Fig. [26] is a longitudinal cross-section cut of rotor and both lower and upper casing. Referring to Fig. [26] the steam comes in from the steam-pipe at C and passes through the main throttle or regulating valve D, which is a balanced valve operated by the governor. Steam enters the cylinder through the passage E.
Turning in the direction of the bearing A, it passes through alternate stationary and revolving rows of blades, finally emerging at F and going out by way of G to the condenser or to atmosphere. H, J, and K represent three stages of blading. L, M, and Z are the balance pistons which counterbalance the thrust on the stages H, J, and K. O and Q are equalizing pipes, and for the low-pressure balance piston similar provision is made by means of passages (not shown) through the body of the spindle.
R indicates a small adjustable collar placed inside the housing of the main bearing B to hold the spindle in a position where there will be such a clearance between the rings of the balance pistons and those of the cylinder as to reduce the leakage of steam to a minimum and at the same time prevent actual contact under varying temperature.
At S and T are glands which provide a water seal against the inleakage of air and the outleakage of steam. U represents the flexible coupling to the generator. V is the overload or by-pass valve used for admitting steam to intermediate stage of the turbine. W is the supplementary cylinder to contain the low-pressure balance piston. X and Y are reference letters used in text of this chapter to refer to equalizing of steam pressure on the low-pressure stage of the turbine. The first point to study in this construction is the arrangement of "dummies" L, M, and Z. These dummy rings serve as baffles to prevent steam leakage past the pistons, and their contact at high velocity means not only their own destruction, but also damage to or the wrecking of surrounding parts. A simple but effective method of eliminating this difficulty is found in the arrangement illustrated in this figure. The two smaller balance pistons, L and M, are allowed to remain on the high-pressure end; but the largest piston, Z, is placed upon the low-pressure end of the rotor immediately behind the last ring of blades, and working inside of the supplementary cylinder W. Being backed up by the body of the spindle, there is ample stiffness to prevent warping. This balance piston, which may also be plainly seen in Fig. [25], receives its steam pressure from the same point as the piston M, but the steam pressure, equalized with that on the third stage of the blading, X, is through holes in the webs of the blade-carrying rings. Entrance to these holes is through the small annular opening in the rotor, visible in Fig. [25] between the second and third barrels. As, in consequence of varying temperatures, there is an appreciable difference in the endwise expansion of the spindle and cylinder, the baffling rings in the low-pressure balance piston are so made as to allow for this difference. The high-pressure end of the spindle being held by the collar bearing, the difference in expansion manifests itself at the low-pressure end. The labyrinth packing of the high-pressure and intermediate pistons has a small axial and large radial clearance, whereas the labyrinth packing of the piston Z has, vice versa, a small radial and large axial clearance. Elimination of causes of trouble with the low-pressure balance piston not only makes it possible to reduce the diameter of the cylinder, and prevent distortion, but enables the entire spindle to be run with sufficiently small clearance to obviate any excessive leakage of steam.
Detail of Blade Construction
In this construction the blades are cut from drawn stock, so that at its root it is of angular dovetail shape, while at its tip there is a projection. To hold the roots of the blades firmly, a foundation ring is provided, as shown at A in Fig. [27]. This foundation ring is first formed to a circle of the proper diameter, and then slots are cut in it. These slots are accurately spaced and inclined to give the right pitch and angle to the blades (Fig. [28]), and are of dovetail shape to receive the roots of the blades. The tips of the blades are substantially bound together and protected by means of a channel-shaped shroud ring, illustrated in Fig. [31] and at B in Fig. [27]. Fig. [31] shows the cylinder blading separate, and Fig. [27] shows both with the shrouding. In these, holes are punched to receive the projections on the tips of the blades, which are rivetted over pneumatically.
