Diagnose and Prediction of Erosion-Corrosion Damage in a Pipeline Transporting Reduced Crude Oil from the Heater to the Vacuum Distillation Tower

Abstract

In this work a computational fluid dynamics study (CFD) was carried out with the aim to diagnose and mitigate the causes of damage in the pipeline that transport reduced crude oil from the heater to the vacuum distillation tower in the YPF Petrochemical plant at Ensenada. The reduced crude oil is mainly a two phase flow (liquid and vapor) that contains significant amounts of naphthenic acids. The operating conditions (temperature, flow velocity, annular flow, presence of drops and possibly solid coke particles) become it susceptible to erosive-corrosive attack, being the distillation tower frequently shut down. The two phase flow was assumed as a homogeneous mixture and the drop-mean size and the net mass exchange from the annular film to the gas core were estimated. The NavierStokes equations and the Lagrangian particle tracking were applied to solve the mixture and the drop trajectories. Two damage mechanisms were investigated: 1- mechanical pick-up of material by drop-impact erosion enhanced by corrosion (DIEC) 2- loss of material by corrosion enhanced by relatively high flow velocity (NAEC). Numerical results allowed identified regions of impact of drops (DIEC) and high wall shear stresses (NAEC) take place, these being in good agreement with the burst zones found in the pipeline. Nevertheless, due to the fact that there are no works reporting DIEC and scarce information about NAEC, the uncertainty about the damage mechanisms made impossible to accurately predict the erosion-corrosion rate. So, a comparative analysis in order to propose geometric modifications and operating condition changes to mitigate the damages was performed. It was proposed to modify a part of the pipeline, replacing several elbows by one large radius curve, reducing drop impacts and wall shear stresses. On the other hand, due to the corrosive composition of the reduced crude, a change on the duct material, replacing the currently carbon steel by stainless steel (317 L or similar), was also recommended based on bibliographic data.