The Effects of Multiphase, Multi-Viscosity Fluids on Axial Thrust and Cooling Flow Performance of a Canned Motor Pump

A canned motor pump for technology demonstration in subsea oil and gas systems was tested to provide data to understand the design, modeling and performance characteristics of a canned motor pump operating in multiphase flow. This paper discusses the impact of multiphase flow and fluid viscosity on axial thrust and motor cooling flow characteristics. The technology demonstrator is a two-stage, low specific speed (Ns∼550) centrifugal pump designed to deliver 140 gpm at 600 feet of head at 3930 rpm. The 61 hp canned motor is cooled by a small portion of the pump discharge fluid, which is drawn downward through the motor by pump out vanes on the hub of the second stage impeller. The multiphase test loop is equipped for both water and light oil operation in low pressure and ambient temperature conditions. Testing occurred over a range of conditions to simulate varying fluid properties and operating scenarios. Shaft rotational speed varied between 2000 and 4250 rpm with pump liquid flow rates from 25 to 250 gpm. These operating scenarios were repeated for both water and light oil (∼2 cP) with multiphase flow ranging from 0–20% gas volume fraction (GVF) using injected air. Testing results indicate a detectable impact from the different fluids and GVF’s tested, which can be related to features such as the second stage impeller pump-out vane and regions within the motor cavities. In water-air tests, increasing GVF led to the following: motor input power reduced by 5%; axial thrust increased by 100%; motor cooling fluid temperature rise increased by 100%; and pressure rise in the second stage pump out vanes reduced by 30% - directly impacting motor cooling flow rate, temperature rise, and axial thrust. In the oil-air tests, multiphase flow showed similar tendencies with reduced magnitude. Notably, the effects due to air injection do not appear at GVF below 15% with oil-air mixtures, unlike water-air tests which demonstrated effects across all GVFs. The test results provide insight into the behavior of variable viscosity, multiphase flow in the canned motor pump cooling passages, as driven by the second stage impeller pump out vanes. These observed characteristics can be used to design flow control features and evaluate operational impacts, while the performance data obtained can be used to assess the behavior of flow models for this application.