Why water, and what the test shows
Water barely compresses, so it stores little energy under pressure. If a part fails during a hydro test, the water escapes with far less force than compressed air would. That is why water is the default and a pneumatic test is used only where the product standard or the order calls for it.
The test pressure comes from the product standard or the drawing. For steel valves, ASME B16.34 sets the shell test at 1.5 times the pressure rating at 38 °C, and API 598 and ISO 5208 set out how valve pressure tests are done. Pipe standards often require a hydro test on each length, or allow an electric NDT method instead. The test finds through-wall porosity, cracks and weak joints. It does not find small internal flaws that do not leak; that is the job of NDT.
What the inspector checks while witnessing
- Parts: the inspector picks them at random from finished stock, before painting if the order says so, because paint can hide a weeping casting.
- Gauge: a calibration certificate in date, and a range that suits the test pressure, so the reading is not at the very bottom or top of the dial.
- Venting: air is bled out before pressurising.
- Pressure and time: the gauge reaches the stated pressure and holds it for the full time without topping up.
- Result: no drops or wet patches on the outside, no visible distortion, and no pressure fall.
- Record: serial or heat numbers, pressure, hold time and photos of the gauge.
Worked example: a pump casing that weeps
An order for pump casings states a hydro test at 24 bar held for 10 minutes. The inspector selects eight casings. Seven hold 24 bar with dry outer faces. On the eighth, a damp patch appears near a boss after about six minutes, although the gauge drop is small. The casing is marked as a failure and set aside. The inspector records the heat number so the buyer can ask whether other casings from the same pour need testing.
How Clear To Load uses it
We do not run pressure tests or own test rigs. In a witness inspection the inspector watches the factory's own hydro test on parts the inspector selects and records the results. See industrial valves, pumps and steel pipes and tubes.