INTRODUCTION

Within the turbine steam path and for major components of the turbine, many forms of both static and dynamic loading introduce mechanical stress. In addition, many components that operate at high stress levels do so continuously at elevated temperatures, which while they are sensibly constant, will under certain conditions change at rates capable of causing changes within the material.

Under such conditions of load, temperature, and temperature variation, these components are, or can be subject to various forms of material property degradation and material life consumption. The design process evaluates these conditions and attempts to define the individual components so they are suitable for operation with the stresses induced by the operating conditions, enabling them to endure for the anticipated life of the unit.

However, these phenomena are capable of damage to the extent that component failure occurs. Circumstances can occur during operation where the unit is operated beyond its design limits. These circumstances represent conditions under which stresses are induced beyond those anticipated. Similarly, material properties inferior to the design specification can be introduced. While failure may not be instantaneous, there is an accumulation of damage that ultimately forces the unit from service and requires corrective action, either immediately or for a future outage.

Steam turbines are intended and designed to operate for considerable periods possibly between six and eight years between major inspection outages. Therefore, the internal portions of the unit, including the steam path "hot sections" will not be available for damage assessment and correction between these...