Pressure Distribution in Progressing-Cavity Pumps: Performance and Run Life

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 153944, ’Pressure Distribution in Progressing-Cavity Pumps: Test Results and Implications for Performance and Run Life,’ by Evan Noble, SPE and Lonnie Dunn, SPE, Weatherford, prepared for SPE's Progressing Cavity Pumps 2011 Digital Edition. The full manuscript is available as a supplement to this article. Conventional thought has assumed that pressure builds linearly along the length of progressing-cavity pumps (PCPs). Rotor sizing and pump-lift selection have been based on this assumption. A series of laboratory tests investigated the buildup of pressure and temperature along the length of a PCP. The test results help explain common field-failure modes and can be used as a basis for selecting pump assemblies for challenging applications. Introduction Since being introduced to the oil field several decades ago, PCPs have been assigned a pump-pressure rating. This rating is the theoretical pressure difference at which a given PCP model can operate while maintaining acceptable performance and life. The pump-pressure rating, also called lift, generally is listed in terms of differential pressure across the entire pump and in terms of meters or feet of fluid head, psi, or kPa. Generally, a pump is selected with a higher pump-pressure rating than is required for the application, to provide a safety factor. The pump-pressure rating is based on the number of pitches in the pump and the cavity-pressure rating. The cavity-pressure rating is the theoretical pressure difference at which a sealed cavity is recommended to operate to ensure acceptable performance and life. The cavity-pressure rating is the difference between the pressure in a sealed upstream cavity and the pressure in the sealed cavity immediately downstream in the pump. The cavity-pressure rating is a manufacturer-established value and can range from 50 psi/pitch to more than 140 psi/pitch. Cavity-pressure ratings can vary with pump geometry, elastomer strength and design, and manufacturer preference. Cavity-pressure ratings assume that pressure builds inside the pump in a linear fashion, and the cavity-pressure rating is the rate of constant pressure buildup. To determine the pump-pressure rating, the cavity-pressure rating is multiplied by the number of pitches. To create a pump with a higher pump-pressure rating (also called increasing the lift), more pitches are added to the pump. All ratings and specifications that relate to pressure in a PCP are based on the cavity-pressure rating, which is an assigned value. The cavity-pressure rating is not modified or adjusted for rotor sizing, operating speed, or fluid viscosity. The cavity-pressure rating dictates the pump model rating, but it does not indicate the capability of the pump model.