Velocity guidelines for preventing pitting of carbon steel piping when the flowing medium contains CO2 and sand

Abstract

In carbon steel piping systems handling fluids containing water and dissolved CO2, under a range of conditions, an FeCO3 (iron carbonate) scale can form on the pipe wall and provide a measure of protection against CO2 corrosion. If the fluid also contains sand, particles of sand impinging the pipe wall can remove the protective scale or prevent it from adhering to the pipe wall. Without the scale to protect the pipe, CO2 corrosion can progress at an accelerated rate. This process is referred to by many as "erosion-corrosion". Whether erosion-corrosion occurs and how severe it will be depend on environmental factors such as pH and temperature, and on how erosive the conditions are, i. e., the "erosivity". Factors that determine erosivity include fluid properties, flow parameters, and sand conditions. For other factors held constant, when flow velocities are low, FeCO3 scale provides protection for all pipe surfaces. In high velocity flows, scale is not able to adhere anywhere on the pipe wall, and CO2 corrosion proceeds uniformly over the surface of the pipe. But, at intermediate velocities, pitting can occur at sand particle impingement points resulting in extremely high penetration rates. Flow velocities defining the boundaries between these three erosion-corrosion behaviors are called "threshold velocities". In previous work, threshold velocities were determined for laboratory and representative oilfield conditions. In this paper, threshold velocities are determined and compared for conditions with and without a commercial imidazoline type inhibitor. Threshold velocities presented are based on experimental results from a flow loop circulating water, oil, inhibitor and sand and on a computer program for predicting sand erosion.