TURBINE SPEED REDUCING METHODS AND GOVERNING SYSTEM

Steam turbines compounding methods :

In order to increasing thermal efficiency of power plant, high pressure and high temperature steam will be used in plant . If total pressure drop carried out in one stage only, then the velocity of steam entering into the turbine will be extremely high. It will make the turbine rotor to run at a very high speed  ( up to 3000 rpm ) .In order to reduce the rotor speed , some types of methods are employed .All of these methods consist of multiple system of rotors, in keyed to a common shaft the steam pressure or the jet velocity is absorbed in stages as it flows over the rotor blades,this process is known as compounding ,the following three methods are commonly employed for reduce rotor speed.

1. Velocity compounding
2. Pressure compounding 
3. Pressure -velocity compounding  

Velocity compounding : In this an impulse turbine , the expansion of steam takes place in nozzle form the boiler pressure to condenser pressure . The impulse wheel carries two or three rows of moving blades 

velocity compounding

No pressure drops occurs either in fixed or moving blades . All pressure drop occurs in nozzles only . this turbine can run at about one third of speed of De-Laval turbine , for the same pressure drop and diameter of wheel 

Pressure compounding : in this an impulse turbine , this rings moving blades ,each having a ring of fixed nozzle are keyed to turbine shafts in series , The pressure drop of the steam , does not take place in the first nozzle ring , but is divided equally among all the nozzle ring  
In this arranging by small pressure drop per stage , the velocity of steam entering the moving blades and hence the speed of the rotor is reduced 

Pressure compounding

Pressure and velocity compounding : In this both of two methods are utilized.The total pressure drop divided into to parts. it may be noted that the diameter of the turbine is increased at eah stage, to allow the increasing volume of steam at the lower pressures A ring of the nozzle is fixed at  the commencement of each stage.
 

Pressure and velocity compounding

Governing of steam turbines : 

Function of governor to maintain its speed constant irrespective of flow whenever there is reduction of load, at the turbine , this balanced throttle valve of arrangement  is operated by servomotor controlling by a centrifugal governor.

Types of governing methods :

1. Throttle governing 
2. Nozzle control governing 
3. By- pass governing