The short answer
A hydro test verifies the pressure boundary, typically at 1.25 to 1.5 times design pressure. It should confirm what your NDT already told you. If it is discovering defects, the inspection regime upstream has failed.
The hydro test is the moment the pressure boundary is proven. It is also, on a badly run job, the moment a fortnight of schedule disappears.
Before you fill
- All pressure-part welds complete, NDT cleared and PWHT recorded
- Temporary supports checked for the water weight — a large boiler full of water is a very different load case
- Vents at every high point, drains at every low point
- Safety valves gagged or blanked per procedure
- Water quality and temperature within specification to avoid brittle fracture risk
- At least two calibrated gauges with valid certificates
Pressurise slowly
Raise pressure in stages with a hold and a walk-down at each. Rushing to test pressure means a leak is found at maximum stored energy rather than at a third of it.
What failure actually looks like
| Symptom | Likely cause | Consequence |
|---|---|---|
| Weeping at a weld | Porosity or lack of fusion | Cut out, re-weld, re-NDT, re-test |
| Drip at a flange | Gasket or bolt torque | Usually recoverable same day |
| Pressure decay, no visible leak | Passing valve or trapped air | Re-vent and verify isolations first |
| Deformation | Support or design issue | Stop — engineering decision required |
After the test
Drain completely and dry out. Water left standing over a shutdown causes corrosion that gets blamed on erection quality later. Where the plant cannot commission immediately, agree a preservation regime in writing.
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