TABLE OF CONTENTS During operation, the stationary blade rows are subjected to a pressure differential from inlet to discharge sides. This pressure differential is applied to impulse and reaction designed stages. However, because the pressure drop across an impulse stage is much higher, and because the inner surface of the stationary blade or vane is normally attached to the inner web of a diaphragm, the consequences of this condition can be much more severe on these impulse stages. Both impulse and reaction stages can also be operating at high temperatures, which modifies the mechanical strength of the material.
Two stationary blade cases must be considered in terms of the amount of axial deflection: the diaphragm type stages (as used in impulse designs) and some low-pressure stages, which have high levels of reaction on them. There is also the need to consider the stationary blading of high reaction stages, where blade roots are assembled into a blade carrier or casing.
The high pressure, high-temperature diaphragm consists of an outer ring, located either in a groove produced on the inner surface of the casing or on a ligament carried from the casing. Because of this location, the outer ring cannot deflect. However, the blade row and the inner web develop high pressures across them, and will deflect axially.
This effect is shown in Figure 2.6.1. The pressure drop across the vanes will deform by some amount shown as "dxv" and the inner web which acts as...
