Total mass flowrate measurement in multiphase flow by means of a Venturi flowmeter

In recent years, the needs of the oil industry ask for a simple, compact and maintenance-free device to measure the mass flowrate of two-phase and three-phase (e.g. oil-gas-water) mixtures, to be used mainly for well testing. In this work the possibility of using a device based on a Venturi nozzle for industrial multiphase mass flowrate measurements has been investigated. In case of multiphase systems, the relationship between the overall mass flowrate and the pressure drop in a Venturi nozzle is not unique and includes also the flow quality, so that measurements of quality or holdup have to be included. An experimental facility for the study of any combination of oil, air and water flow has been built and operated. It consists of a vertical perspex pipe of 50 mm internal diameter and about 7 m length. The facility can be operated at atmospheric pressure and at the present makes use of air, water and diesel-fuel oil. The Venturi meter has been tested in air-water and air-oil-water flow; the superficial velocities ranged up to 1.8 m/s for oil and water and up to 20 m/s for air. Conductance probes, made of two stainless steel rings mounted flush to the pipe wall and placed at different positions in the pipe and in the Venturi throat allowed the measurement of the local liquid holdup for the case of air- water flow. Conventional measurements included pressure drop and absolute pressures. The results show that a unique relationship can be inferred among the pressure drop across the Venturi, normalized with respect to the pressure drop relative to the liquid phase flowing alone in the nozzle, and the flow quality in any combination of the three phases