Trailing Edge Erosion Prevention in Steam Turbine Last Stage Rotating Blades

Abstract

Abstract In recent years, the operational requirements for steam turbines used for heat and/or power applications have changed significantly. Especially, highly flexible operation regimes and extended operating ranges are required including frequently requested start-stop cycles and “parking” of units at low load conditions. Here, low load operation potentially comes with a set of availability relevant aspects for the low-pressure steam turbine’s last stage rotating blade row such as pronounced blade vibrations at windage or trailing edge erosion. Unlike the well-understood leading-edge erosion of last stage rotating blades in steam turbines, trailing edge erosion is a phenomenon which in the past was nearly exclusively known from steam turbines for district heating power plants and some industrial applications. With the intensified flexible operation of power plants, however, trailing edge erosion is more likely to be observed also for non-extraction units. It is assumed that trailing edge erosion occurs during windage operation, when the flow through the last stage is low enough such that the recirculation vortex in the exhaust area transports moisture back towards the trailing edge of the last stage rotating blade. The subsequent water impingement eventually may cause material degradation [1] at a mechanically loaded area of the blade’s foil above the root plate. Sources of moisture can be, e.g., inherent steam wetness or water injection systems. To enable an estimation of trailing edge erosion susceptibility for certain applications and load conditions, an evaluation scheme is proposed in the present paper. Its objective is to indicate and detect the built-up of trailing edge erosion early enough to enable a timely and pre-emptive condition-based service intervention and/or to advise a change in operation mode, as applicable. The developed evaluation scheme is subsequently implemented into a Combined Heat and Power Plant featuring an extraction machine. The resulting inspection results substantiate the potential trailing edge erosion prevention capability of the developed scheme.