Wedge design for high-temperature ultrasonic flow rate measurement

Abstract Measuring flow of liquids in pipes is crucial for process control operations, including custody exchange, chemical manufacture and manufacture of foodstuffs. However, many of these processes are at elevated temperatures (up to 350 °C) where current commercially available ultrasonic systems are unable to operate or require shut-down of plant for flow-meter installation, such as with in-line, or wet, systems. Clamp-on ultrasonic flow meters are able to achieve flow measurements at high-temperatures, often with complex or heavy waveguides and stand-offs to keep the Pb(Zr,Ti)O3 (PZT) element away from the hot, process end, however with the introduction of commercially available high-temperature piezoelectric ceramics, this is starting to change. The final major design consideration is the ultrasonic wedge, which traditionally made from polymers or slower speed of sound materials, is not suitable for use above ∼200 °C. Presented here is a metallic wedge design, which offers a shear mode velocity closer to the longitudinal sound velocity of polymers but with exceptional temperature resistance offering a permanently installed flow monitoring solution for demanding industrial environments. A 1 MHz transducer with integrated 40-degree incident angle wedge in stainless steel exhibits a signal to noise of 29.3 dB in a Z mode path configuration, at temperatures up to 380 °C. This was shown to give a repeatable and stable flow rate measurement when connected to a commercial flow converter, on to a pipe containing mineral oil flowing at approximately 14.4 m3/hr.