Effect of incident angle on ultrasonic transmission in steam flow for use with clamp-on ultrasonic flowmeter

Energy management in industrial plants requires measurements of the steam flow rates at each usage location. A clamp-on ultrasonic flowmeter can be used to effectively measure the steam flow rates in existing pipes. An ultrasonic flowmeter is used to calculate the transit time of ultrasonic signals between the downstream and upstream sensors, which is affected by the line-averaged velocity along the ultrasonic beam. The intensity of the transmitted ultrasonic signal is crucial for measuring the steam flow rate using the clamp-on ultrasonic flowmeter. The authors focused on the effects of the ultrasonic incident angle on the transmitted ultrasonic signal intensity in steam flow. Ultrasonic transmission experiments were carried out on three pipes (SGP 25A, SGP 50A and SGP 80A) filled with stationary nitrogen, and the transit time in steam flow was measured on SGP 80A pipe by changing the ultrasonic incident angle. The pipes are made of carbon steel and generally used for the steam flow. The results indicate that the appropriate incident angle that allowed the maximum transmitted signal intensity differed depending on the thickness of the pipe wall. Furthermore, a good agreement with the critical angles of the zero-order of symmetric mode in Lamb waves was noted. Thus, the propagated ultrasonic waves can be considered Lamb waves, which increase signal intensity. The transmitted signal intensity decreases because of the turbulent dissipation as the steam velocity increases. Appropriately setting the incident angle depending on the wall thickness, particularly for higher steam flow rates, is essential for evaluating the transit time difference between upstream and downstream transducers.