August 17, 2018
The general public doesn't have time for worrying about pressure equipment. And that's just fine. Anyway, all of the gear has been safely commissioned. They're safely storing high-pressure liquids and state-changing chemicals. High-temperature heat exchangers are at work, and they've also been meticulously inspected. However, although post-fabrication inspections are necessarily rigorous, they don't account for every possible in-service condition. To establish an operational baseline here, we need an in-service inspection.
According to theAS/NZS 3788in-service pressure vessel codes of practice, owners and inspectors must work together to establish a schedule for an in-service inspection program. The checks and balances here are established as a redundancy-checking mechanism. Granted, pressure vessels and their fittings will already have been exposed to demanding tests, but the equipment is now in the field, where it could cause catastrophic damage if it were to leak.
If the schedule were to be ignored, the vessel owners would be caught up in a chronically dangerous operational scenario. The tank might operate forever, but that's not a realistic way to view this weighty safety issue. After all, if the worst were to happen, large areas could be totally devastated. Perhaps a flaw was missed during the factory inspection. Even if the tank is flawless, the processing system it's part of might be exerting unusual transient stresses. Conceivably, the pressure vessel is being employed in a setting it was never designed to cope with, so its weld seams are buckling. Even a recent modification to the processing line could trigger an unforeseen event. Suddenly, there's a leak, and the sealed structure releases its stored energy explosively.
Is a tendency for creating worst-case scenarios considered worry-mongering? For the general public, that's perhaps true. For pressure vessel safety inspectors, however, it's their job to imagine disasters because of the caustic or explosive fluid loads stored under pressure in these constructs. Environmental factors and operational blunders are also possible, though unlikely. Unlikely or not, that in-service inspection uses non-destructive technology to make sure the fabricated construct is still working as designed. If a problem does seem likely, a follow-up with a destructive test model is possible.
Site inspectors use a forward-looking assessment approach. They create an in-service report that explicitly certifies the installed pressure vessel for continued operation. Of course, that forward-looking approach can also switch direction, which then leads the inspector back along the processing line so that unusual product demands can be noted. And these in-service forces have time on their sides. What passed inspection today could fail next time because of a new system modification. With that thought in mind, the inspectors must adhere to a set in-line inspection schedule.
Fusion - Weld Engineering Pty Ltd
ABN 98 068 987619
1865 Frankston Flinders Road,
Hastings, VIC 3915
Ph: (03) 5909 8218
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