Is the 80% Leak Criterion Always Appropriate?

When evaluating corrosion ASME B31G recommends an upper limit of 80 percent of the wall thickness for evaluating metal loss depth. Although corrosion deeper than this can still be acceptable for maintaining a safe pipeline, the primary concern was for the effect of error in the corrosion depth measurement and the need to offer conservative criteria. If the measurement error is understood and the treatment of these errors can be handled in a routine and practical manner, then corrosion depths greater than 80 percent could potentially be acceptable. Examples of well understood measurement error are ILI tools where published values exist for commercial tools, in addition the error can be reassessed using in-the-ditch measurements when remediating occurs. The 80 percent maximum is also limiting for low pressure applications such as barge and tank lines, or natural gas distribution lines, where the pressure on a system is often only a fraction of the pressure carrying capability of full wall thickness pipe. This paper will demonstrate a method for assessing deep corrosion which is acceptable for low pressure lines and accounts for measurement error using other known sources of measurement error.Copyright © 2010 by ASME