Are there any special factors that affect the testing procedures when commissioning PE pipe?
Yes. It is important to follow the correct commissioning procedures in order to avoid false conclusions on the performance of the pipeline.
Factors that affect the testing of PE pipe during commissioning procedures are: temperature variations; the amount of trapped air in the pipeline and the creep characteristic of PE pipe. Failure to take these into account can lead to false test results.
Due to the relatively high co-efficient of thermal expansion of PE pipe it is essential that variations in temperature be minimised during the commissioning procedure. It is recommended, both for safety and to minimise temperature variation, that the trench is backfilled prior to testing. It may be allowable to leave critical joints open to allow for inspection during commissioning.
When carrying out hydrostatic testing it is essential that all air is removed from the pipeline prior to testing. The pipe can be filled using either pigging or gravity fed techniques. If gravity filling is to be used it may be necessary to install tapping's at high points to vent trapped air and at low points to enable all the water to be removed.
As the test pressure is applied to a PE pipeline, the pipe will expand due to the creep characteristics of the material. This will result in a drop of pressure or require the system to be ‘topped up' to maintain the required pressure. The test procedure for PE pipe must include a period of time to allow the pipe to stabilise or should include a method whereby the pressure drop due to pipe expansion is calculated to discriminate from the pressure drop from leakage. If this is not done false test results will be obtained because it will not be possible to determine whether any loss of pressure is due to the expansion of the pipe or to real leakage.
This expansion of the pipe when load is applied is normal behaviour for a plastic material and is not an indication of failure.