Abstract

This paper presents a new pump concept, called the New Progressing Cavity Pump (NPCP). The NPCP is composed of a Progressing Cavity Pump (PCP) and a system of Hydraulic Regulators (HR) installed inside the pump in between the cavities. The HR are self-regulated devices that recirculate the fluid between the cavities in order to control the pump thermo-hydraulic response and to avoid excessive built up of heat, which might result in premature failure of the pump's stator.

It is common knowledge that the traditional Progressing Cavity Pumps (PCP) have shown significant problems when dealing with multiphase mixtures, with high gas void fraction (GVF). We propose a new concept, NPCP, which is able to handle much better such conditions, as its internal hydraulic regulators (HR) recirculate fluid inside the pump. The benefits are multiple. First, it uniformizes the pressure across the pump length, which stabilizes the temperature. Second, it compensates the compressed gas volume of progressing cavities. Furthermore, it protects the stator and therefore, improves the pump's performance.

The relatively small dimensions of hydraulic regulators HR and their distribution along the pump result in an efficient multiphase design. Compared to traditional PCP, the new NPCP system better controls the reliability parameters, such as temperature and friction torque, reduces energy consumption and enhances hydraulic performance (flow rate, delivered pressure).

Several industrial NPCP have been manufactured and tested in liquid, both water and oil, as well in multiphase flow (air and liquid). This paper presents and describes the bench test results and shows that NPCP improves both reliability and hydraulic performance over existing pumps.

Introduction

The recent developments of new production systems clearly indicates that there are considerable industrial interests in pumping multiphase fluids, in particular the combination of high gas volume and oil or water, or small amounts of wax and sand. Traditional Progressing Cavity Pump (PCP) often faces up with reliability problems, mostly because of gas phase compression causing heat to build up, resulting in premature failure of elastomeric stator and pump disfunctioning.

We have recently published a study that analyzes the behavior of traditional PCP in multiphase flow (see [1]) and the reasons for traditional PCP poor performance are presented. The thermo-hydraulic process is the result of fundamental gas laws, which explains that as the pressure increases inside a cavity with constant volume, the temperature does increase too. Typical pressure distribution along the PCP is the result of gas volume compression and slippage flow between the rotor and the stator. Close to the end of the discharge, the gas compresses and the volume is compensated by the slippage flow. Multiple tests have shown that a disproportionate amount of pressure is developed by the discharge stages, which causes excessive heat build-up. Another thermo-mechanical process occurs as a result of disproportionate pressure gradient between two contiguous cavities. Because of differential pressure, the stator material is strained inside the low pressure cavity. Thus, the compression stress of the rotor increases the friction torque (viscous) and the increasing temperature becomes a measure of rotor-stator viscous-friction torque. In multiphase flow conditions, the excessive pressure gradient in the discharge stages causes both thermo-hydraulic and thermo-mechanical physical processes which reduce the pump reliability and performance. Furthermore, the traditional PCP reliability is related to pressure distribution which also depends upon the pump design and production conditions, i.e. gas flow rate, delivered pressure and rotational velocity. Among the multiphase pump designs, there are two major classes:

  • Class 1: Pumps that keep the actual PCP design and adapt the compression fit between the rotor and stator, which is gradually reduced with the distance from suction end, as developed by Mirza K and Wild A [2]. Slipping flows introduce a trade-off between delivering pressure and flow rate requirement, and compensating the compressed volume of gas cavities. This design trade-off is difficult to control.

  • Class 2: The New PCP (NPCP), which includes the hydraulic regulators (HR) installed inside the rotor between the cavities, the function of which is to control the reliability parameters, i.e. pressure distribution and the developed temperature as well as the viscous torque.

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