Hydrostatic Testing

The strength, leak and spike phases, and what each one proves

A pressure test does two distinct things: it proves the pipe can carry the pressure, and it proves the pipe does not leak. The regulations treat them as separate phases because they answer separate questions.

Strength

The strength phase applies the highest sustained pressure the pipe will see. Its job is to establish a margin above operating pressure and to force any defect large enough to threaten that margin into failing now, under controlled conditions, with a crew present and the line full of water rather than product.

For gas transmission, 49 CFR 192.505 requires the strength test to be held at least eight hours where operating stress is 30 percent SMYS or more, at the applicable class-location test factor.

Leak

The leak phase runs lower and longer. It is looking for loss of inventory, not loss of strength, and inventory leaves slowly. Small leaks produce pressure decay that is easily masked by a falling water temperature, which is why the analysis matters as much as the hold.

This is the phase where an unexplained residual has to be defended, and where a test either gets accepted or gets argued about.

Spike

The spike phase is a short excursion to a pressure above the strength test, held for a brief period at the start of the test. 49 CFR 192.506 added the spike hydrostatic pressure test to the gas transmission rules in 2019.

The logic is time-dependent: pressure-cycle-sensitive defects such as stress corrosion cracking and certain seam anomalies may survive a conventional strength hold and then grow in service. A brief, higher excursion is intended to fail those features during the test rather than during operation.

Order mattersThe spike is applied first, before the strength and leak holds. Applying it after a long hold does not achieve the same thing, and the sequence should be explicit in the procedure.

Why they are not combined

It is tempting to ask why the line is not simply held at the highest pressure for the longest time. Two reasons.

First, sustained high pressure is not free. It consumes the fatigue life you are trying to protect, and on a line already near its class limits it can do harm.

Second, the phases are diagnostic. A failure during the spike, a failure during the strength hold and an unexplained decay during the leak hold tell you three different things about the pipe. Collapse them into one hold and you lose the ability to say which happened.

The pressure ladder

In practice the phases are planned as a ladder: a sequence of target pressures, hold times and acceptance criteria worked out for the specific segment, its elevation profile and its class location.

Building that ladder is engineering work done well before the water arrives. It is the subject of a full day on our engineering workshop, and of Chapters 1, 6 and 11 of the field book.

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.