HYDRAULIC TURBINES
By
Mrs. J. Emeema
Associate Professor
Mechanical Engineering Department
VJIT
Topics Covered:
1. Hydraulic Turbines
2. General Layout of Hydro-electric power plant
3. Classification of Hydraulic Turbines
4. Pelton Wheel
- Design of Pelton Wheel
- Pelton Turbine Hydro-Electric Set Up
- Working Principle of Pelton Turbine
- Applications of Pelton Wheel
Hydraulic Turbine is a machine which converts the energy of an elevated water supply (hydraulic energy) into mechanical energy. This mechanical energy is used in running an electric generator which is directly coupled to the shaft of the turbine. Thus the mechanical energy is converted to electrical energy. The electric power which is obtained from the hydraulic energy is known as Hydro-electric power.
GENERAL LAYOUT OF HYDRO-ELECTRIC POWER PLANT
CLASSIFICATION OF HYDRAULIC TURBINES
Based on flow path
1. Axial Flow Hydraulic Turbines: This category of Hydraulic Turbines has the flow path of the liquid mainly parallel to the axis of rotation. Kaplan Turbines has liquid flow mainly in axial direction.
2. Radial Flow Hydraulic Turbines: Such Hydraulic Turbines has the liquid flowing mainly in a plane perpendicular to the axis of rotation.
3. Mixed Flow Hydraulic Turbines: For most of the Hydraulic Turbines used there is a significant component of both axial and radial flows. Such types of Hydraulic Turbines are called as Mixed Flow Turbines. Francis Turbine is an example of mixed flow type, in Francis Turbine water enters in radial direction and exits in axial direction.
None of the Hydraulic Turbines are purely axial flow or purely radial flow. There is always a component of radial flow in axial flow turbines and of axial flow in radial flow turbines.
Based on pressure change
1. Impulse Turbine: The pressure of liquid does not change while flowing through the rotor of the machine. In Impulse Turbines pressure change occur only in the nozzles of the machine. One such example of impulse turbine is Pelton Wheel.
2. Reaction Turbine: The pressure of liquid changes while it flows through the rotor of the machine. The change in fluid velocity and reduction in its pressure causes a reaction on the turbine blades; this is where from the name Reaction Turbine may have been derived. Francis and Kaplan Turbines fall in the category of Reaction Turbines.
In this article, Pelton wheel is discussed.
PELTON WHEEL
· The Pelton wheel is an impulse turbine which is among the most efficient types of water turbines.
· It was invented by Lester Allan Pelton in the 1870s.
· Pelton wheel is a high head turbine. It is used with heads of more than 500 m.
· A head is the distance by which the water falls before it strikes the turbine blades
· The flow of water is tangential to the runner. So it is a tangential flow impulse turbine.
· A Pelton’s runner consists of a single wheel mounted on a horizontal shaft.
· Water falls towards the turbine through a pipe called penstock and flows through a nozzle.
· The high speed jet of water coming out from the nozzle hits the buckets (vanes) on the wheel and causes the wheel to rotate producing torque and power.
· The Pelton wheel extracts energy from the impulse (momentum) of moving water as opposed to its weight like traditional overshot water wheel.
Comments|Flag
· 7
· 7
· 1
· 4
· 22
DESIGN OF PELTON WHEEL
The Pelton Turbine has a circular disk mounted on the rotating shaft or rotor. This circular disk has cup shaped blades, called as buckets, placed at equal spacing around its circumference. Nozzles are arranged around the wheel such that the water jet emerging from a nozzle is tangential to the circumference of the wheel of Pelton Turbine. Water flows along the tangent to the path of the runner. According to the available water head (pressure of water) and the operating requirements, the shape and number of nozzles placed around the Pelton Wheel can vary.
Old Pelton Wheel from Walchensee Power Plant, Germany Plan View of a Pelton turbine installation
PELTON TURBINE HYDRO-ELECTRIC SET UP
A typical setup of a system generating electricity by using Pelton Turbine will have a water reservoir situated at a height from the Pelton Wheel. The water from the reservoir flows through a pressure channel to the penstock head and then through the penstock or the supply pipeline to the nozzles, from where the water comes out as high speed jets striking the blades of the Pelton Turbine. The penstock head is fitted with a surge tank which absorbs and dissipates sudden fluctuations in pressure.
For a constant water flow rate from the nozzles the speed of turbine changes with changing loads on it. For quality hydroelectricity generation the turbine should rotate at a constant speed. To keep the speed constant despite the changing loads on the turbine water flow rate through the nozzles is changed. To control the gradual changes in load servo controlled spear valves are used in the jets to change the flow rate. And for sudden reduction in load the jets are deflected using deflector plates so that some of the water from the jets do not strike the blades. This prevents over speeding of the turbine.
Working Principle of Pelton Turbine
High speed water jets emerging from the nozzles (obtained by expanding high pressure water to the atmospheric pressure in the nozzle) strike a series of spoon-shaped buckets mounted around the edge of the pelton wheel. High pressure water can be obtained from any water body situated at some height or streams of water flowing down the hills.
As water flows into the bucket, the direction of the water velocity changes to follow the contour of the bucket. These jets flow along the inner curve of the bucket and leave it in the direction opposite to that of incoming jet. When the water-jet contacts the bucket, the water exerts pressure on the bucket and the water is decelerated as it does a "u-turn" and flows out the other side of the bucket at low velocity.
The change in momentum (direction as well as speed) of water jet produces an impulse on the blades of the wheel of Pelton Turbine. This "impulse" does work on the turbine and generates the torque and rotation in the shaft of Pelton Turbine.
To obtain the optimum output from the Pelton Turbine the impulse received by the blades should be maximum. For that, change in momentum of the water jet should be maximum possible. This is obtained when the water jet is deflected in the direction opposite to which it strikes the buckets and with the same speed relative to the buckets.
For maximum power and efficiency, the turbine system is designed such that the water-jet velocity is twice the velocity of the bucket. A very small percentage of the water's original kinetic energy will still remain in the water. However, this allows the bucket to be emptied at the same rate at which it is filled, thus allowing the water flow to continue uninterrupted.
Often two buckets are mounted side-by-side, thus splitting the water jet in half (see photo). The high speed water jets emerging form the nozzles strike the buckets at splitters, placed at the middle of the buckets, from where jets are divided into two equal streams.
This balances the side-load forces on the wheel, and helps to ensure smooth, efficient momentum transfer of the fluid jet to the turbine wheel.
Because water and most liquids are nearly incompressible, almost all of the available energy is extracted in the first stage of the hydraulic turbine.
Therefore, Pelton wheels have only one turbine stage, unlike gas turbines that operate with compressible fluid.
Applications of Pelton Wheel
Pelton wheels are the preferred turbine for hydro-power, when the available water source has relatively high hydraulic head at low flow rates. Pelton wheels are made in all sizes. There exist multi-ton Pelton wheels mounted on vertical oil pad bearings in hydroelectric plants. The largest units can be up to 200 megawatts. The smallest Pelton wheels are only a few inches across, and can be used to tap power from mountain streams having flows of a few gallons per minute. Some of these systems utilize household plumbing fixtures for water delivery. These small units are recommended for use with thirty meters or more of head, in order to generate significant power levels. Depending on water flow and design, Pelton wheels operate best with heads from 15 meters to 1,800 meters, although there is no theoretical limit.
Thus, more power can be extracted from a water source with high-pressure and low-flow than from a source with low-pressure and high-flow, even though the two flows theoretically contain the same power. Also a comparable amount of pipe material is required for each of the two sources, one requiring a long thin pipe, and the other a short wide pipe.