In assessing dimensional and tolerance requirements of the steam path, it is necessary to consider the objectives of the blade system, relate these objectives to principal dimensions, and then consider their possible influence on various performance criteria. This process establishes blade geometry and certain radial alignment requirements, assists in the selection of various elements, and provides guidance in selecting arrangements suited to achieving these concurrent objectives.

In designing systems to achieve these two objectives, sufficient information must be generated concerning mechanical loads and thermal conditions within the steam path, so component selection is facilitated. Other concerns such as stress levels within the local environment and frequency effects, can also be investigated. After components have been selected and sized, manufacturing techniques are chosen and tolerances applied to ensure a satisfactory product.

For the purpose of this section, the information that the design process should generate and specify (and the owner should be familiar with) concerns the relationships existing at various locations and areas, and the rates at which it varies throughout the steam path. In defining the form the various components take, the designer selects or defines tolerances within which the individual components must be produced, how they are assembled, and the tolerances they provide to ensure area requirements are met.

The area relationships within the steam path that should be considered are:

• The area of the passage formed between stationary and rotating blade profiles, and its variation from inlet to discharge

• The discharge area of individual...