3D CFD Simulation of ESP Rotary Gas-Separator Performance Under Two-Phase-Flow Condition

Abstract

Abstract The performance prediction of a Rotary Gas Separator (RGS) is very important for ESP systems applications. The pump performance is severely affected when it handles high Gas Void Fraction (GVF) at its intake. The function of the RGS is to separate both liquid and gaseous phases, and to expel the gas through a crossover section to the annular area between casing and tubing. Typical designs use separation efficiency values based on an empirical standpoint of view. Also, the literature references regarding an important parameter like the inducer head is very scarce. The performance analysis of a RGS (540 series separator), under two-phase flow conditions, has been conducted using 3D-CFD simulation tools (CFX 5.6). Water-air mixtures were used as working fluid and the mixture GVF was varied from 10% up to 30%. The results shows that the RGS separates efficiently the phases, but the inducer head is insufficient to overcome the friction losses in the crossover and the liquid column static pressure in the annular space. As a consequence, a new inducer design is necessary to create a higher head value to push the gaseous phase out of the RGS and not to be dragged by the liquid phase. The simulation could be an alternative tool for selecting the depth of the downhole equipment as a function of the liquid level. This could help the designer to properly obtain the minimum submergence of the equipment that satisfy the phases separation and gas expulsion of the RGS, getting a lower GVF at the PIP. Introduction Electrical Submersible Pump (ESP) is one of the artificial lift methods commonly used, which is a multiple stage centrifugal pump. When a centrifugal pump handles liquid with free gas at the intake it experiments performance degradation. Depending on free gas amount, the consequences can vary from light pump performance degradation up to pump gas locking. Typical solutions to reduce the free gas amount at the pump intake are the installation of gas separators or gas handling devices (special geometries). The Rotary Gas Separators (RGS), has been traditionally used, but its performance under two-phase flow is still not well understood. The major efforts to analyze RGS's performance has been done by several authors1–4, through experimental works and theoretical models. The separation phenomenon has two domains; natural gas separation in the annular space and gas separation process inside the separator device. The separation efficiency of the RGS depends on the centrifugal force and the inducer head. The last parameter is very important to overcome the crossover losses and annulus' backpressure producing the gas expelling to the annular space. The objective of the present work is to study the inducer performance (separation phases process and head developed) using 3D two-phase numerical flow simulation. A commercial code was used (CFX 5.6) to simulate the two-phase flow inside of RGS inducer. Theoretical Background In this section, the basic definitions and some RGS components will be discussed. Inducers. An inducer is a low-head axial flow impeller with few blades, basically used to provide little booster pressure to a conventional impeller. The purpose of the inducers in rotary separators is to supply the centrifugal force necessary for phase's segregation and the required gas energy to overcome the crossover losses and annulus' backpressure. Intake Section. In the design of ESP installations, one key factor is the free gas amount at the intake. This parameter sets the criteria to choose the pump "intake section" completion among: simple intake, gas separator intake or advance devices for handling the gas. Gas separators are devices employed to expel the gas into the annulus reducing the gas handled amount by the pump. Depending on the separation concept, there are two kinds of equipment: a) The reverse flow or static separators (gas anchor) based on pressure drops and changes on flow direction (momentum) whose efficiency is reduced by the gas dragged by the liquid and b) The rotary separators' based on the phase's segregation induced by the huge centrifugal forces.