A hydrotest that fails due to trapped air or an unvented pocket wastes a full shift and risks damaging the test package. Proper vent placement and hold time discipline are as critical as the pressure calculation itself.
The mechanism is simple: air trapped at high points compresses under pressure, masking true test pressure and creating a dangerous energy store. When the test pressure is released, the compressed air expands rapidly, potentially blowing out gaskets or even rupturing the spool. This is why ASME B31.3 requires vents at all high points and a minimum 10-minute hold time after stabilization.
The Fabricator has published shop-floor articles stressing that hydrotest hold time, venting, and golden weld minimization are operational KPIs that matter as much as WPS qualification on paper. A well-planned hydrotest includes a vent schedule, a pressure-hold log, and a clear pass/fail criterion: zero pressure drop after stabilization, with no visible leaks.
Key decisions for the piping coordinator:
- Place vents at every high point in the spool, typically at flanged connections or using threaded plugs.
- Use a calibrated pressure gauge with a range no more than 4x the test pressure for accurate reading.
- Hold test pressure for at least 10 minutes per ASME B31.3; for critical services, industry practice extends this to 2 hours with zero pressure drop.
- Fill from the lowest point and vent continuously until a steady stream of water (no air bubbles) exits the highest vent.
- After test, drain completely and dry if the spool will not be immediately commissioned, to prevent MIC.
A common pitfall is relying on a single vent at the fill point. If the spool has multiple high points, each must be vented individually. Missing one can cause a false pass and a catastrophic failure during commissioning.
What is your standard hydrotest hold time for critical piping spools?