Velocity compensation and practical aspects for high-temperature ultrasonic testing

Increasingly, the need for on-stream asset integrity, where inspections are carried out while components are still operating to reduce the disruption and impact of outages, is required at elevated temperatures. For example, a typical hydrocarbon refinery has process units with surface temperatures in excess of 500°C, where internal corrosion of the typically steel components needs to be monitored by testing, to maintain safe and reliable operation. This is ubiquitously carried out by means of ultrasonic testing (UT). As high-temperature tools increasingly become available, the accuracy of thickness measurements is often questioned due to the intrinsic link between the measured time-of-flight (TOF) and the characteristic speed of sound of the material. The velocity, in turn, is a factor of the elastic modulus of the material, which is inversely proportional to temperature in steel. Historically, international standards and practices publish correction factors for generic steel materials, but increasingly the desire for more accurate determination is required with more advanced tools and techniques. Here, the velocity as a function of temperature is determined through experimental study for two common carbon steels that may see service at elevated temperatures, and the correction factor is shown by application to a typical corrosion mapping survey at elevated temperature. The errors associated with variable velocity are shown to be effectively minimised below the normal variability of ultrasonic testing.