Thickness monitoring online by welded ultrasonic waveguide for pressure piping at high temperature

Pipe thickness should be on-line monitored when it suffers severe erosion, wear and corrosion under high pressure and high temperature. UT (Ultrasonic Testing), the most effective method in thickness measuring, is inconvenient for insulation removing and the need of special coupling medium for high temperature. In this paper, a stud-welded ultrasonic waveguide was suggested for high temperature pipe thickness on-line monitoring. The ultrasonic waveguide, a short rod, made of the same material as the pipe, was welded vertically on the pipe wall with full penetration to eliminate the shoulder pulse. It was water cooled with a short jacket at the end attaching UT sensor. The waveguide was welded to flat plates to verify the feasibility of the method. It is shown that the water jacket is effective enough to keep the sensor attaching end at ambient temperature when plates are heated up to 400 degrees C. The weld connection has little effect on the thickness measuring with proper weld technique. But the temperature does change the ultrasonic propagation velocity in the plate and its adjacent rod which increases the thickness readings. The influence of temperature on the thickness reading was discussed and correction formula was worked out. Both low carbon steel and austenitic steel were used for verifications. And the results are encouraging.