Separation Friendly Produced Water Pumps
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Produced water is the largest by-product of the oil and gas industry and its handling represents an important challenge. The fluid is mainly constituted by water, but its high content of sand, salt, sediments and especially the presence of dispersed oil makes its treatment necessary before its disposal or its use. The content of oil, in form of dispersed droplets surrounded by a continuous water phase, is the main environmental issue and must be reduced to the lowest possible level. The most common de-oiling treatments are gravity based, i.e. they use the difference in density between oil and water to remove the dispersed phase. All the gravity based systems are based on the Stokes’ Law for dispersed fluid and it is therefore of capital importance that the droplet dimension is not reduced during the fluid handling, so that the components of the circuit (valves and machines) do not cause an excessive droplet breakage. The solutions adopted today are mainly based on the use of volumetric pumps, such as screw pumps. These machines ensure a low breakage level, but the nature of the fluid and the working conditions significantly reduces their operating life. Operating life would be longer if dynamic machines were used, but they introduce a higher shear in the fluid because of their very operating principle. Typhonix AS is developing multistage centrifugal pumps for produced water treatment which couple the longer life of a centrifugal pump with the low droplet breakage typical of a volumetric machine. By means of a special hydraulic design, these pumps have also demonstrated a coalescing effect on the droplets which translates in an increase in the efficiency of downstream removal system. The present work is aimed to the study of the flow characteristics in the machine passage to understand what promotes the droplets growth in order to improve the design. A particular attention was focused on the recurring turbulent flow structures in the machine passages, and the evaluation of the possible influence these can have on the droplets breakage and on the coalescing promotion. To study the internal flow a numerical model was prepared and several computer simulations were carried out. In order to evaluate the accuracy of the simulation, the CFD code was validated by means of comparison with experimental results. An experimental rig was designed and built at the NTNU Waterpower laboratory and the flow velocity distribution was measured with laser techniques. The validation of the numerical model gave satisfactory results and the model considered appropriate for a good flow prediction. In particular recurrent turbulent structures were found in the passage between the diffuser and the return vanes, caused by the interaction between the flow and the machine surfaces. The influence of these structures on the dispersed phase was simulated by means of numerical particle tracking which showed how they have a retaining effect on the oil droplets. The retaining effect demonstrated to have a higher effect on the smaller droplets, increasing their residence time in the machine and therefore increasing the possibility of coalescence. Moreover, because of the difference in density between the dispersed and the surrounding phase, the droplet will tend to migrate in the center of these vortexes under the action of centrifugal forces. This phenomenon means that the vortexes not only have a retaining effect, but represent the active driving force for the coalescing effect.