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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. 

Fluid Transients in Piping Systems

 
Part 2 – Pressure Transients in Liquid-Gas Lines

This is the second in a series of articles on fluid transients in piping systems.

A trapped gas pocket in a liquid system can occur in several configurations, as illustrated in Figure 1. Consider the first case where the gas pocket is trapped in-line at a dead-end, at an initial pressure P_g0. As the pump starts, the gas pocket is compressed, causing an oscillatory pressure in the gas.

The gas pressure can be calculated by closed-form solution based on mass conservation and momentum principles, with gas state equations. A closed-form solution to the differential equations (DE) is plotted in Figure 2, labeled “DE Model”.

A more realistic pressure can be calculated by computational fluid dynamics, taking into consideration the compressibility of the liquid. Figure 2 shows a comparison of the computational fluid dynamics numerical solution with liquid compressibility (labeled “Numerical”) to the rigid column differential equation closed-form solution (labeled “DE Model”). The CFD method goes one step further than just modeling compressibility; it also models the propagation of pressure waves; this is why smaller precursor pressure spikes representing the pump start-up pressure can be found before the main pressure peak.