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Inspectioneering Journal

Piping Inspection Thickness and Corrosion Monitoring With Profile Radiography

Part 1

By John Reynolds at Intertek. This article appears in the September/October 1997 issue of Inspectioneering Journal

Introduction

This is the fourth in a series of articles on piping inspection that I'm writing for the Journal. One of the previous ones dealt with improving thickness data taking accuracy with digital ultrasonic methods. This article is a "sister article" that deals with improving the accuracy of profile radiographic inspection techniques, also called isotope radiography, wall shots, or tangential radiographic inspection.

With a little bit of developmental effort focused on improving procedures and utilizing a spherical comparator, we significantly improved the quality of our thickness measurements taken from pipe wall radiographs. As a direct result, we now have much less rework and much higher confidence in our inspection scheduling associated with these radiographic thickness measurements.

Profile radiography has several sources of inherent errors when used for measuring pipe wall thickness. When used only as a corrosion detection (qualitative) tool, these errors do not normally interfere with our ability to understand the general condition of a piping component. However, when we use the tool quantitatively to measure wall thickness as a means of determining corrosion rates (and the resulting projected renewal and inspection dates), the errors can result in incorrect and expensive assumptions.

Determining Geometric Blow-Up

The primary sources of error in profile radiographic thickness measurement result when the radiation beam, which impinges on the wall of the pipe, continues to diverge as it passes onto the radiographic film. This divergence causes the image on the film to be larger than the actual thickness of the pipe wall (Figure 1).

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