Damage Mechanisms in Double-Screw Pumps for Heavy Crude Oil

This paper summarizes the surface damage mechanisms that were observed in four double-screw pumps used for the transfer of high viscosity crude oil. To study the surface morphology of the damaged pump-components, replicas were extracted and analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The observed morphological features were related to pumps operation conditions, flow characteristics and material defects.

Generally speaking, erosion-corrosion governs screw’s degradation; however the balance between chemical and mechanical damage varied according to local flow pattern and content of formation water, gas and solids. Along the screw, the thickness of the fluid film between threads and the inner surface of the pump case varies when excessive screw deflection occurs. Consequently, at screw ends, where the fluid film is relatively thick, intense corrosion by chlorides, contained in the associated water, develops, while in the strongly deflected middle zone, the fluid film thickness decreases up to a few microns, leading to abrasion with solid particles. Flow instability, mainly local gas compression - decompression, enhances the erosive attack with solid particles, at largely variable impingement angles. Surface imperfections in the Ni-Cr-B-Si coatings, deposited by plasma-transfer arc welding (PTAW) on thread’s face, initiate channeling, which is further oriented toward the flow direction. Deficient welding of the thin end-threads induces early fatigue failures, which occurred in two of the analyzed case.