Kaplan turbine

Kaplan turbine

Working principle

Figure: Kaplan turbine

The water from the scroll casing flows over the guide vanes. It is deflected through 90° between guide vanes and runner. Then, it flows axially into the runner. The blades are shaped such that water flows axially in the runner. The force exerted on the blades causes the runner shaft to rotate. This rotation is transmitted to the generator which is couple to the runner shaft. After passing through the runner, the water enters the tailrace through a draft tube.

Governing of Kaplan turbine

A Kaplan turbine has adjustments in both guide vanes and runner vanes. Generally, double regulators are provided in a Kaplan turbine. The governor regulates the guide blade opening as well as the runner vane angles simultaneously. The adjustment of guide vanes is similar to that of a Francis turbine. The runner vanes are regulated by a separate Servomotor. The control valves for both the runner and guide vanes are interconnected to ensure a define runner vane angle for the given vane opening.

Figure: Governing of Kaplan turbine.

The runner vane angles may be adjusted while the turbine is in motion. The piston rod of the servomotor (of the runner vanes) pass through the hollow turbine shaft as shown in figure. The movement of the piston is transmitted to the runner of the piston is transmitted to the runner vane by a small crank connected to cross head. The servomotor acts as the coupling between the turbines shaft and the generator shaft. Oil from the governor is admitted to the upper or lower side of the servomotor piston through pipes. This will reduce or increase the blade angle. The governor actuates simultaneously both the guide vane and the runner vane. Thus for all loads, the turbine is able to maintain high efficiency.

Compare Impulse and reaction turbines

S.No Impulse turbine

1. Head:

The machine is suitable for high installation. (H=100 + 200 m).

2. Nature of input energy to the runner:

The   nozzle   converts   the   entire hydraulic  energy  into  kinetic energy before water strikes the runner.

3. Method of energy transfer: 

The  buckets  of  the  runner  are  so  shaped that they extract almost all the  kinetic energy of the jet.

4. Operating pressure:

 The turbine works under atmospheric  pressure.   Which  is  the  difference  between the  inlet  and exit points of  the runner.

5. Admission of water to the wheel:

 Only a few buckets comprising a part  of the wheel are exposed to the water  jet.

6. Discharge:  They  are  essential  low  discharge turbines.

7. Speed  of  operation:  The  speed  are  invariably high.

8. Size : These are generally small size.

9. Casing: It prevents splashing of water.  It has no hydraulic function to serve.

10. Turbine  setting:  The  head  between the wheel and race is lost.

11. Maximum  efficiency:  The  highest  efficiency (=88%) is less than that of  reaction turbine.

12. Part load operation: From about 20%  to   100%   of   design   output,   the  efficiency  remains  nearly  the  same.  Hence  the  machine  is   ideal   for  generating  small  loads  over  long  periods of time.

13. Cavitation:  These  machine  are  not  susceptible to cavitation.

14. Civil engineering works: Civil works  like  excavation  and  concreting  are  much simpler and economical.

S.No Reaction turbine

1. Reaction turbine The machines can be used for medium heads (H=50 to 500 m) and low heads (less than 50 m)

2. The head is usually inadequate to produce high velocity jet.  Hence  water  is  supplied  to  the runner  in  the  forms  of  both  pressure  and kinetic energy.

3. The wicket gates accelerate the flow a little and direct  the  water  to  runner  vanes  to  which energies of water are transferred.

4. The runner works is a closed system under the action of reaction pressure.

5. The entire circumference of the wheel receives water  and  all  passages  between  the  runner blades are always full of water.

6. Since power is a product of head and weight of the rate of flow, these turbines consume large quantities  of  water  in  order  to  develop  a  reasonable power under a relatively low head.

7. Although the specific speeds of these turbines is  high,  their  actual  running  speeds  are comparatively low.

8. The turbines sizes is much larger than impulse wheels,  in  order  to  accommodate   heavy discharge.

9. The spiral casing has an important role to play; it   distributes   water   under   the   available pressure uniformly around the periphery of the runner.

10. The draft tube ensures that the head of water below tail race level is not lost.

11. The  maximum  efficiency  (=95%)  of  design output is higher than that of impulse wheels.

12. With  the  exception  of  a  Kaplan  turbine,  all reaction   turbines   give   poor   part   load performance i.e., appreciably low efficiency at less than design output.

13. Runner  blades  and  draft  tube  invariably undergo cavitation on damage.

14. Civil works are more expensive on account of spiral casing and draft tube.