Test pressure factor

Pipeline engineers are familiar with the requirement that the hydrostatic test pressure of a pipeline must be at least 1.25 times the MAOP – see AS 2885.1 Clause 4.5.4 and Equation 4.5.4(1).  What seems to be not so well known is why the test pressure factor has that particular value of 1.25.

In the early days of pipeline construction in the USA (1950s and earlier) hydrostatic pressure testing seemed to be done rarely if ever.  The consequence was numerous failures in service due to manufacturing defects in the pipe.  Extensive research in the 1960s lead to an understanding of the effects and benefits of hydrostatic testing and as a result it became standard practice and incorporated in the pipeline standards of the time.

The 1.25 test pressure factor arose from that research, although not as a clear-cut and unambiguous outcome.  I have a copy of a 1968 AGA report “Study of feasibility of basing natural gas pipeline operating pressure on hydrostatic test pressure” (PRCI catalog no. L30050).  While it documents a great deal of detailed investigation its conclusion that the MAOP should not be more than 80% of test pressure (ie. same as 1.25 test pressure factor) was derived by “exercising engineering judgement so as to provide reasonable assurance of structural reliability”.  It was a well informed judgement, but a judgement nonetheless.

Interestingly, later work in 1980 provided a sort of retrospective justification for the 1.25 factor, as summarised in the Pipeline Rules of Thumb Handbook (Chapter 5 of the 7th edition, some pages of which are accessible via Google Books).  During the earlier testing of pipelines constructed from pipe containing numerous manufacturing defects it was often necessary to do multiple hydrostatic tests as failures occurred and were repaired.  Normally the pressure on each re-test would be higher than the last because the weakest defect had failed and been removed.  However on rare occasions a failure would occur at a pressure lower than the previous maximum.  These “pressure reversal” failures were due to time-dependent growth of flaws.

There were sufficient pressure reversals to allow statistical analysis.  For example, a 2% reversal would occur about once in every 100 repressurisations and larger reversals have a rapidly declining probability.  The conclusions was that a 20% reversal, which would cause failure at MAOP, had a truly minute probability.  It is interesting to note that the probability of such a reversal is not zero – apparently there was a well-documented failure at only 62% of a previous test pressure.

All this was based on pipe from 40 or more years ago.  Manufacturing standards have been improving and the likelihood of pipe containing serious mill defects has been declining, provided of course that the pipe is well-specified and subject to a good inspection and test plan.  Hence the likelihood of pressure reversals in modern pipe is probably even lower than at the time of the work that established the 1.25 test pressure factor.

Perhaps one of the most interesting things about this is that something as conceptually simple as a hydrostatic pressure test can be the subject of extensive research for over 50 years (from the 1960s to the present) and we still don’t understand every aspect of it.

A caution about reading some of the old reports on this subject:  While they are undoubtedly important they need to be read with a reasonable understanding of subsequent developments because some of the assumptions made are no longer considered valid, notably the belief that testing above 100% SMYS required no more control than a limit on the total volumetric strain.  APIA RSC and EPCRC work in recent years and the development of PIPESTRAIN software has demonstrated this clearly.

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