Using a visualization prototype built from original Electrical Submersible Pumps (ESP) components and with minimal geometrical modifications, a pioneer experimental procedure was developed and conducted to address the viscous effect on liquid-gas two-phase flow through these types of pumps.
Based on dimensionless groups that govern centrifugal pump single-phase performance, two-phase experiments were conducted at different shaft speeds (15, 25, 30 Hz), non-slip void fractions (up to 5%) and viscosity values (46 to 161 cP), while liquid rates were kept constant at 60% of the maximum rate at the defined shaft speed. High speed video footage was taken from the entire impeller flow channel and stage incremental pressure was measured.
The authors identified four liquid-air flow patterns inside the impeller channels: Agglomerated Bubbles, Gas Pocket, Segregated Gas and Intermittent Gas. By comparing the images with the differential pressure data, it was concluded that the Agglomerated Bubble pattern is responsible for the initial head degradation and that the surging event coincides with the Gas Pocket structure, indicating that this is an interface instability problem. Another conclusion made was that the increase in viscosity caused surging to occur at lower void fractions which could be compensated by increasing rotational speed.
The significance of this work is given by the fact that several authors have investigated centrifugal pump performance under two-phase flow; however, previous experiments have been conducted only with water as the liquid, thus neglecting the viscous effect on the two-phase flow mixture. In most of the petroleum industry´s applications ESPs operate with oil and natural gas. Thus presenting a knowledge gap between scientific research and field applications, this starts to be addressed in this work.
Keywords: Centrifugal Pump, Flow Visualization, Two-Phase Flow, Surging, Flow Patterns, Viscosity
Electrical Submersible Pump (ESP) is one of the many existing artificial lift techniques used in the petroleum industry. It consists of a series of small diameter diffuser type casing centrifugal pumps, mainly known for their capacity of handling high volumes and widely used. Their applications vary from producing high productivity oil wells onshore and offshore to dewatering coal bed methane and gas wells.
The use of ESPs, or any other centrifugal pump for that matter, on liquid single phase applications is well understood and the knowledge necessary for these situations is consolidated. However, as it is common in the petroleum industry, the presence of a compressible phase, such as natural gas, will also modify the hydraulic performance of these types of pump. The presence of a seccond phase will oftenly degrade the pumps ability of delivering pressure and depending on the amount of gas flowing through the system, instabilities may occur. These, also refered as surging, eventually lead to extremely low pressure generation also known as gas lock.
Several studies have been conducted to visualize and simulate pump performance under the presence of free gas; the majority of these studies focus on centrifugal pumps used by the nuclear industry, motivated by the flux of water and water vapor through their systems. Murakami and Minemura (1974) used a semi-open type pump with transparent casing. The observed flow patterns were recorded photographically, and bubble diameter measurements could be made inside the impeller. Patel et. al. (1978) also observed two-phase flow in centrifugal pumps by testing a 1/20 scale prototype of a primary coolant pump used in nuclear reactors, to measure bubble sizes. Sekoguchi et. al. (1984) used a pump equipped with a closed radial-flow impeller to visualize the flow patterns and also installed eight needle tip void probes to measure bubble frequency and later examined the slip ratio between the fluids.