Progressing Cavity (PC) Pump Design Optimization for Abrasive Applications

Abstract

Abstract The progressing cavity (PC) pump is well established as the pump of choice for handling abrasive solids. PC pump design can be optimized to achieve the best wear performance available for a given size. The wear optimization of the PC pumps is achieved through geometric design for minimum internal fluid velocities and by selecting proper materials of construction. Wear causes PC pump failure by gradually reducing the volumetric efficiency and increasing pump slippage. This paper focuses on the parameters that influence pump wear, describes wear mechanisms, reviews design techniques for wear optimization, and presents field data to support some of the claims. Background Pump Design Parameters. Figure 1 shows the cross section of a progressing cavity pump. Definitions: Ps Stator Pitch D Rotor/Stator Minor Diameter Ecc Rotor Eccentricity A progressing cavity pump consists of a helical steel rotor which turns within a stationary tube with a helical elastomeric lining (stator). As the rotor turns inside the stator, fluid moves through the pump from cavity to cavity. As one cavity diminishes, the opposing cavity increases at exactly the same rate which results in a pulsationless positive displacement flow through the pump. The cavities are separated from each other by a series of seal lines which are created between the rotor and stator. The pressure capability of a pump is a function of the number of times the progressing seal lines are repeated. PC pump manufacturers rate the pressure capability of a pump as a function of the number of pump stages. Although somewhat arbitrary, each stage is between one to one and a half of a stator pitch length and is capable of handling 100 psi differential. If cavity pressure increases beyond the seal limits, the seal lines will open, and fluid will "slip" from one cavity to the other at a very high speed. The PC pump slippage is generally a function of pressure differential across the pump and it changes depending on the compression fit of the rotor and stator. Flow Rate. Pump flow rate is a function of design parameters such as stator pitch (Ps), rotor diameter (D), and pump eccentricity (Ecc). Equation 1, defines this function: Q = K*Ps*4*Ecc*D*N where: Q flow rate N number of revolutions per unit time K conversion factor Equation 1. Fluid Velocity. Nominally, for each rotation of the rotor, fluid will move one pitch length of the stator. Therefore, fluid nominal velocity in the axial direction of the pump is defined by Equation 2: Equation 2. Assumes that the fluid particles travel along a Vfluid= C*Ps * N Where: Vfluid nominal fluid velocity N = number of revolutions per unit time C = conversion factor Equation 2. P. 547^