Modeling Surface Roughness-Related Uncertainties of Leaky Lamb Wave Clamp-on Ultrasonic Flowmeters

Transit time clamp-on ultrasonic flowmeters (UFMs) are widely used in industry due to their ease of installation. However, these ultrasonic clamp-on flowmeters are also known to be less accurate than ultrasonic inline flowmeters because of the uncertainties induced by the installation process. Amongst the installation-related parameters that influence the measurement uncertainties, internal pipe wall roughness is one of the most significant but uncontrollable parameters. The effect of roughness on accuracy can be reduced by operating the flowmeter at a longer wavelength. This article investigates the effect of roughness on a clamp-on UFM using low-frequency (200 kHz) leaky Lamb waves. This results in operation at roughly five times lower frequency compared with conventional clamp-on UFMs. The ultrasonic signals of this leaky Lamb wave UFM were simulated using the 2-D finite-element (FE) analysis. Using the simulated signals, the roughness effects on the uncertainties were quantified. The simulation results show that the uncertainty related to pipe wall roughness of leaky Lamb wave UFMs is approximately half that of conventional UFMs for corroded pipe walls with rms value larger than 0.1 mm (0.2, 0.35, and 0.5 mm). Demonstration experiments were also carried out to detect leaky Lamb wave using an electromagnetic acoustic transducer (EMAT). The experiment shows that the simulation correctly captures all the physics of the wave propagation and that we, therefore, can trust the simulation results with incorporated roughness.