Impulse Turbine. It may be defined as a system in which all steam expansion takes place in fixed nozzles and none occurs in pas­sages among moving blades.

A Single-stage or Simple-impulse Turbine. Here the steam expands from its initial to its final pressure in one nozzle (or one set of noz­zles, all working at the same pressure), resulting in a steam jet of high velocity which enters the blade passages and, by exerting a force on them due to being deflected in direction, turns the rotor.¹ Energy of all forms remaining in the steam after it leaves the single row of blading is lost.

The steam volume increases whenever the pressure decreases, but the resulting velocity changes depend on the type of turbine. As a matter of fact, these velocity changes are distinguishing characteristics of the different types.

A Velocity-stage Impulse Turbine has one set of nozzles, with sev­eral rows of blades following it. In passing from the nozzle exit through one set of blades, the velocity of the steam is lowered by virtue of the work done on the blades but is still high. It then passes through a row of fixed guide blades which change the direction of the steam until it flows approximately parallel to the original nozzle direction, discharging it into a second row of blading fixed to the same wheel. This second row again lowers the steam velocity by virtue of the work delivered to the wheel. A second set of guide blades and a third row  of moving blades are sometimes used.

The steam enters through a steam strainer and governor valve into a steam chest supplying the various nozzles spaced around a portion of the periphery.² Individual nozzles may be opened or closed by a hand-wheel on the stem of the nozzle-control valve. The turbine wheel is mounted on a shaft which passes through the casing to bearings out­side, carbon packing being used in the shaft glands of this turbine to maintain steam tightness. The governor is mounted on the right-hand end of the shaft and operates the balanced-piston governor valve through a lever and link. On the left end of the shaft goes the coupling for attaching the driven machinery.

A Pressure-stage Impulse Turbine of Three Stages. Each of a se­ries of chambers formed by parallel disc-shaped partitions called dia­phragms has a simple-impulse turbine inclosed in it, all wheels being fastened to the same shaft. Each chamber receives the steam in  turn through groups of nozzles placed on arcs,³ the last chamber discharg­ing to the condenser. The pressure drop is divided into as many steps as there are chambers,⁴ each being called a pressure stage. The result­ant steam velocity in each stage is relatively small, allowing reason­ably low blade velocities and preventing excessive loss by steam fric­tion. The steam pressure drops in each stage and the steam volume increases; the steam velocity is high at exit from the nozzles and is low at exit from the blades. This arrangement is sometimes termed a Rateau turbine, and the separate stages, Rateau stages.

Turbines with combinations of pressure and velocity staging are widely used and are of several sorts. The wheel in each pressure stage might have two (or even three) rows of blades instead of one. The turbine has as many pressure stages as there are wheels, and each pressure stage has as many velocity stages as there are rows of blades on the wheel in that stage. This arrangement results in a small, short, and cheap turbine, at more or less sacrifice of efficiency. Com­mercial turbines of this type are called Curtis turbines, after the orig­inal inventor, and the individual pressure stages, each with two or more velocity stages, are often called Curtis stages.

Reaction or Parsons Turbine. In the turbines so far described, the steam expands only in fixed nozzles and flows through passages between blades arranged in rows, transferring its kinetic energy to these rows of blades and causing them to rotate against resistance. In the widely used reaction turbine proposed and first built by Sir Charles Parsons, the steam decreases in pressure and expands while it is passing through the moving blades as well as in its passage through the fixed nozzles. There are no wheels, but blades are set in rows on the surface of a rotor in the form of a cone or in the form of a cylinder combined with one or more cones. In any of the forms, the rows of blades on the rotor pass between rows of similar blades (called vanes) set on the internal surface of the cylinder or casing. As the steam flows from inlet to exhaust it expands greatly; hence both moving blades on the rotor and fixed vanes on the stator increase in height from row to row and the diameters of the rows also progressively increase. The balancing pistons at the right and near the foreground are for the purpose of counterbalanc­ing the axial thrust due to steam pressure.

 Impulse-reaction Turbines. This is the usual combination in all central-station turbines except certain machines which handle very large quantities of steam, in which case the two-row wheel may be omitted. The principal reasons for using the less efficient two-row wheel in place of the appropriate number of single-row stages are that it shortens the turbine, decreases the portion of the cylinder exposed to high pressure and temperature, and offers certain advantages in connection with governing.

(Church E. F.)