Thus far we have considered predictable factors that influence the hot running axial and radial clearances within the steam path. These are factors the design engineer examines during the various stages of design, to the accuracy with which components can be set at cold stationary conditions. The goal is to enable the unit to operate with a minimum of risk of running interference, and the performance of each stage maximized (within the limitations set by mechanical considerations).

There are, however, two groups of considerations influencing the actual running clearances. The first have been considered; the second group is those factors that are unpredictable. These phenomena occur normally as a consequence of operating conditions, operating excursions, or major transients of inlet steam parameters. There are, however, other considerations to these unpredictable phenomena that occur as a consequence of stress relief, extended exposure to the high temperatures and pressures of the steam, and possibly other factors of design or manufacture.

Unpredictable phenomena can modify the shape of the components by various forms of permanent deformation, which as a consequence affects the initially selected dimensional settings. What is particularly difficult with these phenomena is that while the probability of their occurrence once or twice during the life of a unit is almost certain, and some allowance can be made for them, their extent is unpredictable. The severity of the deformation or the modifications that result is normally dependent upon the type and severity of the condition causing...