Pressure testing, especially hydrostatic pressure testing is used to determine and verify pipeline integrity of its welds and joints. In fact. pressure testing is assumed to be the best alternative to determine integrity of a pipeline in place of other inspection methods.
Limits of the pressure Tests
The pressure tests are carried out to induce a pre-determined stress level, by pressurizing the pipeline and equipment and observe their performance. Valuable information can be obtained through this verification process. However it is essential to identify the limits of the test process and obtainable results. There are several types of flaws in pipeline material that can be detected by hydrostatic testing. For new pipe - poor welds and longitudinal seam failures are the major ones discovered during hydrostatic tests. Pipe that has been in operations may have corrosion-related failures.
There are other flaws that cannot be detected by hydrostatic testing alone such as the sub-critical material flaws which cannot be detected by hydro testing, but the test has profound impact on the post-test behavior of these flaws. The active corrosion cells, localized hard spots that may cause failure in the presence of hydrogen etc. cannot be easily detected. In pipe material, there may exist some other flaws that cannot be detected by hydrostatic testing, these for example may be the sub-critical flaws.
Planned and predetermined TESTing
Given that the test will play a significant role in the non-destructive evaluation of pipeline, it is important to utilize the test pressure judiciously. The pressure testing involves predetermined and planned steps. That includes determining test objective(s), pressure calculations, pressure medium, test monitoring and recording and disposal of test medium in environmentally safe way.
The maximum test pressure should be so designed that it provides a sufficient gap between the test pressure and the maximum allowable operating pressure (MAOP). In other words, the maximum test pressure shall be greater than MAOP. Necessary calculations are carried out to establish the test pressure to meet the objective of the test.
The determination of test pressure presupposes that after the test, the surviving flaws in the pipeline shall not grow when the line is placed in service at the maintained operating pressure. The entire process of hydrostatic testing should be documented in a procedure that describes all aspects of the process from objective of the test, testing medium (usually water), venting and draining, test pressurization and depressurization, leak review, how documentation will be prepared, responsible person in charge, safety barriers, etc.
While plastic pipe does not have Stress Corrosion Cracking (SCC) issues, it too should subject to a well-run hydrostatic testing program as well. Procedures for plastic pipe are to be rigorous as for steel pipelines.