Hydrostatic Testing

What a pressure test cannot prove

An honest account of the limits is more useful than a confident account of the strengths, because the limits are where operators get surprised.

It proves a margin at a moment

A passed test establishes that, on the day, at the temperature and pressure applied, the segment carried the load without failing and without losing measurable inventory.

It says nothing about tomorrow. Corrosion continues, cycles accumulate, and third-party damage is not predicted by anything that happened during the hold.

A short hold can miss a very slow leak

Detection is a function of leak rate, hold duration and instrument resolution. A leak small enough to fall below the resolution of the instrumentation, or slow enough that its signal over the hold window is smaller than the thermal noise, will pass.

This is not a defect in the method; it is the arithmetic of measurement. It is also why the leak hold is long and why resolution is specified in advance rather than discovered afterwards.

Pressure reversals are possible on bleed-down

A defect that survives the hold can, in some circumstances, be worsened by the test itself and fail at a lower pressure later. The mechanism is associated with the depressurisation part of the cycle.

It is uncommon, it is well documented, and it is a reason to treat a passed test as evidence rather than as a guarantee.

It does not locate anything

A pressure test is a pass/fail on a whole segment. It does not tell you where a defect is, how big it is, or whether the one that failed was the only one.

That is what in-line inspection is for, and the two methods answer different questions. Operators who treat them as alternatives are usually about to be disappointed by one of them.

Why the honest version sells better

Chapter 1 of the field book closes with this material deliberately. An operator who understands what the test does not cover writes better acceptance criteria, asks better questions of a contractor, and argues less at the end.

The people who get surprised are the ones who were sold certainty.

Standing noteThis is educational reference material, not engineering advice. Regulations are revised continually; the editions in force at the time of a project govern, and every project requires its own qualified engineering analysis and the operator’s governing specifications.

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From Proper Pigging, the hydrostatic and nitrogen test field book, and the HydroTech certification courses built on it.