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

RAMAJO DAMIAN; RAVICULE MARCELA; BENNINI ROBERTO; MARQUEZ DAMIAN, SANTIAGO; STORTI MARIO; NIGRO NORBERTO

Tandil

Congreso; ENIEF 2009; 2009

Asociacion Argentina de Mecanica Computacional (AMCA) Universidad Nacional del Centro

<!-- @page { margin: 0.79in } P { margin-bottom: 0.08in } --> In this work a computational fluid dynamics study (CFD) was carried out with the aim to diagnose and mitigate the causes of damage in a pipeline that transport reduced crude oil from the heater to the vacuum distillation tower in the YPF Petrochemical plant at Ensenada. The processed reduced crude oil is a two phase flow (liquid and vapor) with a high amount of naphthenic acids and sulphurs. The operation conditions (temperature, flow velocity, annular flow, presence of drops and maybe solid coke particles) become the pipeline susceptible to severe erosive-corrosive attack, so the vacuum equipment has been frequently shut down several times during the last years for reparation. Simulation of annular flow is very complex because of the liquid phase moves as a thin film over the duct wall (annular) dragged by the gas phase that flows at higher velocity though the centre of the duct (core). Due to flow instabilities the liquid pass from the annular film to the gas core and vise versa, so small drops are continuously entering to the fast gas flow. In this work the two phase flow was assumed as homogeneous (with mixed values for density, viscosity and velocity). By means of suitable correlations the mean size of drops and the exchange of mass from the annular ring to the gas core were estimated. Then, the Navier-Stokes equations were applied to solve the homogeneous mixture while the trajectory of drops was estimated by Lagrangian particle tracking. Two damage mechanisms were considered; 1-corrosion enhanced by drop impact erosion DIEC (ductile and fragile material behaviors were considered) and 2- corrosion enhanced by erosion due to the enhancement of mass transfer and the acceleration of the reaction as well as the rapid spallation of corrosion products from the metal surface (NAEC: naphthenic acid corrosion-erosion, SEC: sulphur corrosion-erosion). Numerical results allowed to find the particle impact regions (DIEC) and high wall shear stresses (NAEC and SEC), these being in agreement with the burst zones found in the pipeline. Nevertheless, there are no works reporting DIEC and scarce information about NAEC and SEC, so the uncertainty about the damage mechanisms made impossible to predict the erosion-corrosion rate. Due to this reason a comparative analysis was performed in order to propose geometric and operation changes to mitigate the damages. From CFD results, it was proposed to modify a part of the pipeline, replacing several elbows by only one curve, reducing the drop impacts and the wall shear stresses. On the other hand, due to the nature of the corrosive crude, a change on the duct material replacing the currently employed carbon steel by stainless steel (317 L or similar) was also recommended in order to reduce corrosive attack.