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PRCI NG-18 Report 111
- Study of the Causes of Failure of Defects that have Survived a Prior Hydrostatic Test
- Report / Survey by Pipeline Research Council International, 11/01/1980
- Publisher: PRCI
$198.00$395.00
L51398e
Battelle Memorial Institute
Need: The observance in service or during hydrostatic testing of an occasional pressure reversal has raised a question as to what causes these reversals and what is their magnitude. A pressure reversal occurs when a defect survives a given pressure level only to fail at a lower pressure level upon subsequent pressurization from 0 pressure.
Benefit: The objective of this study is to investigate the causes of pressure reversals and their magnitude. In addition, the probability of the occurrence of a pressure reversal was investigated along with the identification of the factors in hydrostatic testing that tend to eliminate or reduce the occurrence of pressure reversals.The causes and magnitudes of pressure reversals were studied in this investigation using a number of full-scale experiments on line pipe. The experiments involved artificially induced defects that were purposely subjected to several cycles of near failure load and various hold times. Also, the pressure testing experience on numerous pipelines and the documented associated pressure reversals have been factored into this investigation. The major factors defined as contributing to pressure reversals in line pipe of normal toughness are the subcritical (stable) crack growth during prior loading, the proximity of the defect to failure, and compressive yielding at the flaw tip during unloading. The maximumpressure reversal observed in normal toughness line pipe in the experiments was a 25.2 percent reduction from the previous pressure level as a leak. The maximum recorded service pressure reversal of 62 percent was for a rupture that occurred during testing of old low toughness line pipe.
Result: Research results indicated that the three following factors should be controlled when conducting hydrostatic tests. The first is that the margin between test pressure and operating pressure should exceed 10 percent. Secondly, the hold time at maximum pressure should be minimized since it causes remaining subcritical defects to grow. While long hold periods are required for a leak check, this can be performed at a lower pressure than the maximum test pressure such as approximately 90 percent of the maximum test pressure. The third factor is that repeated cycles of proof testing are detrimental since they cause more flaw growth than is caused by holding at constant pressure levels. This increases the probability of a pressure reversal occurring. In the final analysis, hydrostatic testing has proven to be a very useful and indispensable tool to the pipeline industry.
Battelle Memorial Institute
Need: The observance in service or during hydrostatic testing of an occasional pressure reversal has raised a question as to what causes these reversals and what is their magnitude. A pressure reversal occurs when a defect survives a given pressure level only to fail at a lower pressure level upon subsequent pressurization from 0 pressure.
Benefit: The objective of this study is to investigate the causes of pressure reversals and their magnitude. In addition, the probability of the occurrence of a pressure reversal was investigated along with the identification of the factors in hydrostatic testing that tend to eliminate or reduce the occurrence of pressure reversals.The causes and magnitudes of pressure reversals were studied in this investigation using a number of full-scale experiments on line pipe. The experiments involved artificially induced defects that were purposely subjected to several cycles of near failure load and various hold times. Also, the pressure testing experience on numerous pipelines and the documented associated pressure reversals have been factored into this investigation. The major factors defined as contributing to pressure reversals in line pipe of normal toughness are the subcritical (stable) crack growth during prior loading, the proximity of the defect to failure, and compressive yielding at the flaw tip during unloading. The maximumpressure reversal observed in normal toughness line pipe in the experiments was a 25.2 percent reduction from the previous pressure level as a leak. The maximum recorded service pressure reversal of 62 percent was for a rupture that occurred during testing of old low toughness line pipe.
Result: Research results indicated that the three following factors should be controlled when conducting hydrostatic tests. The first is that the margin between test pressure and operating pressure should exceed 10 percent. Secondly, the hold time at maximum pressure should be minimized since it causes remaining subcritical defects to grow. While long hold periods are required for a leak check, this can be performed at a lower pressure than the maximum test pressure such as approximately 90 percent of the maximum test pressure. The third factor is that repeated cycles of proof testing are detrimental since they cause more flaw growth than is caused by holding at constant pressure levels. This increases the probability of a pressure reversal occurring. In the final analysis, hydrostatic testing has proven to be a very useful and indispensable tool to the pipeline industry.
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