EXPERIMENT NO -3.10

AIM OF THE EXPERIMENT: Study of Impluse turbine.

REQUIREMENTS:

(i) Model of pelton wheel(ii) Map of pelton wheel

THEORY: Hydraulic turbines convert the hydro energy into mechanical energy. These are also known as hydraulic motors or prime movers. The mechanical energy developed by a turbine is used in running an electric generator, which is directly coupled, to the shaft of the turbine. The electric generator thus develops electric power. A turbine can be operated under high head and at high speed. Turbines are of two types i.e. impulse or velocity turbines and reaction turbines. An impulse turbine as the name indicates is a turbine runs by the impulse of water. The water from a dam is made to flow through a pipeline and then through guide mechanism and finally through the nozzle. So total energy of water is converted into kinetic energy while passing through the nozzle. The jet of water issuing from nozzle strikes the buckets, are provided on a runner. Thus runner is keyed to a shaft. The shaft is coupled to a generator, which produces electricity. A pelton turbine is a impulse turbine. It named after pelton L.A(1829-1908) who was an engineer of California. He made Large-scale experiments and developed a turbine used for high heads and low discharge.

CONSTRUCTION: A pelton turbine in its simplest form consists of a nozzle that is a circular guide mechanism, guide the water to flow at a designed direction. A conical needle knows as spear operates inside the nozzle in an axial direction. A conical needle knows as spear operates inside the nozzle in an axial direction. It regulates the quantity of water flow through the nozzle. The movement of spear is regulated by hand automatic governing arrangement depending upon the requirement.

A runner consists of a circular disc with a number of buckets evenly spaced round its periphery. The bucket are in the shape of double semi-ellipsoidal cups. Each bucket is divided into two symmetrical parts by a splitter. The jet of water impinges on splitter gets divided into two equal portions and leaves the bucket with an angle of 165°. The surface of the buckets is made very smooth. For low head the bucket are made of cast iron & for high head they are made of bronze, stainless steel or other alloys. The advantage of making a double cup shaped buckets is that the axial thrust neutralized each other, being equal & opposite, & hence the bearings supporting the wheel shaft are not subjected to any axial or end thrust. The width & depth of buckets of a pelton turbine are 5d & 1.2d respectively. Here “d” indicates the diameter of the jet. The number of buckets provided on the runner is given by

N=D/2d+15

Where D=Mean dia of bucket

d=diameter of the jet

Whenever a pelton turbine has to stop, the nozzle must be closed. But due to the inertia the runner still rotates in spite of closing the nozzle. In order to bring the runner to rest in a short time a small nozzle is provided in such a way that it will direct a jet of water on the back of buckets. So the speed can be reduced. For this purpose a breaking jet is provided as shown.

A casing made of cast iron prevents splashing of water & leads water to the tail race. It also acts as a safe guard against accidents.

WORING PRINCIPLE: Pelton turbine is a parallel flow impulse turbine. The water is conveyed from the reservoir to the turbine through the pen stock. The branch pipe of penstock is connected with a nozzle that is provided with a spear mechanism. A powerful jet that issued out of the nozzle impinges on the buckets provided on the periphery of a runner. This jet strikes the double semi ellipsoidal cups at the splitter. The water after imparting its energy to the turbine is discharged into the tailrace. So in this turbine the impulsive force of water is converted into mechanical energy. So this mechanical energy is fed to a generator that produces electricity.

PRACTICAL APPLICATION: pelton turbine is a high head turbine it is used in hydroelectric power plants where a high head (250m to 1770m) of water is available. It requires small discharge.

PROCEDURE: take the model of pelton turbine. Draw its diagram from map locate its parts. Sharply observe the mechanism of spear control system. Take the runner. Note its construction of hemi spherical cups (buckets). Collets its manufactured list. Note the hydro power stations in which this type of turbine is used.