Hydrostatic and pneumatic pressure testing: procedure, test pressure and calculator
Deskely Completions team Singapore
A pressure test proves that a piping system or vessel is strong enough and does not leak before it is put into service. Most are hydrostatic tests, with water; where water cannot be used, a pneumatic test with air or nitrogen.
This guide covers the test pressures in ASME B31.3 and the other common codes, a calculator, what goes into a test pack, the procedure step by step, and the reinstatement that follows. Every project has its own test procedure, which takes precedence.
- ASME B31.3 hydrotest: at least 1.5 times the design pressure, times the stress ratio St/S (not more than 6.5), held for at least 10 minutes.
- ASME B31.3 pneumatic test: at least 1.1 times the design pressure, with a preliminary check and a stepped rise.
- Tests are run by test pack: the lines in one test, with their isometrics, weld records, punch status and test limits.
- Reinstatement after the test is where many punch items come from: blinds left in, instruments not refitted, wrong gaskets.
What a pressure test proves
A pressure test loads a closed system above its design pressure and holds it there while every joint is examined. It proves the strength of the pipe, fittings and welds, and that the joints do not leak. On a capital project, no line is put into service until it has passed.
Hydrostatic vs pneumatic testing
| Hydrostatic test | Pneumatic test | |
|---|---|---|
| Test medium | Water, sometimes with inhibitor, or another suitable liquid | Air, nitrogen or another non-flammable gas |
| ASME B31.3 test pressure | At least 1.5 x P x St/S | At least 1.1 x P |
| Stored energy | Low: water barely compresses | High: a failure releases far more energy |
| When used | The default for almost all piping | Where water cannot be tolerated: systems that cannot be dried, that cannot carry the weight of water, or where water would harm the process |
| Extra precautions | Venting, support for the water weight, freezing | Safety review, exclusion zone, stepped pressure rise, relief device |
Test pressure by code
| Code | Hydrostatic test pressure | Pneumatic test pressure |
|---|---|---|
| ASME B31.3, process piping | 1.5 x design pressure x St/S, with St/S not more than 6.5 | 1.1 x design pressure |
| ASME B31.1, power piping | 1.5 x design pressure | See the code |
| ASME Section VIII Division 1, vessels | 1.3 x MAWP x the lowest stress ratio | 1.1 x MAWP x the lowest stress ratio |
St is the allowable stress at the test temperature and S at the design temperature. For carbon steel lines designed for moderate temperatures the ratio is often 1, so the hydrotest pressure is simply 1.5 times the design pressure. B31.3 also requires the test pressure to be reduced if it would stress the pipe beyond yield at the test temperature.
Codes are revised, and project specifications often add their own rules. Always take the test pressure from the approved test pack.
Test pressure calculator
Choose hydrostatic or pneumatic, enter the design pressure and, for a hydrotest, the allowable stresses and the height of the system. The calculator gives the minimum B31.3 test pressure and the pressure at the low point due to the head of water.
- Minimum test pressure
- 30 bar g
- Stress ratio St / S used
- 1.00
- Static head of water
- 1.18 bar
- Pressure at the low point, if the high point is at the test pressure
- 31.18 bar g
ASME B31.3 hydrostatic test: at least 1.5 times the design pressure, times St / S (not more than 6.5), held for at least 10 minutes. Check that the test pressure does not overstress the pipe or any component, and use the stresses from the code tables for your material. Your project test procedure takes precedence.
Test packs built from the line list, tracked to reinstatement: Deskely Testpack
The test pack
Pressure tests are planned and run by test pack: the set of lines tested together, with everything needed to prove they are ready and to record the result. A typical test pack holds:
- The P&IDs and isometrics, marked up with the test limits, blinds, vents, drains and gauge points. See piping isometric symbols.
- The line list extract: design pressure and temperature, test medium and test pressure.
- The weld summary: every weld in the test, welded, NDT done and accepted, and any heat treatment.
- The punch list for the lines, with all items that must be cleared before the test closed. See the punch list template.
- Calibration certificates for the gauges and the relief valve used.
- The test record, the reinstatement checklist and the sign-off sheet.
Hydrostatic test procedure
- 01Walk down the lines
Check the system against the isometrics: supports, joints, welds complete and exposed (not painted or insulated), and the punch items that must be closed before the test.
- 02Isolate and protect
Fit blinds at the test limits. Remove or isolate instruments, control valves, relief valves and equipment that must not see the test pressure.
- 03Fit vents, drains and gauges
Vents at the high points, drains at the low points, calibrated gauges and a relief device to protect the system from overpressure.
- 04Fill and vent
Fill from the low point and vent all air from the high points. Trapped air makes the test unstable and dangerous.
- 05Pressurise in stages
Raise the pressure slowly, pausing to check for leaks, up to the test pressure.
- 06Hold and examine
Hold the test pressure for the required time, at least 10 minutes under B31.3 and often longer by procedure, and examine every joint for leaks.
- 07Depressurise and drain
Release the pressure slowly, open vents before draining so no vacuum forms, and drain and dry as the procedure requires.
- 08Reinstate and sign off
Remove temporary blinds and fittings, refit instruments and valves with the right gaskets, and record it all on the reinstatement checklist.
Pneumatic test procedure
A pneumatic test follows the same preparation, with more care because of the stored energy. Under B31.3 the pressure is first raised to a preliminary check at the lesser of half the test pressure or 170 kPa (25 psi), where joints are checked. It is then raised in steps to the test pressure, held, and reduced to the design pressure for the leak examination. An exclusion zone is kept for the whole test.
Safety
- Set and keep an exclusion zone, with barriers and signs, for the whole test.
- Never tighten or hammer a joint under pressure. Depressurise first.
- Protect the system with a calibrated relief device, and use calibrated gauges.
- Check the minimum metal temperature for the test, especially on carbon steel, to avoid brittle fracture.
- Treat pneumatic tests as high-risk work with their own risk assessment.
Reinstatement and records
Reinstatement puts the system back the way the P&IDs show it: every temporary blind out, every spade turned, every instrument and valve refitted with new gaskets. It is checked against a list, line by line, because a blind left in or a gauge connection left open is found at start-up at the worst possible moment.
The signed test record and the reinstatement checklist close the test pack and are the evidence for mechanical completion. See mechanical completion vs commissioning.
Deskely Testpack builds test packs from the line list and the drawings, records the test in the field, and tracks every spade, gasket and instrument back in before the pack is closed, with the status of each pack feeding the subsystem in Deskely Core.
The Deskely products for this
Frequently asked questions
What is the hydrotest pressure for ASME B31.3?
At least 1.5 times the design pressure, multiplied by the ratio of the allowable stress at test temperature to that at design temperature (St/S, not more than 6.5). For many carbon steel lines this is 1.5 times the design pressure.
How long is a hydrotest held?
ASME B31.3 requires the test pressure to be held for at least 10 minutes before the leak examination. Many project procedures ask for longer.
What is the pneumatic test pressure for B31.3?
At least 1.1 times the design pressure, with a preliminary check at the lesser of half the test pressure or 170 kPa (25 psi).
Why is a pneumatic test more dangerous than a hydrotest?
Gas compresses, so a pressurised gas system stores far more energy than the same system full of water. If it fails, that energy is released at once.
What is a test pack?
The set of documents for one pressure test: the marked-up P&IDs and isometrics, the line data, weld and NDT records, punch status, calibration certificates, the test record and the reinstatement checklist.
Should welds be painted or insulated before the hydrotest?
No. Joints are normally left exposed, unpainted and uninsulated, so leaks can be seen during the test.
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