During operation, the stationary and rotating components of the turbine absorb and reject heat energy from the steam and expand and contract with the temperature changes this heat introduces. The rotating and stationary portions accept and reject this heat, both in different amounts and at different rates, depending upon their total mass and the extent to which they can radiate this heat to other media. Because these stationary and rotating components expand and contract by different amounts and at different rates, there will be a relative positional shift between them from the cold stationary position during normal operation. This difference becomes even more pronounced during transient conditions, when dramatic temperature changes and (in emergency conditions) heat generation can occur within the steam path due to frictional heating caused by rotor rotation in a partially evacuated or stagnant steam atmosphere.

Because they are anchored to the turbine foundation block at various locations, the casings normally one for each section will expand in an axial direction away from these anchor points. The extent of this expansion in any one casing is influenced only by the amount of heat absorbed, rejected, and retained by individual casings. The extent of expansion in any one casing is unaffected by other casings and components of the unit.

The rotating portions are solidly coupled and have a total movement from their one anchor point. The anchor point is the thrust bearing and the contact point at the active thrust face. The position of the thrust bearing is...