INTRODUCTION

The rotating blade is the steam path element that seems most susceptible to damage in normal operation. This is partially because both water and any mechanical debris impacting the blades will, because of the high tangential velocities of the rotating blades, develop sufficient force to possibly cause severe mechanical deformation. In addition, the stress levels in the rotating blades are so great that when combined with irregularities of steam flow, they create the possibility of causing dangerous levels of vibration. There are also discontinuities in the blade shape, creating a potential problem of stress concentration in many regions. These factors, together with the possible presence of aggressive chemical compounds and the probability of externally imposed shock loads, indicate the continual potential for failure or serious damage to these components, resulting in an interruption of service. Many millions of rows of blades are in service, and their failure is relatively rare. However, they do remain the component causing more outages than any other in the unit.

Some of the performance-deteriorating mechanisms do not require repair or replacement. However, they may require the turbine be cleaned. Even this upgrading action must be performed to accepted standards to prevent deterioration of the unit.

This chapter considers the damaging mechanisms and deteriorating performance (discussed in chapters 4 to 7), and any other phenomena, in terms of their ability to be repaired. In some instances these procedures are described in general terms. However, many of these repair and refurbishment techniques represent relatively new techniques,...