Essay: HYDRAULIC MACHINES

A hydraulic machine is a device in which mechanical energy is transferred from the liquid flowing through the machine to its operating member (runner, piston and others) or from the operating member of the machine to the liquid flowing through it.
Hydraulic machines in which receives energy from the flowing liquid it and energy of the liquid is greater at inlet than the outlet are referred as turbines.
Hydraulic machines in which energy is transferred to the flowing liquid and the energy of the liquid at the outlet is more than the energy at inlet are referred to as pumps.
A. Hydraulic Turbines
A hydraulic turbine converts potential energy and kinetic energy of water into usable mechanical energy. The mechanical energy made available at the turbine shaft can be used to run an electric power generator or a machinery.
Hydraulic Turbines are classified According to the type of energy at inlet:
‘Impulse turbine: – In the impulse turbine, the total head of the incoming fluid is converted in to a large velocity head using a nozzle. That is the total energy of the flow is converted in to kinetic energy. That is the flow strikes the turbine runner has only kinetic energy. Eg: Pelton wheel turbine.
‘Reaction turbine: Reaction turbines on the other hand, are best suited -for higher flow rate and lower head situations. In this type of turbines, the rotation of runner or rotor (rotating part of the turbine) is partly due to impulse action and partly due to change in pressure over the runner blades; therefore, it is called as reaction turbine.
Since, the flow from the inlet to tail race is under pressure, casing is absolutely necessary to enclose the turbine. In general, Reaction turbines are medium to low-head, and high-flow rate devices. The reaction turbines in use are Francis and Kaplan
CLASSIFICATION BASED ON DIRECTION OF FLOW THROUGH RUNNER:
Tangential flow turbines: In this type of turbines, the water strikes the runner in the direction of tangent to the wheel. Example: Pelton wheel turbine.
Radial flow turbines: In this type of turbines, the water strikes in the radial direction.
Axial flow turbine: The flow of water is in the direction parallel to the axis of the shaft. Example: Kaplan turbine.
Mixed flow turbine: The water enters the runner in the radial direction and leaves in axial direction. Example: Francis turbine.
1. FRANCIS TURBINES
Francis Turbine is an Inward Flow Reaction Turbine having Radial Discharge at Outlet. It is a mixed flow type turbine (i.e. Water enters the runner of the turbine in the radial direction and leaves the runner in the axial direction).
Radial Flow Turbines are those turbines in which the water flows in the Radial Direction. In Francis Turbine the water flows from outwards to inwards through the runner (Inward Flow Radial Turbine). Reaction Turbine means that the water at the inlet of the Turbine possesses Kinetic Energy as well as Pressure Energy.
Francis turbines are the most common water turbine in use today. They operate in a water head from 10 to 700 meters and are primarily used for electrical power production.
The turbine powered generator power output generally ranges from 10 to 750 megawatts
The speed range of the turbine is from 80 to 1000 rpm. Wicket gates around the outside of the turbine’s rotating runner adjust the water flow rate through the turbine for different water flow rates and power production rates.
Francis turbines are almost always mounted with the shaft vertical to keep water away from the attached generator and to facilitate installation and maintenance access to it and the turbine.
CONSTRUCTION: –
Figure 1: Francis turbine
MAIN PARTS
CASING: The runner is completely enclosed in an air-tight spiral casing. The casing and runner are always full of water.
GUIDE MECHANISM: It consists of a circular wheel all-round the runner of the turbine. The stationary guide vanes are fixed on the guide wheel. The guide vanes allow the water to strike the vanes fixed on the runner without shock at inlet. Also width between the two adjacent vanes can be altered so that amount of water striking the runner can be varied.
RUNNER: It is a circular wheel on which a series of Radial Curved Vanes are fixed. The vanes are so shaped that the water enters and leaves the runner without shock.
DRAFT TUBE: The pressure at the exit of the runner of Reaction Turbine is generally less than atmospheric pressure. The water at exit cannot be directly discharged to the tail race. A tube or pipe of gradually increasing area is used for discharging water from the exit of turbine to the tail race. This tube of increasing area is called Draft Tube. One end of the tube is connected to the outlet of runner while the other end is sub-merged below the level of water in the tail-race.
APPLICATION
” Francis turbines may be designed for a wide range of heads and flows. This, along with their high efficiency, has made them the most widely used turbine in the world.
” Francis type units cover a head range from 20 to 700 meters
” Large Francis turbines are individually designed for each site to operate with the given water supply and water head.
” The highest possible efficiency, typically over 90%.
2. KAPLAN TURBINE
The Kaplan turbine is a propeller-type water turbine which has adjustable blades. Its invention allowed efficient power production in low-head applications that was not possible with Francis turbines.
�� The head ranges from 10-70 meters and the output from 5 to 200 MW.
�� Runner diameters are between 2 and 11 meters. The range of the turbine rotation is from 80 to 430 rpm.
�� Kaplan turbines are now widely used throughout the world in high-flow, low-head power production.
Kaplan-type hydraulic turbine in which the positions of the runner blades and the wicket gates are adjustable for load change with sustained efficiency, it is a purely axial flow turbine. It have 3 to 8 number of blades
DIAGRAM
MAIN PARTS:
Components of the Kaplan turbine:-
Scroll casing: – It is the casing in which we pass the water to the runner in the turbine.
Guide vanes: – It is the blade which guides the water and control the water passage (i.e. how much the water flow goes in the turbine).
Draft tube: – After passing through the runner, the water is discharged to the tail race through a gradually expanding tube.
Runner: – It is an important part of the turbine which is connected to the shaft of the generator and consist movable vanes and hub (boss).
Hub (Boss):- Lower end of the shaft is made larger and is called ‘Hub’ or ‘Boss’. The vanes are fixed on the hub and hence Hub acts as runner for axial flow turbine.
WORKING:
The Kaplan turbine is an inward flow reaction turbine, which means that the working fluid changes pressure as it moves through the turbine and gives up its energy. The design combines radial and axial features.
The inlet is a scroll-shaped tube that wraps around the turbine’s wicket gate. Water is directed tangentially, through the wicket gate, and spirals on to a propeller shaped runner, causing it to spin.
The outlet is a specially shaped draft tube that helps de-accelerate the water and recover kinetic energy.
The turbine does not need to be at the lowest point of water flow, as long as the draft tube remains full of water. A higher turbine location, however, increases the suction that is imparted on the turbine blades by the draft tube. The resulting pressure drop may lead to capitation.
Variable geometry of the wicket gate and turbine blades allows efficient operation for a range of flow conditions. Kaplan turbine efficiencies are typically over 90%, but may be lower in very low head applications.
APPLICATIONS:
�� Kaplan turbines are widely used throughout the world for electrical power production.
�� They cover the lowest head hydro sites and are especially suited for high flow conditions.
�� Inexpensive micro turbines are manufactured for individual power production with as little as two feet of head.
�� Large Kaplan turbines are individually designed for each site to operate at the highest possible efficiency, typically over 90%.
�� They are very expensive to design, manufacture and install, but operate for decades.
3. PELTON WHEEL
The Pelton wheel is a water impulse turbine. The Pelton wheel extracts energy from the impulse of moving water. Pelton’s paddle geometry was designed so that when the rim runs at �� the speed of the water jet, the water leaves the wheel with very little speed, extracting almost all of its energy
DIAGRAM: –
Pelton turbine
MAIN PARTS
NOZZLE AND FLOW REGULATING ARRANGEMENT
The amount of water striking the buckets is controlled by providing a spear in the nozzle. The spear is a conical needle operated in the axial direction depending up on the size of the unit. When the spear is pushed forward, the amount of water striking the runner is reduced and when the spear is pushed back, the amount of water striking the runner increases.
RUNNER AND BUCKETS
Runner consists of a circular disc on the periphery of which a number of buckets evenly spaced are fixed. The space of the buckets is of a double hemispherical cup or bowl. Each bucket is divided into two symmetrical parts by dividing wall which is known as Splitter. The Splitter divides the jet into two equal parts. The buckets are shaped in such a way that the jet gets deflected through 160�� or 170��. The buckets are made of cast iron, cast steel bronze or stainless steel depending upon the head at the inlet of the turbine.
3. CASING
The function of water is to prevent the splashing of water and to discharge water to tail race. The casing of Pelton Wheel does not perform any Hydraulic function.
BREAKING JET
When the nozzle is completely closed by moving the spear in the forward direction, the amount of water striking the runner reduces to zero. But the runner due to inertia goes on revolving for a long time. To stop the runner in a short time, a small nozzle is provided which directs the jet of water on the back of vanes. This jet of water is called Breaking Jet.
WORKING
The water flows along the tangent to the path of the runner. Nozzles direct forceful streams of water against a series of spoon-shaped buckets mounted around the edge of a wheel. As water flows into the bucket, the direction of the water velocity changes to follow the contour of the bucket.
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. In the process, the water’s momentum is transferred to the turbine.
This “impulse” does work on the turbine. 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 it is filled
APPLICATIONS
” Pelton wheels are the preferred turbine for hydro-power, when the available water source has relatively high hydraulic head at low flow rates, where the Pelton wheel is most efficient.
” Pelton wheels are made in all sizes. 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.
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.