During hydrostatic testing of pipelines, leaks in the test section will result in a loss of test pressure, and a reduction in water temperature during the test hold time will also typically result in a pressure decrease. This paper reports an analysis of the comparative effects of temperature, leakage, and trapped air on the pressure during hydrostatic testing.
Equations were developed to quantify the effects of temperature changes and leakage rates on the pressure during hydrostatic testing. Additional analysis was performed to investigate the effect of trapped air in the system on the test pressure. By comparing the relative magnitudes of these factors for a given pipeline geometry, insight can be gained into the minimum size of a leak that can be reasonably detected using hydrostatic testing.
The results of this analysis show that small leaks are most easily detected in short test sections with stable temperatures and when very little air remains trapped the piping. The magnitudes of pressure changes resulting from temperature variations and leaks depend on the starting temperatures and test pressures. The variations of the compressibility and the thermal expansion of the fluid are often neglected in test calculations, but can be important. The effect of temperature changes on the test pressure are minimized for cold temperature tests with water near 40°F, and increase as the test temperature increases. The presence of relatively small volumes of trapped gas can significantly affect the apparent compressibility of the test fluid. As a result, if a small leak is present, the pressure loss due to a leak may be less than expected. This effect becomes smaller at higher test pressures.
Matta, L., “Collective Effects of Leakage, Temperature Changes, and Entrapped Air during Hydrostatic Testing,” Proceedings of The 29th International Pipeline Pigging & Integrity Management Conference, Houston, TX, February 27 – March 2, 2017.