Clamp-on transit-time ultrasonic flowmeters (UFMs) suffer from poor accuracy compared with spoolpiece UFMs due to uncertainties that result from the in-field installation process. One of the important sources of uncertainties is internal pipe wall roughness which affects the flow profile and also causes significant scattering of ultrasound. This paper purely focuses on the parametric study to quantify the uncertainties (related to internal pipe wall roughness) induced by scattering of ultrasound and it shows that these effects are large even without taking into account the associated flow disturbances. The flowmeter signals for a reference clamp-on flowmeter setup were simulated using 2-D finite element analysis including simplifying assumptions (to simulate the effect of flow) that were deemed appropriate. The validity of the simulations was indirectly verified by carrying out experiments with different separation distances between ultrasonic probes. The error predicted by the simulations and the experimentally observed errors were in good agreement. Then, this simulation method was applied on pipe walls with rough internal surfaces. For ultrasonic waves at 1 MHz, it was found that compared with smooth pipes, pipes with only a moderately rough internal surface (with 0.2-mm rms and 5-mm correlation length) can exhibit systematic errors of 2% in the flow velocity measurement. This demonstrates that pipe internal surface roughness is a very important factor that limits the accuracy of clamp on UFMs. Index Terms-Clamp-on flowmeter, roughness, transit time, ultrasound, uncertainties. I. INTRODUCTION T RANSIT-time ultrasonic flowmeters (UFMs) are widely used in many industrial sectors, such as oil and gas, power, nuclear, process, water distribution, and chemical plants. These flowmeters measure flow velocity by calculating the difference in arrival time between the ultrasonic signals that are traveling with the flow (downstream signal) and against the flow (upstream signal). There are two common ways to install the ultrasonic transducers, "inline" and "clampon" (Fig. 1). The installation of the inline UFM requires cutting the pipe and subsequent insertion of a premanufactured and calibrated spool piece that contains integrated ultrasonic transducers. The clamp-on flowmeter only requires transducers to be mounted on the outside of the pipe wall to take a measurement. This clamp-on flowmeter has a number of Manuscript
Transit time clamp-on ultrasonic flow meters (UFMs) are widely used in industry due to their ease of installation. However, these ultrasonic clamp-on flow meters are also known to be less accurate than ultrasonic inline flow meters 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 flow meter at longer wavelength. This paper investigates the effect of roughness on a clamp-on UFM using low frequency (200 kHz) leaky Lamb waves. This results in operation at roughly 5 times lower frequency compared to conventional clamp-on UFMs. The ultrasonic signals of this leaky Lamb wave UFM were simulated using 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 of that of conventional UFMs for corroded pipe walls with RMS value larger than 0.1 mm (0.2 mm, 0.35 mm and 0.5 mm). Demonstration experiments were also carried out to detect leaky Lamb wave using an EMAT (electromagnetic transducer). 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.
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