Assessment of Pipe Wall Thickness Radiation Technique

Introduction

Degradation of materials with time during service is a common phenomenon for all engineering components. Hence periodic inspection is required to ensure structural integrity and availability for service. During in-service inspection (ISI), wall thickness measurement of insulated and non-insulated pipe is a typical non-destructive evaluation technique in the oil & gas, chemical, petrochemical and nuclear industries. Ultrasonic testing is available for wall thickness measurement, but in some cases, it may not be the preferred technique. For ultrasonic testing, accuracy is dependent on the temperature of pipe, which may carry fluid at high temperature. Hence shutdown of the installation is required. Moreover for insulated pipe, insulation has to be removed before ultrasonic testing. The radiation technique is a complementary testing method which can be carried out without disturbing the installation. In this technique electromagnetic radiation passes through the object of inspection and is finally recorded in a recording medium. The recording medium is either an industrial X-ray film or a radiation detector. This paper is devoted to detection of pipe wall thinning by the radiation technique. Two different methods i.e. radiography and radiometry, are discussed with their relative merits and demerits.

The pipeline is the most economical way of carrying fluid from one location to other under pressure. These fluids are sometimes corrosive in nature. Even the most corrosion resistant pipe material is eaten away slowly with time. Therefore in-service inspection (ISI) is carried out periodically to detect corrosion, erosion and deposit, which if not detected may cause problems such as leaking and sometimes bursting, causing considerable economical loss. Therefore pipe wall thickness is regularly monitored in industry to avoid such catastrophes. Many established techniques such as ultrasonic testing, magnetic flux leakage and eddy current testing are available for inspection. All these techniques have their own limitations such as high magnetic permeability in ferromagnetic materials limits conventional eddy current testing, removal of insulation material is required for ultrasonic testing, etc. Therefore the radiation technique may be used as an alternative or complementary method. In the radiation technique, an attenuated electromagnetic ray is detected in a recording medium. Depending on the recording medium, it is classified as either radiography, which produces an image, or, radiometry, which counts photon flux with a radiation detector. Both these techniques can be carried out without shutting down the plant and the radiation beam does not interfere with operation .