The uncertainties induced by internal pipe wall roughness on the measurements of clamp-on ultrasonic flow meters

Clamp-on ultrasonic flow meters (UFMs) have lower accuracy compared with spool piece UFMs because of the uncertainties introduced during the in-field installation process. Internal pipe wall roughness, one of these uncertainties, distorts the flow profile and causes the scattering of ultrasound. The objective of this paper is to carry out a parametric study to quantify the effect of scattering of ultrasound on the uncertainties of clamp-on UFM measurements without considering the flow disturbances. 2D finite element analysis was used to simulate the upstream and downstream signals of the clamp-on UFM based on some simplifying assumptions which were made about the effect of the flow. This simulation method was then verified by experiments which measure the uncertainties relating to the placement of ultrasonic probes at different separation distances. The simulation and experimental results were in good agreement. Then we applied this verified simulation method to investigate the uncertainties caused by the internal pipe wall roughness on the flow measurements. For ultrasonic waves at a frequency of 1 MHz and corroded internal pipe wall surfaces (0.2 mm RMS) it was found that systematic errors of 2 percent can result from the roughness induced scattering. This is a significant part of the measurement uncertainty range (1-5 percent) that is often quoted by manufacturers of clamp-on UFMs. This study therefore demonstrates that the accuracy of the clamp-on UFMs can be limited by the effects of internal pipe wall roughness.