Before considering the individual elements that determine performance of the steam path, it is first necessary to consider its definition, and review those elements that comprise and influence it. The factors of efficiency and availability were briefly discussed in the introduction to this chapter. However, in that discussion no effort was given to defining or quantifying those factors or to discussing those factors that affect the steam path's overall contribution to the total performance of the unit.

The steam path can be defined as the annular space bounded by imaginary lines extending axially through the inner and outer circumferential surfaces of the stationary and rotating blade passages. For completion, this should also include the attachment portion of these elements and paths through which the steam/water can expand or leak.

Figure 3-1 shows an expansion of the requirements of performance, efficiency, and availability. It can be seen that the basic considerations of these requirements can be further divided to provide a fuller description. In the case of efficiency, there are both short- and long-term factors, and, in the case of availability, there are important considerations that go into the original selection and design of the components to aid in the effective maintenance of the unit.

Figure 3-1: Factors Defining Performance of the Steam Turbine Unit

Efficiency is of importance in terms of both its original or initial value, which represents the design predicted performance, and the value which is most nearly measured in a performance test, and its...