Pipeline pressure testing: what a hydrotest actually checks, and what makes one fail

Before a new or repaired pipeline carries product, it has to prove two things: that it is strong enough for the pressure it will see, and that it does not leak. The field pressure test is where that is demonstrated, witnessed and recorded. It is also a point where projects lose days, so it is worth understanding what the test is actually checking.

Which standard governs the test

The method and the acceptance criteria come from whichever standard the pipeline was designed to, so the first question is always which one applies.

• AS 2885.5 covers field pressure testing of gas and liquid petroleum pipelines designed under the AS 2885 series. It sets the strength and leak test requirements, the hold periods and the records.
• AS/NZS 2566.2 covers installation of buried flexible pipelines (polyethylene, PVC and similar, typically water, sewer and slurry) and includes the field test procedures, which account for the pipe material stretching under pressure.
• AS 4041 covers pressure piping within plant, with its own hydrostatic and pneumatic test provisions.

Mixing these up is a real problem. A test that is acceptable under one standard can be rejected under another, and the test procedure should name the standard, the design pressure and the test pressure derived from it before anyone opens a valve.

Hydrostatic or pneumatic

Water is the default test medium. It is nearly incompressible, so the stored energy at test pressure is low and a failure is a leak rather than an explosion. Pneumatic testing with air or nitrogen stores vastly more energy at the same pressure and volume, and is only used where water cannot be tolerated in the line or cannot be removed afterwards. It needs exclusion zones, permits and a specific risk assessment, and the standard will set a lower limit on how much pressure and volume can be tested that way.

The sequence

1. Test plan. Written before the test and reviewed by the RPEQ responsible: the standard, the section limits, the test pressure, hold durations, the medium, instrument requirements and the acceptance criteria. Anything the test will pressurise that is not rated for the test pressure (instruments, relief valves, equipment nozzles) is identified and isolated or removed.
2. Pre-test inspection. Welding complete and non-destructive testing signed off, supports in place, blinds and test ends installed and rated, valves in the correct position, gauges and recorders calibrated with certificates in hand.
3. Fill and vent. Fill from the low point, vent from the high points, and keep venting until the air is out. Trapped air is the most common cause of a test that will not stabilise.
4. Stabilise. Let the water and pipe reach a steady temperature. On a buried line this can take hours; on an above-ground line in the sun it may never happen during the day, which is why many tests run overnight.
5. Pressurise in stages with a hold at each stage to check for gross leaks and to let the system settle.
6. Strength test. A hold at the strength test pressure, above the maximum allowable operating pressure by the margin the standard requires, for the required period.
7. Leak test. A longer hold, often at a reduced pressure, with pressure and temperature recorded throughout. This is where the judgement is: a pressure drop with a temperature drop is physics, a pressure drop at constant temperature is a leak.
8. Evaluate. Compare the corrected pressure change, or for flexible pipes the make-up water volume, against the allowance in the standard.
9. Depressurise, dewater and dry. Controlled release, disposal of test water in line with the environmental approval, and drying to the specification if the product cannot tolerate moisture.
10. Records. The chart or logged data, calibration certificates, the signed test certificate, and the test recorded on the as-built drawings.

Why tests get rejected

Temperature misread as a leak. Water expands and contracts with temperature and so does the pipe. Without a temperature record alongside the pressure record, the evaluation cannot separate the two, and a test on a hot afternoon will look like a leak by evening. Record both, and correct for it.

Trapped air. Air compresses, so the pressure creeps as it dissolves and the test will not hold. The fix is patience during venting, not more pumping.

Testing against the wrong boundary. A closed valve is not a test end unless it is rated for the test pressure in that direction and its seat is proven tight. Blind it.

Instruments out of calibration. A gauge without a current certificate invalidates the test regardless of what it read.

Incomplete records. A test that was done but cannot be evidenced was, as far as the regulator and the client are concerned, not done.

Safety during the test

Even a hydrotest can hurt people. Test ends, temporary blinds and hoses fail, and a jet of water at pipeline pressure is dangerous. Exclusion zones around the section under test, a competent person in charge, no work on the line while it is pressurised and a controlled depressurisation are minimum requirements. For pneumatic tests the exclusion distances grow with the stored energy and the whole test is treated as a high-risk activity.

What the engineer's role is

Writing or reviewing the test procedure, confirming the test pressure against the design, witnessing the test, evaluating the pressure and temperature records, and signing the certificate. That certificate is the document that lets the line go into service, and it is a professional engineering service under Queensland law.