clearly confined somewhere inside the fluid-containment process. That information is passed onto the system engineer. However, if the problem is a weld defect, that nozzle joint must be repaired with great haste.
Fusion - Weld Engineering Pty Ltd
ABN 98 068 987619
1865 Frankston Flinders Road,
Hastings, VIC 3915
Ph: (03) 5909 8218
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