FIG. 37
It is not necessary that the blades shall run close together, and the axial clearance, that is the space lengthwise of the turbine between the revolving and the stationary blades, varies from 1/8 to 1/2 inch; but in order that there may not be excessive leakage over the tops of the blades, as shown, very much exaggerated, in Fig. [38], the radial clearance, that is, the clearance between the tops of the moving blades and the casing, and between the ends of the stationary blades and the shell of the rotor, must be kept down to the lowest practical amount, and varies, according to the size of the machine and length of blade, from about 0.025 to 0.125 of an inch.
In the passage A (Fig. [34]) exists the initial pressure; in the passage B the pressure after the steam has passed the first section or diameter of the rotor; in the passage C after it has passed the second section. The pressure acting upon the exposed faces of the rows of vanes would crowd the rotor to the left. They are therefore balanced by pistons or "dummies" P P P revolving with the shaft and exposing in the annular spaces B1 and C1 the same areas as those of the blade sections which they are designed to balance. The same pressure is maintained in B1 as in B, and in C1 as in C by connecting them with equalizing pipes E E. The third equalizing pipe connects the back or right-hand side of the largest dummy with the exhaust passage so that the same pressure exists upon it as exists upon the exhaust end of the rotor. These dummy pistons are shown at the near end of the rotor in Fig. [35]. They are grooved so as to form a labyrinth packing, the face of the casing against which they run being grooved and brass strips inserted, as shown in Fig. [39]. The dummy pistons prevent leakage from A, B1 and C1 to the condenser, and must, of course, run as closely as practicable to the rings in the casing, the actual clearance being from about 0.005 to 0.015 of an inch, again depending on the size of the machine.