The first is necessary in order to make it possible to limit the leakage area through which steam can escape, that is to say, to enable the two surfaces to be run close up to one another at suitable points, without danger of irreparable damage, in the event of accidental contact.
The second is necessary in order to limit further the leakage by compelling the steam to flow in a tortuous path.
In another well-known form of packing, which is said to have been originated much later in Germany, rings of graphite or car bon are placed round the shaft. Each ring is cut up into several segments, and all are held together by a "garter" spring. A pin, projects into the fixed turbine casing on one side in order to pre vent the carbon ring being carried round with the rotating shaft. The carbon ring does not actually seal the shaft against all leak age ; if it pressed sufficiently hard on the shaft to do so it would rapidly become overheated, as explained above. In practice, the length of the individual segments is made such that when they are pressed together butt and butt by the garter spring they are just clear of the shaft, and a thin film of leakage steam separates them from actual contact.
Parsons' Compound Reaction Steam Turbine.—Although Parsons' master principle of pressure compounding has since its introduction been applied to all types of modern efficient turbines, there is another distinctive feature of design which he introduced in 1884 and which renders his steam turbine different from all others, with the one exception of the Ljungstrom (1912). This feature (see fig. 7) is the use of blades which give a nozzle shape to the spaces between them in both the fixed and rotating ele ments of the turbine (fig. 8). In the modern "reaction" tur bine, in fact, the efficiency of the "blading" is practically equal to that of well-formed nozzles. It has therefore a higher intrinsic efficiency than "impulse" blading, because in the latter there are losses due to friction in the moving blades in addition to the losses in the nozzles.
As will be made evident later, in a Parsons reaction turbine, the function of the fixed nozzles is to bring the steam jets smoothly into the moving nozzles and the turbine is driven or rotated al most entirely by the reaction of the steam jets issuing from the moving nozzles. Hence the name "reaction turbine." This reaction is analogous to that of machine guns. The moving blades them selves in a reaction turbine rep resent the guns which receive a continuous backward "kick" or "reaction" from the shots fired from them, causing them to recoil in the opposite direction.
The most modern form of Parsons reaction blading however is shown in fig. I°. This is known as "end-tightened" or axial clearance reaction blading. It will be seen that the clearance in the radial direction is large, giving mechanical safety. The blades are fitted with shrouding having an overhanging sharp edge in close proximity to the roots of the blades of the adjacent row. The axial clearances between the edge of the shrouding and the blade roots are adjustable by endwise motion of the rotor, and be cause of the positive nature of the control of the end-position of the "rotor" by the thrust block, the axial clearances can be made small with greater safety.
Another feature of reaction blading, due to the pressure drop over the moving blades, is the evident existence of an unbal anced steam thrust on every row of the latter. There is also a steam pressure gradient along the rotor so that this also contributes to a resultant steam thrust on the rotor and its blading of considerable magnitude in a direction down the turbine, i.e., towards the exhaust end. This resultant thrust may amount to several tons and may be too heavy to carry entirely on the thrust bearing. Parsons therefore uses dummy pis tons to balance the end thrust by receiving steam pressure in the opposite direction. One such piston could be made to balance the whole, but in order to ensure proper control of the end pres sure at all loads and under all conditions it is usual to use two or more dummy pistons, each connected to that portion of the rotor blading which it balances, by means of large bore equalizing pipes. Sometimes the low pressure dummy piston is omitted and the residual end thrust carried by the thrust bearing. This prac tire saves the steam leakage past the low pressure piston and thus improves the economy.