Site tests check the stability and leaktightness of a main before it goes into service.
A successful hydraulic test is a prerequisite for the acceptance of a newly installed pipeline. One of the objectives of that test is to check the leaktightness of the joints.
The test must be carried out as soon as possible after laying.
The instructions of the job’s Technical Specifications should be followed, or those of other valid regulations.
Test section length
The test section length depends on the layout configuration.
As generally recommended and unless otherwise stipulated by the job’s Technical Specifications, test lengths should not exceed 2,000 m.
Detection of leaks is more difficult the longer the test section. In practice short lengths can be tested at the beginning of a job and then extended to longer lengths.
Responsibility for this lies with the contractor.
Partial joint anchoring
In the event of partial joint anchoring using the pipes to provide their own support, the anchored areas are tested at the same time as the non- anchored areas.
Fully anchored section
When testing a fully anchored section, special arrangements must be made to allow pipe traction under the effect of the pressure applied. The ends of these sections must not be mounted on thrust blocks.
Testing a section length (non-anchored, partially or fully anchored) can generally be carried out after backfilling the trench, unless specified otherwise in the public works specifications or by the contractor.
In the case of tests before final backfill, saddles are made over the pipe barrel, leaving just the joints exposed.
Block the ends of the test section with blank flanges (A and B) equipped with valves for water filling and air venting.
Calculate the hydraulic forces developed at the ends of the main and install a suitably sized restraint system. The forces are absorbed by timbers laid across the trench, or by an equivalent arrangement (sheet piling for example).
Do not use the end of a previously laid hydraulically tested pipe section as a buttress.
The ends of a main under test may also move sideways, therefore lateral support must also be provided.
The main is gradually filled with water, preferably from the low points.
It is important to ensure that the main has been completely purged of air at any high spots in the section before it is pressurized.
During pressurization, the temporary end restraints become compressed. If necessary any play has to be taken up to restore the main to its initial length. Jacks are used for this purpose, allowing precise adjustment.
- If a pumping main is involved, pumps are used to fill from the bottom end, limiting the flow rate.
- In the case of a large diameter siphon it is preferable to fill it from the lowest point by means of a small diameter feed tube.
The water then rises steadily in both legs without turbulence.
Whenever possible, wait 24 hours before pressure testing, for the main to reach equilibrium.
Thorough removal of air from the main is essential. The vital importance of this has already been indicated.
- Check that the air valves are functioning,
- Check that the valves at the base of the air valves are open,
- Use the wash out valves to check the progressive arrival of water.
First ensure that the test pressure is compatible with the pressure each pipeline component in the test section can withstand. If not, isolate those items.
The pressure must be increased gradually, to allow checking of the end restraints and for jack adjustments. The pressure test must not only reveal any joint leakage but provide a positive check on the main for any damage caused during transport and laying.
The test pressure (STP) used for the section of pipe installed shall be equal to the section's maximum design pressure (MDP). The MDP corresponds to the static level under gravity flow, or dynamic flow when on a discharge line, plus the effects of temporary status changes.
The maximum amplitude for the temporary status changes is calculated taking into account the protection system installed, if any. The above test pressure is determined by the public works specifications, and is the result of calculations made by the contractor.
It is highly recommended that “the section shall be maintained at this pressure for 30 minutes, during which the pressure drop, measured using a suitable instrument, should not exceed 20 kPa”.
Commissioning the main
Empty the main, remove the test equipment and connect up the section.
Flush out the main thoroughly to remove any stones or soil trapped during laying.
In the case of a potable water main, sterilize it before it goes into service.