Pressure tests are used by pipeline operators as a means to determine the integrity of the pipeline immediately after construction and before placing the pipeline in service, as well as during a pipeline’s operating life. The post-construction pressure test verifies the adequacy of the pipeline materials and construction methods.
Integrity Management regulations require pipeline operators to periodically conduct integrity assessments of certain pipelines. Pressure testing is one acceptable method of performing these assessments. An integrity assessment pressure test is intended to determine whether a hazardous liquid or gas pipeline has adequate strength — integrity — to prevent leaks or ruptures under normal operation and upset conditions.
In a pressure test, a test medium (gas or liquid) inside the pipeline is pressurized by the use of pumps or compressors to a pressure that is greater than the normal operating pressure of the pipeline. This test pressure is held for a number of hours to ensure there are no leaks in the pipeline. Any indication of leakage requires the identification and repair of the leak. The pipeline is then re-pressurized and the test is repeated. The operational integrity of field welds and of the pipe itself is assured if the pressure test is successfully completed.
Hydrostatic (non-flowing water) Testing:
Hydrostatic pressure testing is generally used for the post-construction testing of hazardous liquid pipelines and higher stress (> 30% SMYS) natural gas pipelines.
Hydrostatic testing is also widely used to periodically assess the integrity of hazardous liquid and gas transmission pipelines (particularly when the use of inline inspection tools is not feasible). The hydrocarbon products are displaced from the section or sections being tested and replaced with water in order to minimize environmental damage that might result from leaks or ruptures. For a variety of reasons, including service interruptions and other system impact factors, hydrostatic testing may not be feasible for all pipelines. When neither inline inspection tools nor hydrostatic testing are possible, operators typically conduct integrity assessments using a process called “direct assessment”.
If a pipeline successfully passes a hydrostatic pressure test, it can be assumed that no hazardous defects are present in the tested pipe. This is especially important when dealing with pipe sections that were manufactured prior to 1970 using low-frequency electric resistance welding (LFERW) and lap welding (LW) of the longitudinal seam. Experience has shown that, in some instances, some of the factory-welded seams in these types of pipe can be susceptible to failure.
Hydrostatic testing requirements and restrictions for hazardous liquid pipelines are specified in 49 CFR 195, Subpart E; requirements and restrictions for natural gas pipelines are specified in 49 CFR 192, Subpart J.
Hydrostatic testing of hazardous liquid pipelines requires testing to at least 125% of the maximum operating pressure (MOP), for at least 4 continuous hours, and an additional 4 hours at a pressure of at least 110% of MOP if the piping is not visible. If there is concern with latent cracks that might grow due to a phenomenon known as "pressure reversals", then a “spike” test at the maximum pressure of 139% of MOP for a short period (~1/2 hour) may be conducted. The spike test will serve to “clear” any cracks that might otherwise grow during pressure reductions after the hydrostatic test or as a result of operational pressure cycles. Studies have been performed that demonstrate the acceptability of the pipeline for extended service after a hydrostatic pressure test, if there are no factors present that would accelerate crack growth such as corrosion or aggressive pressure cycles.
Pneumatic Pressure Testing:
Pneumatic pressure testing of pipelines refers to the use of air or inert gas (such as nitrogen) as a test medium instead of water. Pipeline operators rarely use pneumatic pressure testing for systems that operate at pressures above 100 psig due to safety concerns. The amount of energy stored in a compressed gas greatly exceeds the energy stored in a compressed liquid. If a failure of the pipeline were to occur during a high-pressure pneumatic pressure test, the sudden release of this large amount of energy could be dangerous to personnel conducting the tests.
Hydrostatic Testing: Where can I learn more?
Additional information on hydrostatic testing and LFERW piping can be found in the following references:
Date of Revision: 12012011