November 5, 2019
Although it's true that pressure vessels are put through the proverbial wringer when subjected to an inspection, potential fatigue points aren't quick to make themselves known. Of some concern, especially if the stress has only recently made itself known, vessel fatigue predicaments can elude an inspector's keen eye. To prevent one from flying under the radar, a change in approach is probably a good idea.
Understanding the Three-Step Fatigue Sequence
It's not that an inspection will miss stress-weakened areas, not when the inspection team has access to many scientific resources. Trained to recognize the smallest, most innocuous vessel Achilles heel, top-notch engineering professionals rarely miss anything. All the same, we need to understand the fatigue process so that we can anticipate and even intercept a flaw before it develops into something nastier. To begin with, where does the fatigue point begin? It starts at a wall or weld area that can't handle cyclical stress. With high fluid pressures and temperatures causing vessel expansion and contraction, tiny submicroscopic changes begin to appear deep within the alloy shell. The metal lattice weakens. Those weaknesses accumulate, and then a hairline crack occurs as a stress dumping mechanism. From here the crack propagates. If the cyclical stress continues unabated, well, the pressure containment vessel will likely rupture, perhaps explosively.
Identifying Containment Shell Fatigue Points
Viewed with a magnifying glass or an electronic instrument, the cracks become visible as the stress stretches the vessel walls. Liquid penetrant dyes help to reveal the cracks, too. Granted, the alloys used to fabricate pressure vessels possess augmented elasticity coefficients, plus plenty of tensile strength, but time and stress are patient antagonists. Material discolourations and patches of corrosion may signal the beginnings of a containment rupture. Material pitting is another issue. In summary, inspectors are trained to recognize many variants on a theme. With corrosion cracking and pitting, there will be microscopic cues, which reveal whether the damage is stress or chemically induced. With thermal expansion weakness, the same thing is true. With these signs and symptoms open to interpretation, it takes a skilled inspector to interpret an elusive surface irregularity on a fatigue-weakened pressure vessel.
In the three-step fatigue point process, the very best inspection teams can identify fatigue weaknesses straight away. Maybe an initial crack hasn't even appeared yet. The stress is cycling, and the expansion effect has been recognized early, but the weak area can't be confirmed until a radiographic or ultrasonic instrument scans the suspected fatigue area. Stage two detection is more common. A magnifying glass is placed close to a wall or weld point that's wet or corroded. Caused by thermal shock or chemical corrosion, the damage acts as a big red flag which points to a pressure vessel fracture, hopefully, one that hasn't begun to propagate yet.
Fusion - Weld Engineering Pty Ltd
ABN 98 068 987619
1865 Frankston Flinders Road,
Hastings, VIC 3915
Ph: (03) 5909 8218
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