Transducer design for clamp-on guided wave flow measurement in thin-walled pipes

Clamp-on ultrasonic transit time difference flow meters provide opportunities for metering where it is impractical or undesirable to cut into an existing pipeline to install an alternative flow meter. Up until now, it has been difficult to perform this type of measurement on thin-walled metal pipes, due to the difficulty of interpreting the guided wave modes in the combined pipe wall and internal fluid system, but a new method has been reported recently that utilises these guided wave modes for flow measurement. Through computational modelling, and construction and testing of different transducers, the design considerations for clamp-on transducers are highlighted and their impact on guided wave flow rate measurement is evaluated. The design features considered include a curved contact face to provide focusing of the ultrasound within the pipe and a scattering surface to reduce internal reflections. It is found that additional unwanted ultrasonic modes can be minimised by ultrasonic transducer wedge design features such as profiling the curvature of the transducer to conform to the pipe wall or creating a scattering edge to minimise internal wedge reflections. It is also observed that minimising these unwanted modes does not offer any advantage for the transit time difference measurement used in calculating flow.