Hydraulic Turbine

Hydraulic Turbine

Hydraulic turbine is a mechanical component which converts hydraulic energy (i.e. kinetic energy and potential energy) of water in mechanical energy. The mechanical energy so produced is utilized in running an electrical generator which is connected to shaft of the turbine. Thus mechanical energy is converted into electrical energy.

This principle is utilized in the production of hydroelectric power.

Turbine is broadly classified in two types:

Ø  Impulse turbine

Ø  Reaction turbine

Impulse Turbine:

A turbine si said to be as impulse turbine if the runner of the turbine revolves by impact or impulse action of water. To increase the impact of water the entire available energy of water is converted in kinetic energy by passing it through a nozzle. The jet of water smoothly impinges on the bucket through the nozzle due to which the turbine rotates. There is no pressure difference between the inlet and outlet of runner.

Example: Peton turbine

              Cross flow Turbine

              Turbo Turbine

This turbine is also known as free jet turbine. In such type of turbine, it can be written that

Pressure of inlet (p1) = Pressure of outlet (p2)

Velocity of inlet (v1) is very greater than velocity of outlet (v2). (i.e. v1>>v2)

Weight of inlet water (w1) = weight of outlet water (w2)               (neglecting losses in bucket)

Reaction turbine

A turbine is said to be reaction turbine if at the inlet of the turbine water possesses kinetic energy as well as pressure energy. In a reaction turbine runner rotates due to the reaction of pressure difference between the inlet and outlet of the runner. In this case entering water has pressure as well as kinetic energy and when it moves over the runner/blade/bucket both kinetic & pressure energy provides the turning movement of the wheel.

Example: Francis turbine

              Propeller Turbine

              Kaplan Turbine

For this type of turbine, it can be mentioned that;

P1>>P2                                V1>V2                                              & W1<W2

Difference between Impulse and Reaction turbine

Impulse Turbine	Reaction Turbine
The wheel rotates due to the impact of water.	The wheel rotates due to the pressure difference between inlet & outlet.
The turbine is installed over a tail race.	The turbine is entirely submerged on fluid below tail race.
Cashing has no hydraulic function to perform.	Cashing has hydraulic function to perform. It is one of the main which create pressure difference at inlet and outlet.
The flow of fluid can be without loss.	The flow of fluid can’t be regulated without loss.
The wheel does not run full.	The wheel must run full.

Pelton Turbine:

Pelton Turbine is a tangential flow impulse turbine. It is the mostly widely used turbine. The impact of water on the bucket causes the runner to rotate thus develops mechanical energy & is called impulse turbine.

Characteristics of Pelton turbine:

It requires high head above 200m but small quantity of water.

The flow is tangential

It has specific speed varying between 10-25 for single jet and 50 for double jet.

Main components of Pelton turbine

Nozzle

Nozzle is provided at the end of penstock. The pressure energy of water passing through the penstock is converted into kinetic energy and is further increased by nozzle at atmospheric pressure in the bucket. When the water trikes the bucket fitted on the periphery of runner mechanical energy is produced.

Bucket

The bucket of Pelton wheel has a shape of a double hemispherical cup. Bucket is divided by a common dividing straight edge. The two hemispherical cups gives the advantage that the axial force in both the cups neutralizes each other and avoids axial thrust.

These buckets are made of cast iron (for low head) cast steel or stainless steel is used for high head

Cashing

Cashing in Pelton turbine has no hydraulic function but it is provided to prevent splashing of water. It is made strong enough to resist reaction of jet.

Hydraulic brake

The turbine goes on revolving for a long time, even if the jet is closed. Thus to stop the rotation small brake nozzle is provided which opposes the rotation of bucket and stops.

Francis Turbine

Francis turbine is a radial flow reaction turbine. The flow in Francis Turbine is radial at both entry and exist of head which is less than 200m, Francis turbine is more suitable than Pelton as Pelton becomes slow and unwidely.

 Characteristics of Francis turbine

It operates in medium head (between 30-200m) and needs medium quantity of water.

Water flows in radial direction.

Francis turbine has specific speed varying from 50-300 rpm.

Main components of Francis Turbine:

Scroll Cashing:

The water from penstock is supply to the turbine casing which is spiral in shape. The casing plays an important role in hydraulic function. It surrounds the runner and guide blade. As water enters the casing it forces the water to flow into them at high pressure causing runner to rotate.

Guide Mechanism:

It performs two functions:

Ø  To direct the water in correct angle to the runner vane

Ø  To regulate the quantity of water according to load variation

Guide mechanism is held stationary in the casing and surrounds the runner. These vanes are generally made of cast iron.

Runner

It is fitted to the shaft which is coupled to electric generator. It is made of cast iron for small turbine or cast steel for large turbine. It can also make up of stainless steel and sometimes of non-ferrous metal like bronze.

Draft Tube:

It is a tube in passage having gradually increasing cross-sectional area. Its upper end is connected to exist of runner and lower end sufficiently submerged to tail race. It helps in increasing to woe done of turbine by reducing pressure.

Propeller Turbine

It is an axial flow propeller turbine. It has 4-8 vanes fixed to Boss. The shape of the runner is similar to the propeller of a ship hence these turbines are known as propeller turbine.

Characteristics of propeller turbine:

·         It can operate at low head (below 30m) but it requires large discharge

·         The flow in propeller turbine is in axial direction

·         Its specific speed varies from 300-1000 rms.

·         It is vertical turbine shaft.

·         Components are same as Francis turbine.

Kaplan Turbine

It is an axial flow reaction turbine. It is the modified propeller turbine as the blade on Kaplan turbine is adjustable (i.e. blades angle can be changed by means of oil pressure servo motor). This type of turbine can give good efficiency even at different load condition and water flow over the blade without shock.

Characteristics of Kaplan Turbine:

·         It is suitable for low head (3-30m) & it requires large discharge

·         It consists of axial flow

·         Its specific speed varies from 300-1000 rpm.

·         It can give overall efficiency up to 92%

·         It is vertical shaft turbine.