Articles

Study questions specified hydrotest hold times value - Oil and Gas Journal article

21/02/2018

    Originally published on March 2012

    The value of a particular hold time during a hydrotest can- not be determined in terms of pipeline integrity because the consequences of holding cannot be measured, making any specified hold time of the maximum test pressure arbitrary.

    Verifying the safe pressure-containing capability of a pipe- line at its maximum allowable operating pressure (MAOP) requires a pressure test of the pipeline. This pressure test should exceed the MAOP by a safety margin sufficient to validate the stability of remaining anomalies.
     
    Regulations specify the duration for which pressure must be maintained, commonly called the hold time.
    The hold time is intended to allow sufficient certainty that there are no leaks in the pipeline. Starting in 1970, Federal Regulations required a hold time of 8 hr. Before this, when hold times were often less than 8 hr, the use of a shorter hold time introduced no increased risk of a rupture. Al- though a shorter hold time may increase the risk of a leak being overlooked during testing, any subsequent leak of significant volume should be discovered by required operator leak surveys, or by customers who would smell the odorant added to the natural gas stream specifically for the purpose of detecting leaks.
     
    Pressure tests conducted after building the pipeline ensure no inadvertent manufacturing or construction defects are present that could affect the safe operation of the pipeline, having escaped detection before the pipeline was placed in service. Test effectiveness derives from the hoop stress imposed in the pipe wall, irrespective of the testing fluid or medium, meaning water, air, or natural gas can be used to test the pipeline for such defects. Though the consequences of a rupture during testing could vary greatly with the type of fluid used in the test, the test fluid used has no impact on test effectiveness.
     
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