Power Plant Engineering

STEAM TURBINE 203

mounted on the same shaft and the blades are attached on the rotor. The rotor (i.e. disc) may be keyed to

the shaft or it may be integral part of shaft.

The expansion of steam only takes place in the nozzles while pressure remains constant in the

moving blades because each stage is a simple impulse turbine. So it is obvious from the pressure curve

that the space between any two consecutive diaphragms is filled with steam at constant pressure and the

pressure on either side of the diaphragm is different. Since the diaphragm is a stationary part, there must

be clearance between the rotating shaft and the diaphragm. The steam tends to leak through this clear-

ance for which devices like labyrinth packings, etc. are used.

Since the drop in pressure of steam per stage is reduced, so the steam velocity leaving the nozzles

and entering the moving blades is reduced which reduces the blade velocity. Hence for good economy or

maximum work shaft speed is significantly reduced so as be reduced by increasing the number of stages

according to ones need. The leaving velocity of the last stage of the turbine is much less compared to the

de Laval turbine and the leaving loss amounts to about 1 to 2 percent of the initial total available energy.

This turbine was invented by the late prof L. Rateau and so it is also known as Rateau Turbine.

6.6 Simple Velocity-Compounded Impulse Turbine

In this type of turbine, the compounding is done for velocity of steam only i.e. drop in velocity is
arranged in many small drops through many moving rows of blades instead of a single row of moving
blades. It consists of a nozzle or a set of nozzles and rows of moving blades attached to the rotor or
wheel and rows of fixed blades attached to casing as shown in Fig. 6.8.

The fixed blades are guide blades which guide the steam to succeeding rows of moving blades,
suitably arranged between the moving blades and set in a reversed manner. In this turbine, three rows or
rings of moving blades are fixed on a single wheel or rotor and this type of wheel is termed as the three
row wheel. There are two blades or fixed blades placed between Lint first and the second and the second
and third rows of moving blades respectively.

Steam chestpressure

Inlet velocity

Guide bladesMoving bladesLost velocityCondenser pressure

Moving bladesGuide bladesMoving blades

Nozzle

Bearing

Shaft

Steam

entering

Nozzle

Moving blades

Guide bladesCasing

Exhaust

steam

Rotor wheel

Fig. 6.8. Velocity Compounded Impulse Turbine.