Abstract
Clamp-on transit-time ultrasonic flowmeters (UFMs) suffer from poor accuracy compared with spool-piece 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.
Highlights
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
The standard deviation of the estimated flow velocity reaches approximately 2% for a moderately corroded pipe with rms 0.2 mm and correlation length 5 mm. This means that if clamp-on UFM on the pipe with these roughness parameters is used, the systematic errors that result purely from the roughness at the location of installation can be of the order of 2%
This is a large part of the uncertainties that are commonly attributed to clamp-on flowmeters by manufacturers (1%–5%)
Summary
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. The clamp-on flowmeter only requires transducers to be mounted on the outside of the pipe wall to take a measurement. There is a limited amount of information available on the effect of internal pipe wall roughness on the uncertainties. The pipe wall roughness would induce uncertainties in two sets of problems, one is distortion of the flow profile, the other is the scattering of ultrasound [6]. Since the internal pipe wall is inevitably rough, the impact of the wave scattering from these surfaces needs to be studied and quantified. The aim of this paper is to carry out parametric studies to quantify the effect that internal pipe wall surface roughness has on the uncertainties of clamp-on UFMs for nonflow profile related effects (scattering effects). The method was used to quantify the effect of internal pipe wall roughness on uncertainties for different roughness parameters (Section IV)
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