Blade vibration triggered by low load and high back pressure

Abstract

Load and condenser pressure play an important role in overall life of turbine blades especially in the last stages of low pressure (LP) turbine. LP last-stage blades have greatest potential to stall flutter during low steam flow and high backpressure. Exhaust loss refers to the energy lost to the condenser as a result of the steam velocity going into the condenser. The faster the steam leaves the last stage blades, the greater the energy loss. Since the backpressure is a function of the plant cooling system, and the flow is a function of the initial plant design, the only cost-effective way to reduce the velocity is by increasing the area. This is achieved by increasing the length of last stage bucket.The two primary forces acting on the blades are the steady centrifugal force due to rotation and the fluctuating steam bending force. The best estimation of dynamic stresses in the blades due to the two known sources of vibration works out to be a small portion of the allowable value. But still, incidences of blade failure are regularly reported. Published reports on compilation of blade failures in power plants say that cause for large number of failure is not fully understood. One such cause on which very little is published is self excitation in long blades. Self excitation sustain until the condition remain conducive to vibration inside the turbine. In plants operating at low load and high back pressure the blades are most susceptible to self excitation wherein the blade vibrates in its lower mode. The paper deals with monitoring self excitation in an operating power plant. A Single long blade has been modeled to steady fluid structure interaction by Lagrangian and Eulerian meshing approach during low load and high back pressure. The effect of the steam inlet angle on the blade has been studied. The model simulates low steam flow condition to show increased blade response thus validating observation made in operating power plant.