The Pelton wheel is an impulse turbine

Pelton wheel

 The Pelton wheel is an impulse turbine. In the Pelton wheel, the pressure energy of water is fully converted into kinetic energy by passing water through the nozzle. This forms a high-velocity jet that is used for the driving wheel by changing its velocity on the buckets. A Pelton turbine consists of a rotor. On the periphery of the rotor are mounted equally spaced buckets that have either double hemispherical or double ellipsoidal shapes. Water is brought down from a high head source like a dam through a pipe system, Which is called a penstock. A nozzle is provided at the end of the penstock which converts the potential energy of high head water into a high-velocity water jet. The high-velocity water jet emerging from the nozzle impinges on the buckets of the Pelton wheel and it sets the wheel in Function of the casing of a Pelton turbine is…..

 (i) to prevent the splashing of water; 

(ii) to discharge water to tail race; 

 (iii) to safeguard against any accident due to rotating buckets. 

The casing of the Pelton wheel does not perform any hydraulic function as it is exposed to the atmosphere and it does not have the shape of the tapered draft tube as in the reaction turbine. The draft tube in the reaction turbine has a hydraulic function to increase the pressure of discharge water and this hydraulic function is unnecessary in an impulse turbine.

       The shape of the bucket is as shown in Fig. The water jet emerging from the nozzle hits the splitter of the bucket symmetrically. The jet after striking the splitter is equally distributed into two halves of the hemisphere on the bucket. The inlet angle of the jet is assumed to be zero. The ideal angle of deflection of the jet is 180°. This deflection of the jet cannot be achieved in practice as the jet leaving the bucket would strike the back of the wheel will decrease, thereby decreasing the efficiency of the wheel. Hence, the angular deflection of the jet in the bucket is limited to about 160° to 170°. The bucket is therefore shaped slightly smaller than a hemisphere in size.

The speed of motion of the wheel depends upon the flow rate and the velocity of the water. The flow rate of water is controlled by means of a needle in the nozzle as shown in Fig. The turbine operates most efficiently when the wheel rotates at half the velocity of the jet. When the load on the wheel suddenly reduces, then the jet deflector moves up and partially diverts the jet issuing from the nozzle until the jet needle moves forward and appropriately reduces the jet as shown in Fig. This arrangement of reducing the size of the jet is necessary because in the event of sudden fall of load requirement if the jet needle is moved forward in the nozzle to reduce the flow of water hammer phenomenon in the penstock. In practice, the control of the jet deflector is linked to the electric generator coupled which the turbine. The complete potential energy available in the water is converted into kinetic energy before the water jet strikes the buckets on the runner. The pressure on the buckets in the turbine is atmospheric pressure which does not change. Hence, energy transfer occurs purely on the buckets by the impulse action.

 

When the water is completely closed with the help of the needle in the nozzle, the runner of the turbine keeps on rotating due to its inertia for a long time even when no jet is striking the buckets. In order to stop the runner in a short time, a brake nozzle is provided to stop the rotation of the wheel by directing a small braking jet on the buckles in the opposite direction as shown in Fig. Pelton wheel is an impulse turbine. In the Pelton wheel, the pressure energy of water is fully converted into kinetic energy by passing water through the nozzle. This forms a high-velocity jet that is used for driving the wheel by changing its velocity on the buckets. A Pelton turbine consists of a rotor. On the periphery of the rotor are mounted equally spaced buckets which have either double hemispherical or double ellipsoidal shape. Water is brought down from a high head source like dam through a pipe system,Which is called penstock. A nozzle is provided at the end of the penstock which converts the potential energy of high head water into a high-velocity water jet. The high-velocity water jet emerging from the nozzle impinges on the buckets of the Pelton wheel and it sets the wheel in Function of the casing of a Pelton turbine is,,…

 (i) to prevent the splashing of water; 

(ii) to discharge water to tail race; and

 (iii) to safeguard against any accident due to rotating buckets. 

The casing of the Pelton wheel does not perform any hydraulic function as it is exposed to atmosphere and it does not have the shape of tapered draft tube as in reaction turbine . The draft tube in reaction turbine has hydraulic function to increase the pressure of discharge water and this hydraulic function is unnecessary in an impulse turbine.

       The shape of the bucket is as shown in Fig The water jet emerging from the nozzle hits the splitter of the bucket symmetrically. The jet after striking the splitter is equally distributed into two halves of hemisphere on the bucket. The inlet angle of the jet is assumed to be zero. The ideal angle of deflection of the  jet  is 180°. This deflection of the jet cannot be achieved in practice as the jet leaving the bucket would strike the back of the wheel will decrease, thereby decreasing the efficiency of the wheel. Hence, the angular deflection of the jet in the bucket is limited about 160° to 170°. The bucket is therefore shaped slightly smaller than a hemisphere in size.

The speed of motion of the wheel depends upon the flow rate and the velocity of water. The flow rate of water is controlled by means of a needle in the nozzle as shown in Fig. The turbine operates most efficiently when the wheel rotates at half the velocity of the jet. When the load on the wheel suddenly reduces, then the  jet deflector moves up and partially diverts the jet issuing from the nozzle until the jet needle moves forward and appropriately reduces the jet as shown in Fig.  This arrangement of reducing the size of the jet is necessary because in the event of sudden fall of load requirement if the jet needle is moved forward in the nozzle to reduce the flow of water  hammer phenomenon in the penstock. In practice, the control of the jet deflector is linked to the electric generator coupled which the turbine . The complete potential energy available in the water is converted into kinetic energy before the water jet strikes the buckets on the runner. The pressure on the buckets in the turbine is atmospheric pressure which does not change. Hence, energy transfer occurs purely on the buckets by the impulse action.

When the water is completely closed with the help of the needle in the nozzle, the runner of turbine keeps on rotating due to its inertia for a long time even when no jet is striking the buckets. In order to stop the runner in a short time, a brake nozzle is provided to stop the rotation of the wheel by directing a small braking jet on the buckles in the opposite direction as shown in Fig. Pelton wheel is an impulse turbine. In pelton wheel, the pressure energy of water is fully converted into kinetic energy by passing water through the nozzle. This forms a high-velocity jet that is used for driving the wheel by changing its velocity on the buckets. A Pelton turbine consists of a rotor. On the periphery of the rotor are mounted equally spaced buckets that have either double hemispherical or double ellipsoidal shapes. Water is brought down from a high head source like dam through a pipe system, Which is called penstock. A nozzle is provided at the end of the penstock which converts the potential energy of high head water into a high-velocity water jet. The high-velocity water jet emerging from the nozzle impinges on the buckets of the Pelton wheel and it sets the wheel in Function of the casing of a Pelton turbine is,,…..

 (i) to prevent the splashing of water; 

(ii) to discharge water to tail race; and 

(iii) to safeguard against any accident due to rotating buckets. 

The casing of pelton wheel does not perform any hydraulic function as it is exposed to atmosphere and it does not have the shape of tapered draft tube as in reaction turbine. The draft tube in reaction turbine has hydraulic function to increase the pressure of discharge water and this hydraulic function is unnecessary in impulse turbine .

       The shape of the bucket is as shown in Fig. The water jet emerging from the nozzle hits the splitter of the bucket symmetrically . The jet after striking the splitter is equally distributed into two halves of the hemisphere on the bucket. The inlet angle of the jet is assumed to be zero. The ideal angel of deflection of the jet is 180°. This deflection of the jet cannot be achieved in practice as the jet leaving the bucket would strike the back of the wheel will decrease, thereby decreasing the efficiency of the wheel. Hence, the angular deflection of the jet in the bucket is limited about 160° to 170°. The bucket is therefore shaped slightly smaller than a hemisphere in size.

The speed of motion of the wheel depends upon the flow rate and the velocity of the water. The flow rate of water is controlled by means of a needle in the nozzle as shown in Fig. The turbine operates most efficiently when the wheel rotates at half the velocity of the jet. When the load on the wheel suddenly reduces, then the jet deflector moves up and partially diverts the jet issuing from the nozzle until the jet needle moves forward and appropriately reduces the jet as shown in Fig. This arrangement of reducing the size of  the jet is necessary because in the event of sudden fall of load  requirement, if the jet needle is moved forward in the nozzle to reduce the flow of water hammer phenomenon in the penstock. In practice, the control of jet deflector is linked to the electric generator coupled which the turbine . The complete potential energy available in the water is converted into kinetic energy before the water jet strikes the buckets on the runner. The pressure on the buckets in the turbine is atmospheric pressure which does not change. Hence, energy transfer occurs purely on the buckets by the impulse action.

 

When the water is completely closed with the help of the needle in the nozzle, the runner of the turbine keeps on rotating due to its inertia for a long time even when no jet is striking the buckets. In order to stop the runner in a short time, a brake nozzle is provided to stop the rotation of wheel by directing a small braking jet on the buckles in the opposite direction as shown below the sketch.

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