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Key takeaways
- B31.3 hydrostatic testing is normally at 1.5 times design pressure, corrected for temperature, and held for at least 10 minutes.
- Pneumatic testing is at 1.1 times design pressure and is used only when water can’t be tolerated, because compressed gas stores much more energy.
- Offshore, water quality, weight and drying often drive the test method and sequence.
Why pressure test?
A pressure test proves the strength of the system and finds leaks before hydrocarbons are introduced. It is the last check on materials, welding and assembly.
Hydrostatic test
In B31.3 the hydrostatic test pressure is at least 1.5 times the design pressure, multiplied by the ratio of the allowable stress at test temperature to the allowable stress at design temperature (for hot lines this ratio raises the test pressure). The test pressure must not cause yielding, so for some systems it is limited. The pressure is held for at least 10 minutes and then the joints are examined.
Water is used because it is nearly incompressible. If the pipe fails, very little stored energy is released.
Practical points:
- Vent all high points so no air remains.
- Check that supports and spring hangers are designed (or locked) for the water weight. Large gas lines may need temporary supports.
- Use clean water. For austenitic stainless steel, limit chlorides (projects often specify around 50 ppm or less) to avoid chloride pitting and cracking.
- Drain and dry systems promptly, especially gas and instrument air lines.
Pneumatic test
When water would damage the system, such as some gas and refrigerant systems or lines that can’t be dried properly, B31.3 allows a pneumatic test at 1.1 times the design pressure.
Compressed gas stores a lot of energy, so a failure can be violent. B31.3 requires a preliminary check at a low pressure (not more than the lesser of half the test pressure or 170 kPa), then raising the pressure in steps. Projects add safety measures such as:
- calculating the stored energy and setting an exclusion zone,
- extra NDE of welds before the test,
- checking the material’s brittle fracture risk at the test temperature.
Other options
- Hydropneumatic test: liquid with a gas cushion, treated like a pneumatic test.
- Initial service leak test: allowed for Category D fluid service at the owner’s option.
- Sensitive leak test: extra testing for Category M and for some gas systems, using helium or bubble testing.
| Method | Test pressure (B31.3) | Main risk |
|---|---|---|
| Hydrostatic | 1.5 × design pressure (temperature corrected) | Weight, water quality, drying |
| Pneumatic | 1.1 × design pressure | Stored energy |
Onshore
Onshore, test water is easy to get and dispose of, and test packs follow construction sequences around the plant.
Offshore
Offshore, fresh water is limited, the weight of water may overload module steel, and seawater can’t be used for stainless or duplex. Tests are often done in the module yard before sail-away, with hook-up joints tested later.
Code reference
ASME B31.3, para. 345.4 Hydrostatic leak test.
ASME B31.3, para. 345.5 Pneumatic leak test.
NORSOK L-004 Piping fabrication, installation, flushing and testing.
What to take back to your desk
When you prepare a test pack, check two numbers on the isometric: the design pressure and the test pressure. If the test pressure isn’t 1.5 times design (or explained), find out why before the test.
Educational content. Always design to your project specification.
