Introduction to Pelton Wheel or Turbine :
- The turbines, a subgroup of rotodynamic machines, are used to produce power utilizing converting hydraulic energy into mechanical energy. They are of different types according to their specification. Turbines can be subdivided into two groups, impulse and reaction turbines. Moreover due to working fluid used, turbines can be named as steam turbines, gas turbines, wind turbines, and water turbines.
- Pelton wheel, named after an eminent engineer, is an impulse turbine wherein the flow is tangential to the runner and the available energy at the entrance is completely kinetic energy.
- Pelton wheel is preferred at a very high head and low discharges with low specific speeds. The pressure available at the inlet and the outlet is atmospheric.
- This type of turbine was developed and patented by L.A. Pelton in 1889 and all the types of turbines are called by his name to honor
Parts of Pelton Turbine :
The main components of a Pelton turbine are:
- Nozzle and Flow Regulating Arrangement
- Runner with Buckets
- Breaking Jet.
1. Nozzle and flow regulating arrangement :
- Water is brought to the hydroelectric plant site through large penstocks at the end of which there will be a nozzle, which converts the pressure energy completely into kinetic energy. This will convert the liquid flow into a high-speed which strikes the buckets or vanes mounted on the runner, which in turn rotates the runner of the turbine.
- The amount of water striking the vanes is controlled by the forward and backward motion of the spear.
- The spear is a conical needle which is operated either by a hand wheel or automatically in an axial direction depending upon the size of the unit.
- When the spear is pushed forward into the nozzle the amount of water striking the runner is reduced. On the other hand, if the spear is pushed back, the amount of water striking the runner increases.
2. Runner with Buckets :
- Runner is a circular disk mounted on a shaft on the periphery of which several buckets are fixed equally spaced as shown in Fig.
- The buckets are made of cast -iron cast -steel, bronze, or stainless steel depending upon the head at the inlet of the turbine.
- The shape of the bucket is of a double hemispherical cup or bowl. Each bucket is divided into two symmetrical parts by a dividing wall which is known as the splitter.
- The jet of water strikes on the splitter. the splitter divides the jet into two equal parts and the jet comes out at the outer edge of the bucket.
- The water jet strikes the bucket on the splitter of the bucket and
gets deflected through (α) 160-170°.
- Arrangement of jets- In most of the Pelton wheel plants, a single jet with a horizontal shaft is used. The number of the jets adopted depends upon the specific speed required.
3. Casing :
- It is made of cast -iron or fabricated steel plates. The main function of the casing is to prevent the splashing of water and to discharge the water into the tailrace.
- Casing is also acting as a safeguard against accidents.
- It is made of cast iron or fabricated steel plates. The casing of the Pelton wheel does not perform any hydraulic function.
4. Breaking jet:
- Even after the amount of water striking the buckets is completely stopped, the runner goes on rotating for a very long time due to
- To stop the runner in a short time, a small nozzle is provided which directs the jet of water on the back of the bucket with which the rotation of the runner is reversed. This jet is called as breaking jet.
5. Governing mechanism:
- The speed of the turbine runner is required to be maintained constant so that the electric generator can be coupled directly to the turbine. Therefore, a device called governor is used to measure and regulate the speed of the turbine runner.
Working of Pelton wheel:
- The water stored at a high head is made to flow through the penstock and reaches the nozzle of the Pelton turbine.
- The nozzle increases the K.E. of the water and directs the water in the form of a jet.
- The jet of water from the nozzle strikes the buckets (vanes) of the runner. This made the runner to rotate at very high speed.
- The quantity of water striking the vanes or buckets is controlled by the needle valve present inside the nozzle.
- The generator is attached to the shaft of the runner which converts the mechanical energy of the runner into electrical energy.
Velocity Triangles Diagram For Pelton Wheel :
- The jet of water from the nozzle strikes the bucket at the splitter, which splits up the jet into two parts. These parts of the jet glides over the inner surfaces and comes out at the outer edge.
- The splitter is the inlet tip and the outer edge of the bucket is the outlet tip of the bucket.
- The inlet velocity triangle is drawn at the splitter and the outer velocity triangle is drawn at the outer edge of the bucket.
V1= velocity of jet at inlet
u1= velocity of the vane/bucket at inlet
Vr1 = relative velocity of jet at inlet
α = angle between the direction of the jet and the direction of motion of the vane, guide blade angle (Here in this figure it is zero)
θ = angle made by vr1 with the direction of motion of vane at the inlet, vane angle at inlet (=0)
Vw2 = velocity of whirl at outlet
Vf2 = velocity of flow at the outlet
β = angle between v2 with the direction of motion of vane at the outlet
ϕ = angle made by vr2 with the direction of motion of vane at the outlet, vane angle at outlet
Pelton Wheel – Efficiencies and Work done
(i) The work done by the jet on runner per second = ρ aV1 ( Vw1 ±Vw2 )
(ii) The work done per second per unit weight of water striking =1/g ( Vw1 + Vw2 ) *u
(iii) Hydraulic efficiency,
(iv) Mechanical efficiency = shaft power / Bucket Power
(v) Volumetric efficiency, = Volume of water actually striking the runner / total water supplied by the jet to the turbine
(vi) Overall efficiency, = shaft power / water power = P/ ρgQH
Design Of Pelton Wheel :
1. Velocity of jet at inlet V1= Cv √2gH …….. where Cv = coefficient of velocity = 0.98-0.99
2. Velocity of wheel where u = φ √2gH ………..Where φ is the speed ratio = 0.43-0.48
3. The angle of deflection is 165° unless mentioned.
4. Pitch or mean diameter D can be expressed by, u= πDN / 60
5. Jet ratio M =D/d ( 12 in most cases/calculate), d = nozzle diameter
6. Number of bucket on a runner Z = 15 + D/2d = 15 + 0·5 m
7. Number of Jets = obtained by dividing the total rate of flow through the turbine by the rate of flow through the single jet
8. Size of Bucket: Axial Width, radial length, depth
Performance and operating characteristics curves of a Pelton turbine :
The followings are the important characteristic curves of a turbine :
Main Characteristic Curves or Constant Head Curve –
The speed of the turbine is varied by changing the load on the turbine. For each value of the speed, the corresponding values of the power (P) and discharge (Q) are obtained.
Operating Characteristic Curves OR Constant Speed Curves
- Tests are performed at a constant speed.
- Constant speed is attained by regulating the gate opening thereby varying the discharge flowing through the turbine as the load varies.
- Head may or may not kept constant.
Advantages of Pelton turbine
- It has simple construction
- It is easy to maintain
- Intake and exhaust of water takes place at atmospheric pressure hence no draft tube is required
- No cavitation problem
- Its overall efficiency is high
- It can be both axial and radial flow
- It can work on low discharge
Disadvantages of Pelton turbine
- It requires high head for operation
- Turbine size is generally large
- Its efficiency decreases quickly with time
- Due to high head, it is very difficult to control variations in operating head
Applications of Pelton Wheel :
- Pelton wheels are the preferred turbine for hydro-power when the available water source has a relatively high hydraulic head at low flow rates.
- Pelton wheels are made in all sizes. For maximum power and efficiency, the wheel and turbine system is designed such that the water jet velocity is twice the velocity of the rotating buckets.
- There exist in multi-ton Pelton wheels mounted on vertical oil pad bearing in hydroelectric power.
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