April 5, 2019
The descriptive term provides an important clue, so we're ready to take a guess at the role this test technique plays. Being engineers, however, there's no room for guessing, so let's get the explanations down in black-and-white right now. Hydrostatic testing, as utilized on a pressure vessel, uses water as a seal inspection medium.
It is Technically a Non-Destructive Test Technique
Pressurized water is pumped inside the to-be-inspected vessel. That fluid shouldn't cause any damage to the equipment. However, the reason this check is being carried out in the first place is to see if there are any concealed sealing or material defects. If a weakness is exposed by a hydrostatic testing routine, it could occur explosively, after all. A seal rupture is one possible outcome, which is why such flaws should come to light under controlled conditions, not while a system is operational.
Running a Hydrostatic Test
Every keen cyclist knows a little about hydrostatic testing. If an inner tube gets a patch, the cyclist overinflates the tyre, then there's that dip into the water to see if there are any leaks. Industrial-class hydrostatic testing is a far more challenging procedure, but the principle stands. Water is pumped into a pressure vessel, all air is evacuated from that chamber, and now the wait begins. Held at 1.5 times above its design specs, the pressure pushes against the vessel walls and seams. If there's an existing leak, or one that's developing, a visual inspection will soon spot the breach. To improve the likelihood of detection, dyes and tracer colourants help the inspection team track the breach back to its origination point.
An Engineer's Approach to Hydrostatic Testing
Pressure vessels in the field are taken out of service. They're drained and cleaned of any leak-clogging detritus. Okay, now the tests can commence. But wait, there are three different options available. First off, there's the Water Jacket Method, with its added layer of testing media. For smaller tanks and pipeline components, the sealed vessel is filled with water, then it's placed inside a second sealed chamber. This test jacket reacts to small pressure changes. Next, the Direct Expansion Method mathematically differentiates the leakage flow by comparing the system pressurizing volume against the water expelled during the test. Last of all, we have the Proof Pressure Method. With a set liquid pressure established, this test looks for material deformations and/or weld discontinuities.
There's still more to do, especially in a pressure vessel that's expected to operate under adverse conditions. Pneumatic tests can follow, or there's a burst test, which, as the label implies, is applied if a non-destructive test procedure just doesn't yield conclusive results.
Fusion - Weld Engineering Pty Ltd
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