Commissioning A New Multiphase Pump Visualization Test Rig To Investigate The Internal Flow Field And Its Connection With Pump Performance

Abstract

Pumps with helico-axial impellers are used to boost mixtures of gas and liquid, for instance in subsea production of unprocessed oil and gas. Experimental data is essential to fully understand and characterize the internal flow, as well as for validation and improvement of numerical modeling techniques. Here we describe the commissioning of a test rig that enables visualization and measurements of the pump’s internal flow field. The test rig’s pump unit design contains three impeller stages that are 50 percent downscaled relative to a full-scale version previously tested.A reduction in nondimensional head, flow and efficiency relative to the full-scale pump can be attributed to lower Reynolds numbersand an increased relative impeller tip clearance. The tested head curves on single phase water exhibit a transition where the negative slope is replaced by a flat curve when reducing the volumetric flow rate below a certain value. Despite the downscaling, this change in slope occurs at the same relative flow rate as for the full-scale pump. This suggests that the test rig can be used to replicate the characteristics of the full-scale performance and flow field in pumps with helico-axial impellers. Varying the impeller tip clearance allowed for an estimate of pump head with the equivalent clearance as for the full-scale geometry. A Morrison number of 0.03 could then be established for the pump.The impeller tip leakage flow and two recirculation zones in the diffuser channels were identified in a preliminary view of the internal flow field at two percent gas volume fraction and part-load operating conditions. Operation at low relative flow rates and high gas volume fractions led to system surge and slugging in the flow loop. Increasing the inlet pressure and temperature significantly improvedthe situation, allowing stable operation at lower relative flow rates. Modifications to avoid gas coalescence through the pump inlet could also further widen the operational envelope at high gas volume fractions