This paper summarizes the surface damage mechanisms that were observed in three double-screw pumps used for the transfer of high viscosity crude oil and their relations with pump operation conditions, flow characteristics and material defects. Erosion-corrosion governs the degradation of the screws; however the balance between chemical and mechanical damage varied according to local flow pattern and content of formation water, gas and solids. Along the screws, the thickness of the fluid film between threads and the inner surface of the pump case varies when excessive screw deflection occurs. Flow instability, mainly local gas compression - decompression, enhances the erosive attack with solid particles and intensify the fatigue inducing conditions. Contact areas between spindle and bearing or gears are corroded by the pumped fluid leaked through the mechanical seals, which viscosity is reduced due to the contained gas; clearances that developed in these corroded areas enhance fatigue.
Double-screw pumps have an increasing use in handling multiphase flow, mainly for less common hydrocarbon production and transportation processes. Multiphase flow may contain liquids with different viscosities, associated gas and solid particles, including several corrosive agents, such as chlorides, CO2 and H2S carried by liquid and gas phases. The erosive and corrosive flow intensively wears pump internal components, mainly the cylindrical flank of the thread that faces the pump casing, thus leading to pumping efficiency loss [1, 2]. An exhaustive literature review performed by O. Klein  in 2007 shows that few attempts have been done to describe the erosion-corrosion process that originates in the hydrodynamic film, which fills the gap between screw periphery and casing. The present work attempts to approach the fluid induced damage on screw flanks, in terms of processes developing in sliding bearings when worn under lubricant films of variable thickness [5, 6]. The axial variation of damage mechanisms along the screw length is interpreted in the same terms. This analysis regards geared twin screw pumps that are used for the transfer on surface of diluted heavy crude which is produced in a field of the Orinoco Belt in Venezuela. The characteristics of the pumped fluid are shown in the Table 1. It is worth noticing that the presence of associated gas was not prospected in the oil reservoir; the gas was intermittently passing trough pumps and transport pipelines as large bubbles, altering the vibration level of the system and affecting materials integrity. To mitigate these harmful effects, a gas separator, compressor and gas transfer pipeline were installed after the performance of this failure analysis. The drive and driven screw, which are the main components of the pump, are schematically illustrated in the Figure 1. Pumps spindles are made of AISI 8620 HSLA steel and the thread are coated with NiCrBSi wear and corrosion resistant alloy, with a nominal composition of( (wt %): 3 % C; 7.5 % Cr; 1.7 % B; 3.5 Si, Bal. Ni. The coating is deposited by plasma transfer arc plasma (PTAW) over an intermediate (bonding) layer of AISI 309 steel.