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Fitness-for-Service of Corroded Buried Pipes

When planning or performing a wall thickness inspection of buried pipe, it is necessary to establish the procedure to be used in evaluating the thickness readings, and determine the pipe’s fitness-for-service (FFS), which will be the basis of run-or-repair decisions.

We are currently working with EPRI to develop such a procedure. This article is intended to provide a broad brush of the FFS process for buried pipe.

An industry that has come to grips with this process for over 30 years is the oil and gas pipeline industry. They use ASME B31G to assess the integrity of corroded pipelines. The ASME B31G procedure is simple, conservative, and … not applicable to nuclear plant buried pipes. The reason is that B31G contains assumptions regarding the operating loads, the operating pressure stress, and the equivalency to a hydrotest which are not applicable to ASME B31.1 or ASME III lines.

Another reference would be ASME XI Code Case N-597 and API 579 / ASME FFS-1, for safety-related and non-safety related pipes respectively. But neither document explicitly addresses buried pipes. 

For a PDF of our Becht Nuclear Summer 2010 Newsletter,
please click here.

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Code Evolution

Prior to 1969 the design and construction code for piping systems in nuclear power plants was ASME B31.1 Power Piping, with supplementary requirements specific to nuclear power. To this day, ASME B31.1 remains the applicable code for pre-1969 plants, and for non-safety related piping in all plants.

In 1962 a nuclear code case was published (ASA B31.1 Code Case N-1 “General Requirements for Nuclear Power-Plant Piping”) to address the design requirements specific to nuclear piping, what today is referred to as safety class 1, 2 and 3 piping systems.

With time, Code Case N-1 evolved in a new code in 1969, the ASME B31.7 Code for Nuclear Power Piping. This code remained applicable until 1971, when the design and construction requirements for nuclear power plant piping were folded into the ASME Boiler and Pressure Vessel Code, Section III Division 1, which until then was a nuclear pressure vessel code.