Power Plant Engineering

STEAM TURBINE 207

6.11 Capability

The capability of steam turbine is the maximum continuous out put for a clean turbine operating

under specified throttle and exhaust conditions with full extraction at any openings if provided.

The difference between capability and rating is considered to be overload capacity. A common

practice is to design a turbine for capability of 125% nominal rating and to provide a generator that will

absorb rated power at 0.8 power factor. By raising power factor to unity the generator will absorb the full

turbine capability.

6.12 Steam Turbine Governing

Governing of steam turbine means to regulate the supply of steam to the turbine in order to
maintain speed of rotation sensibly constant under varying load conditions. Some of the methods em-
ployed are as follows :
(i) Bypass governing. (ii) Nozzle control governing. (iii) Throttle governing.
In this system the steam enters the turbine chest (C) through a valve (V) controlled by governor.
In case of loads of greater than economic load a bypass valve (Vi) opens and allows steam to pass from
the first stage nozzle box into the steam belt (S).
In this method of governing the supply of steam of various nozzle groups N 1 , N 2 , and N 3 is
regulated by means of valves V 1 , V 2 and V 3 respectively.

In this method of governing the double beat valve is used to regulate the flow of steam into the

turbine. When the load on the turbine decreases, its speed will try to increase. This will cause the fly bar

to move outward which will in return operate the lever arm and thus the double beat valve will get

moved to control the supply of steam to turbine. In this case the valve will get so adjusted that less

amount of steam flows to turbine.

6.13 Steam Turbine Performance

Turbine performance can be expressed by the following factors :
(i) The steam flow process through the unit-expansion line or condition curve.
(ii) The steam flow rate through the unit.
(iii) Thermal efficiency.
(iv) Losses such as exhaust, mechanical, generator, radiation etc.
Mechanical losses include bearing losses, oil pump losses and generator bearing losses. Genera-
tor losses include will electrical and mechanical losses. Exhaust losses include the kinetic energy of the
steam as it leaves the last stage and the pressure drop from the exit of last stage to the condenser stage.

For successful operation of a steam turbine it is desirable to supply steam at constant pressure
and temperature. Steam pressure can be easily regulated by means of safety valve fitted on the boiler.
The steam temperature may try to fluctuate because of the following reasons :

(i) Variation in heat produced due to varying amounts of fuel burnt according to changing loads.

(ii) Fluctuation in quantity of excess air.

(iii) Variation in moisture content and temperature of air entering the furnace.