Abstract
Non destructive Testing (NDT) is commonly used to assess the extent of corrosion within plant, including vessels, piping and pipelines. In particular measuring instruments using ultrasound are used to find locations where loss of wall thickness has occurred and to determine the extent of the loss. Inherent in the measurement of wall thickness is the assumption that from a limited set of measurements it is possible to infer the condition of a vessel, heat exchanger tube bundle or section of piping. The determination of corrosion rate then requires a reassessment of the wall thickness at a later date. For old plants, the variability in corrosion damage both in a spatial sense and in depth of loss causes a difficulty in finding the worst case losses in wall thickness and in determining remaining life. It is also necessary to recognize that the measurement error depends on the skill applied by the inspector during the inspection. This paper discusses the use of probability of detection concepts for finding corrosion damage and illustrates it for a corroding vessel inspected over a 2 year period. It also describes the use of distributions of wall loss to characterize the condition of corroding vessels and pipelines. The incorporation of statistical principles into data analysis is required because of the inherent variability of corrosion damage.
