Two-Phase Flow Measurement
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The use of multiphase flow meters (MPFMs) increase each year. The main reasons for this are the cost benefits connected to the use of such a meter by eliminating exclusive lines to the test separator and the test separator itself and the demand for a meter that can measure the produced fluids in real time without separating the phases. The MPFMs measures the flow rate by use of different measurement principles and techniques, and in combination of these, e.g. by means of Venturi and void fraction sensor. The scope of this thesis is to measure the flow rate of a two-phase flow by means of a flow meter device and a void fraction sensor and to check the applicability of several known calculation methods to the measurements. In order determine the flow rate of such a flow; a flow loop, flow meter, and a void fraction sensor are required to be designed and fabricated. The Venturi tube is a common flow meter and can offer high accuracy, high pressure recovery, a short straight pipe run is required upstream and has no moveable parts. No moveable parts are preferred throughout the industry, owing to expensive maintenance runs of any moveable part. The flow rate can be determined from the pressure drop over the Venturi from well known relationships. An impedance probe is used to determine the void fraction of the flow. This sensor is made from two full-ring electrodes flush mounted to the inner pipe wall making the impedance probe non-intrusive. To determine the mean void fraction from the impedance measurements the liquid holdup equation proposed by Andreussi et al. (1988) is used. Two rotameters are used as reference measurements in the flow loop, one for water and one for air. An electrical water pump and an air compressor are used in order to obtain a wide range of flow rates. The flow rates used in this study are ranging from 1.185 up to 1.975m3/h for water and 0.347 up to 2.1m3/h for air. Flow characteristics that are encountered are defined as bubbly and transition to slug flow, and the mean void fraction ranged from 18% up to 44% and the mass quality obtained ranged up to 0.12%. The uncertainty of the Venturi and impedance probe is obtained by the use of 90% confidence interval, which is a common method used to present the uncertainty of a MPFM, Johnsen (2010). From the measurements the Chisholm correlation proved to achieve the best performance in predicting the flow rate of the two-phase flow and the homogenous model gives reasonable predictions of the flow. The uncertainty of the meters ability to measure the flow rate by the use of Chisholm correlation is ± 3.5%, ± 10.23%, and ± 4.19% for water, air, and total flow rate, respectively. The study shows that the measurement of a low quality (χ < 0.12%) two-phase flow is possible by the means of simple Venturi and impedance probe, when the mean void fraction ranged from 18% up to 44% and the flow is within the bubbly and transition to slug flow.