Hydrostatic testing of steel pipe and pipeline

A hydrostatic test is a way in which pressure vessels such as pipelines, plumbing, gas cylinders, boilers and fuel tanks can be tested for strength and leaks. The test involves filling the vessel or pipe system with a liquid, usually water, which may be dyed to aid in visual leak detection, and pressurization of the vessel to the specified test pressure.

For hydrostatic testing purposes, there are two: one is the process of hydraulic test to test whether the leaking pipe, that test tube sealing performance; 

Another is confirmatory hydrostatic test to test the strength of steel adequacy. In general, low and medium pressure boiler pipe hydrostatic test is part of the process of the hydrostatic test, which is the density of material testing and inspect and whether the material is a continuous dense; it is not verify the strength tests. From the intensity of mechanics of materials theory, seamless steel components are thin and long, and its diameter is very small, even for thin-walled capillary tubes can withstand great pressure. That is to say, when low and medium pressure boiler tubes test pressure, even when there are deep cracks, can not happen leaking.

Hydrostatic test is that many product specification test methods are available. This test can detect leakage of liquid from the inner wall of the case outward can be observed with the naked eye or with a pressure drop to determine. Hydrostatic test can not be found through-wall defects but very close or very close to the wall thickness in depth but not yet fully penetrated defects. Conducted for each pipe eddy current inspection after the low and medium pressure boiler pipe hydrostatic testing is not necessary to replace the hydrostatic test eddy current testing.

Hydrostatic testing of Steel pipe is as follows:

1. test section at the end of block, slow water injection, the tube gas discharge.
2. after the steel tube filled with water, water tightness checks.
3. made slow progress in steel pressure appropriate USES manual pump, booster time should not less than 10 min.
4. stop pressure after test pressure, observe the 10 min, must not exceed 0.02 MPa pressure drop; Then put the testing pressure drop to the working pressure, steel pipe for visual inspection, leak-proof is not qualified.
5. after arrival, steel pipe pressure, leakage and pressure drop more than a specified value, check the steel tube, after the reason to discharge the leakage, as stipulated in the above to test again, until it meets the requirements.
6. the temperature is lower than 5 ℃ under the environment of water pressure and water capacity, reliable antifreezing measures should be taken, after the test, water should be put.

Hydrotesting of pipes, pipelines and vessels is performed to expose defective materials that have missed prior detection, ensure that any remaining defects are insignificant enough to allow operation at design pressures, expose possible leaks and serve as a final validation of the integrity of the constructed system. ASME B31.3 requires this testing to ensure tightness and strength.

Buried high pressure oil and gas pipelines are tested for strength by pressurizing them to at least 125% of their maximum allowable working pressure (MAWP) at any point along their length. Since many long distance transmission pipelines are designed to have a steel hoop stress of 80% of specified minimum yield strength (SMYS) at MAOP, this means that the steel is stressed to SMYS and above during the testing, and test sections must be selected to ensure that excessive plastic deformation does not occur.

Test pressures need not exceed a value that would produce a stress higher than yield stress at test temperature. ASME B31.3 section 345.4.2 (c)

Other codes require a more onerous approach. BS PD 8010-2 requires testing to 150% of the design pressure - which should not be less than the MAOP plus surge and other incidental effects that will occur during normal operation.

Leak testing is performed by balancing changes in the measured pressure in the test section against the theoretical pressure changes calculated from changes in the measured temperature of the test section.

Australian standard AS2885.5 "Pipelines—Gas and liquid petroleum: Part 5: Field pressure testing" gives an excellent explanation of the factors involved.