Hydraulic Machines

UNIT – I & II

By Dr. Subhash Kamal

CLASSIFICAITONS OF FLUID MACHINES AND DEVICES

• 1 Classification Based on

• Direction of Energy Conversion

• 2 Classification Based on

• Principle of operation

• 3 Classification Based on

• Fluid Used

1 Classification Based on Direction of Energy Conversion

1. The device in which the kinetic, potential or intermolecular energy held by

the fluid is converted in the form of mechanical energy of a rotating member is known as a Turbine.

2. The machines, on the other hand, where the mechanical energy from moving parts is transferred

to a fluid to increase its stored energy by increasing either its pressure or velocity are known as

Pumps, compressors, fans or blowers.

2 Classification Based on Principle of operation

The machine producing mechanical energy is known as Reciprocating engine. The machine consuming mechanical energy is known as Reciprocating pump or Reciprocating air compressor. Depending upon the main direction of fluid path in the rotor blade, the machine is termed as

a. Radial flow machine, The flow is towards the centre of the rotor blade Ex. Francis turbine and the centrifugal pump or compressors

Video Francis turbines http://vimeo.com/28238120

b. Axial flow machine The flow is away from the centre Ex. Kaplan turbines and axial flow compressors

C. If the flow is party radial and partly axial, Then it is termed as mixed-flow machine

3 Classification Based on Fluid Used

The fluid machines use either liquid or gas as the working 1.Liquid used by the turbines producing power is water, and therefore, they are termed as

water turbines or hydraulic turbines. 2. Turbines handling gases in practical fields are usually referred to as

steam turbine, gas turbine, and

air turbine depending upon whether they use steam, gas (the mixture of air and products of burnt fuel in air) or air.

Air Turbine

http://www.youtube.com/watch?v=sLXZkn2W-lk

What's inside a wind turbinehttp://www.youtube.com/watch?v=LNXTm7aHvWc

1. Pelton wheel Turbine

General layout of a Hydro-Electric Power Plant

Pelton Wheel Construction Details

• Large heads (from 100 meter to 1800 meter)

• Relatively small flow rate 

• Maximum of 6 nozzles• Good efficiency over a 

vide range

Define (a) Hydraulic efficiency (b) Mechanical efficiency (c) Overall efficiency of turbines.

UNIT-IIReaction Turbines

2. Francis Turbine

Francis Turbine

Construction and working of Francis turbine

Construction:Penstock: It is a large size conduit which conveys water from the upstream to the dam/reservoir to the turbine runner.Spiral Casing: It constitutes a closed passage whose cross-sectional area gradually decreases along the flow direction; area is maximum at inlet and nearly zero at exit.Guide Vanes: These vanes direct the water on to the runner at an angle appropriate to the design, the motion of them is given by means of hand wheel or by a governor.Governing Mechanism: It changes the position of the guide blades/vanes to affect a variation in water flow rate, when the load conditions on the turbine change.Runner and Runner Blades: The driving force on the runner is both due to impulse and reaction effect. The number of runner blades usually varies between 16 to 24.Draft Tube: It is gradually expanding tube which discharges water, passing through the runner to the tail race.

Working:Francis turbine has a purely radiate flow runner. Water under pressure, enters the runner from the guide vanes towards the center in radial direction and discharges out of the runner axially. Francis turbine operates under medium heads. Water is brought down to the turbine through a penstock and directed to a number of stationary orifices fixed all around the circumference of the runner. These stationary orifices are called as guide vanes.The head acting on the turbine is transformed into kinetic energy and pressure head. Due to the difference of pressure between guide vanes and the runner (called reaction pressure), the motion of runner occurs. That is why a Francis turbine is also known as reaction turbine.Pressure at inlet is more than that at outlet. In Francis turbine runner is always full of water. The moment of runner is affected by the change of both the potential and kinetic energies of water. After doing the work the water is discharged to the tail race through a closed tube called drafttube.

Heads between 15 and 700 meter

Medium Flow Rates

Good efficiency =0.96 for modern machines

X blade runnerTraditional runner

3.Kaplan Turbine

Kaplan Turbine

HUBB OR BOSS

VANESSHAFT

• Low head (from 70 meter and down to 5 meter)

• Large flow rates

• The runner vanes can be governed

• Good efficiency over a vide range

Kaplan Turbine Construction Details

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.

Different Types of Draft TubesThe draft tube is an integral part of a reaction turbine. Its principle has been explained earlier. The shape of draft tube plays an important role especially for high specific speed turbines, since the efficient recovery of kinetic energy at runner outlet depends mainly on it. Typical draft tubes, employed in practice, are discussed as follows.

(a)Straight or conical draft tube :- This is known as tapered draft tube and used in all reaction turbines where conditions permit. It is preferred for low specific speed and Francis turbine. The maximum cone angle is 8° (a = 40°). The hydraulic efficiency is 90%.

(b)Simple Elbow Type The elbow type draft tube is often preferred in most of the power plants. If the tube is large in diameter; ‘it may be necessary to make the horizontal portion of some other section. A common form of section used is over or rectangular. It has low efficiency around 60%.

(3) Moody Spreading Tubes-This tube is used to reduce the whirling action of discharge water when turbine runs at high speed under low head conditions. The draft tube has efficiency around 85%.

(4) Elbow with circular inlet and rectangular outlet

This tube has circular cross- section at inlet and rectangular section at outlet. The change from circular section to rectangular section take place in the bend from vertical leg to the horizontal leg. The efficiency is about 85%.

Cavitation in Reaction Turbines

Derivation

Unit quantities applied to hydraulic machines

Selection of water turbines for hydro projectThe selection of turbines depend on the following considerations

1. Operating Head :-    Pelton turbine - Greater than 400 m                                        Francis turbine - 50-400 m                                         Kaplan turbine - Less than 50 m

2. Specific speed-Turbine:-

Turbine having high specific speed is selected. High speed means a smaller size of the turbine. Francis turbines run at higher speeds (50-250) than those of pelton wheels (8—50), Kaplan turbine have the greatest specific speed (250—1000).

3. Cavitation :-Cavitation occurs when the pressure at the runner outlet equals vapour pressure. Francis turbines can not be used for very high heads because of cavitation. Pelton turbines are free from cavitation because the pressure at runner outlet is the atmospheric.

4. Performance characteristics :-Turbines should be selected in such a way that their efficiencies do not fall appreciably when operating under part load. Francis turbines operate efficiently between half and full load. Kaplan turbines are more efficient at low heads.

5. Overall cost :-

The plant should be designed for the minimum cost as cost is the prime consideration in designing a plant

6. Number of units :-

It is better to go in for a larger unit as far as possible, but there must be at least two units at any particular site so that one unit is always available.

Governing of Reaction Turbines-Governing of reaction turbines is usually done by altering the position of the guide vanes and thus controlling the flow rate by changing the gate openings to the runner. The guide blades of a reaction turbine are pivoted and connected by levers and links to the regulating ring. Two long regulating rods, being attached to the regulating ring at their one ends, are connected to a regulating lever at their other ends. The regulating lever is keyed to a regulating shaft which is turned by a servomotor piston of the oil.

Performance Characteristics of Turbine

Impulse turbine Reaction turbine

All the available fluid energy is converted in kinetic energy. Blades are in action only when they are in the front of the nozzle.Water may be allowed to enter a part or whole of the wheel circumference.The wheel does not run full and air has free access to the buckets.Unit is installed above the tail race.There is no loss when the flow is regulated.

Only a portion of fluid energy is converted into kinetic energy.

Blades are in action all the time. 

Water is admitted over the circumference of the wheel. Water completely fills the vane passages throughout the operation of the turbine.Unit is kept entirely submerged in water below the tail race.There is always a loss when the flow is regulated.

Difference between impulse turbines and reaction turbines

MORE ADAPTED TYPE OF TURBINE AS FUNCTION OF THE  SPECIFIC SPEED.

Specific Speed in r.p.m. Turbine type Jump height in m

Until 18 Pelton of an injector 800

From 18 to 25 Pelton of an injector 800 to 400

From 26 to 35 Pelton of an injector 400 to 100

From 26 to 35 Pelton of two injectors 800 to 400

From 36 to 50 Pelton of two injectors 400 to 100

From 51 to 72 Pelton of four injectors 400 to 100

From 55 to 70 Very slow Francis 400 to 200

From 70 to 120 Slow Francis 200 to 100

From 120 to 200 Normal Francis 100 to 50

From 200 to 300 Quick Francis 50 to 25

From 300 to 450 Extra-quick Francis 25 to 15

From 400 to 500 Extra-quick helix 15

From 270 to 500 Slow Kaplan 50 to 15

From 500 to 800 Quick Kaplan 15 to 5

From 800 to 1100 Extra-quick Kaplan Less than 5