Issues of a Combination of Ultrasonic Doppler Velocity Measurement with a Venturi for Multiphase Flow Metering

摘要

Multiphase flowmeters (MPFMs) available today are mostly based on a combination of differential pressure measurement, provided by at least one flow constriction, such as a Venturi device, with phase holdup measurement based on nuclear techniques, electromagnetic techniques, or both. This paper investigates the issues of combining the ultrasonic Doppler velocity measurement with the differential pressure measurement for multiphase flow metering. The intention is to measure at least one “useful” velocity of the flow, such as the bulk liquid velocity or the homogeneous mixture velocity, which can then be combined with a measured differential pressure across a “constriction” device, such as a Venturi, to derive parameters needed to calculate liquid and gas flow rates. An ultrasonic Doppler velocity sensor has been tested in combination with a Venturi under multiphase flow conditions. The results show that, due to the effect of gas bubbles in the liquid phase, the interrogation depth of the ultrasonic wave is often limited to a shallow liquid region near the pipe wall. As a result, the ultrasonic Doppler sensor can measure neither the bulk liquid velocity nor the homogeneous velocity reliably. It is generally difficult to interpret the measured Doppler velocity and to derive the flow rates of a multiphase flow without employing an elaborate flow model. This work was part of a project co-funded by the UK Technology Strategy Board1, with industry partners The University of Manchester, Schlumberger Gould Research, and TUV-NEL.