Abstract
Water droplet erosion has emerged as a reliability concern in steam turbines, resulting from exhaust flow recirculation in combination with the presence of down-stream moisture. Last-stage blades in Low-Pressure (LP) steam turbines interact with water spray systems in the condenser housing, which leads to blade trailing edge erosion. Such erosion can result in unplanned service outages, costly repairs or even blade liberation. It is for this reason that a computational fluid dynamics (CFD) model was developed to predict water droplet erosion for the last stage low pressure turbine blade in certain axial flow turbine configurations. In this study, Lagrangian multiphase physics were employed in a control volume representing an LP turbine configuration with an axial exhaust diffuser and short-coupled water-cooled condensers. Four critical, part load operating points were investigated which enable the conditions required for turbine water droplet erosion including active water spray systems and diffuser recirculation. First, a gaseous steam flow field is established, and then discrete particles representing water droplets are injected into the continuum. Particles are tracked as they redistribute throughout the domain, collide and breakup with boundary walls. An erosion wear model was implemented on blade surfaces of interest which included the influences of wall material, impact speed and incidence angle.
Blade surface erosion rate distribution and surface integrated erosion rates were calculated for each case. Qualitative comparisons have been made between blade impact erosion observed in the field and CFD simulations. Mitigation solutions to reduce blade erosion were also studied for relative change.
