Classification of Water Turbines | Electricity

There are many ways of classifying water or hydraulic tur­bines, for example, according to the type of flow of water, according to action of water on moving blades, according to head and quantity of water available, after the name of origi­nator and according to the specific speed of the machine.

According to the type of flow of water, the water tur­bines used as prime movers in hydroelectric power stations are of four types namely:

(i) Axial flow turbines having flow of water along the shaft axis,

(ii) Inward radial flow turbines having flow of water along the radius,

(iii) Tangential or peripheral flow turbines having flow of water along the tangential directions, and

(iv) Mixed flow (radial inlet and axial outlet) turbines.

Kaplan turbine is an axial flow tur­bine and has adjustable runner blades which can be rotated about pivots fixed to the boss of the runner. If the runner blades of the axial flow turbines are fixed, these are called the propeller turbines. Francis turbine is the mixed flow turbine while Pelton wheel is the tangential flow turbine.

According to the action of water on moving blades, water turbines are of two types, namely impulse and reaction type turbines. When the entire pressure of water is converted into kinetic energy in a nozzle and the jet thus formed drives the wheel, the turbine is of impulse type, whereas if the water pressure combined with its velocity work on the run­ner the turbine is known as the reaction type turbine.

Pelton wheel is an impulse turbine, in which the water flowing over the turbine rotor blades remains constant. In reaction turbines, the rotor of the turbine operates while submerged in water, the turbine casing being full of water. As the water flow through the rotor blades its pressure changes. Francis turbines, Kaplan turbines and propeller turbines are reaction turbines.

According to the head and quantity of water avail­able, the water turbines are of two types viz., high head and low flow and low to medium head and high to medium dis­charge turbines.

According to the name of originator, water turbines are of three types, namely Pelton wheel, Francis turbine and Kaplan turbine. Pelton wheel is an impulse turbine and is suited to high head and low flow plants. Francis turbine is a reaction turbine and is suited to medium head and medium flow plants. Kaplan turbine is a special type of propeller turbine having adjustable blades and is suited to low head and high flow plants.

1. Pelton Wheel:

It is an impulse turbine and is suitable for high head and low flow plants. The potential energy of water in the penstock is converted into kinetic energy in a water jet issuing from a nozzle. The inside pressure is atmospheric one. It consists of a rotor equipped with elliptical shaped buckets along the periphery of the turbine.

The water jet impinges on the buckets and causes the motion of the rotor. After doing useful work, the water discharges into the tailrace. The quantity of water discharged by the nozzle is controlled by controlling the nozzle opening by means of a needle placed in the nozzle tip.

The movement of the needle is controlled by the governor. When the load on the turbine reduces the governor pushes the needle into the nozzle, thereby reducing the quantity of water striking the buckets. In case of increase of load on the turbine, reverse action takes place. For control of speed deflectors are used in addition to the needle.

The Pelton turbines usually have one jet but machines with 2 or 4 jets are also being used. Majority of the Pelton turbines are of horizontal shaft type. Horizontal shaft type impulse turbines have usually two nozzles since there will be interference between them if their number is increased. For using more number of nozzles, vertical shaft arrange­ment will be required. The rotor of the runner is made of cast steel, bronze or stainless steel.

The buckets are bolted on to the runner but integral casting of buckets with the runner is also possible. The runner should be placed as close to the tailrace level as possible since the head between the nozzle and the tailrace is ineffective but at the same time, the runner should not submerge in the water.

Impulse tur­bines have long penstocks due to high heads. If such tur­bines are used in low head power plants, the Pelton wheel runner of large and unwieldy diameter will be required for the given output. As such the turbine of this type is not suitable for water heads below 200 metres.

Apart from Pelton wheel, the Turgo, and the Crossflow turbines are also impulse turbines. The Turgo turbine is similar to the Pelton wheel but the jet strikes the plane of the run­ner at an angle (typically 20°) so that the water enters the runner on one side and exits on the other.

Thus, the flow rate is not limited by the discharged fluid interfering with the incoming jet (as is the case with Pelton turbines.) As a consequence, a Turgo turbine can have a smaller diameter runner than a Pelton for an equivalent power. The cross-flow turbine has a drum-like rotor with a solid disk at each end and gutter-shaped “slots” joining the two discs.

2. Francis Turbine:

It is an inward mixed flow type of reaction turbine and is suitable for medium head and medium flow power plants. Such turbines develop power partly due to velocity of water and partly due to the difference in pressure acting on the front and back of the runner buckets. Such a turbine essen­tially consists of “guide apparatus” consisting of an outer ring of stationary guide blades fixed to the casing of the turbine and an “inner ring” consisting of rotating blades forming a wheel or a runner.

In these turbines, water glides over the blades with a small and fairly constant velocity and exerts a pressure, varying from maximum at the top to a small value at the bottom. The water flows radially inwards and changes to a downward direction while passing through the runner. As the water passes over the rotating blades of the runner, both pressure and velocity of water are reduced causing a reaction force driving the turbine.

After doing work, water is discharged to the tailrace through a closed tube of increasing cross section, called the draft tube. It increases the effective head by an amount equal to difference of runner outlet level and tailrace level. Moreover the water leaving the runner still possesses kinetic energy and this kinetic energy is reduced as the water flows through the draft tube to the tailrace and results in setting up of negative pressure head at the runner outlet. Both of these factors increase the net effective head and, thereby the output and the efficiency of the turbine.

The guide blades of the turbine are each pivoted about an axis in parallel with the turbine axis so that the quantity of water entering the turbine may be regulated by turning them simultaneously in one direction or the other. Like Pelton wheel their motion is automatically controlled by governor. As in all reaction turbines, cavitation is a serious problem in a Francis turbine.

The full-load efficiency of this type of turbine is about 92 per cent but with the part load the efficiency decreases to 65%, at half-full load. Francis type turbines can be con­structed in vertical or horizontal forms. The horizontal con­structions are more accessible and have higher speed, but for large machines generally vertical construction is preferred to effect economy in space. The alternator is mounted above the turbine and thus is free from flooding.

As compared to Pelton wheel a Francis turbine offers the advantage of high efficiency at full load and at 75% of full load. Since this turbine can be designed for a higher speed than Pelton wheel, the dimensions and weights are smaller together with the reduced generator cost.

3. Kaplan Turbine:

It is also a reaction type turbine and has gate and governing mechanism similar to that of a Francis turbine. The differ­ence between Kaplan turbine and Francis turbine is that in the former runner the water strikes the turbine blades axially whereas the latter receives water radially.

Water flows radially inwards through regulating gates all rounds the sides, changing direction in the runner to axial flow and causing a reac­tion force which drives the turbine. Practically all Kaplan turbines are of single runner vertical shaft type.

Kaplan turbine, because of its highest specific speed (2 to 3 times that of a Francis turbine), is suitable for low head and large flow plants. In this type of turbine the draw­back of considerable loss at low loads due to rotary motion of water in Francis turbines is overcome and uniform effi­ciency at all loads is maintained.

Thus part load efficiency of Kaplan turbine is much higher. Kaplan turbine gives high speed (in the range of 400-1,500 rpm) than ordinary Francis turbine resulting in lower cost of runner and alternator and power house structure. The Kaplan turbine, because of adjustable pitch of blades, is capable of operating in a wide range of heads.

All parts except runner of a Kaplan turbine (such as spiral casing, guide mechanism and draft tube) are similar to those of a Francis turbine. The runner blades in Kaplan turbines are less in number (3 to 6) as compared to 16-24 for a Francis turbine. The reduction in the number of blades in Kaplan turbine results in less friction between the blades and water, and hence more efficiency. The characteristic feature of Kaplan turbine is that the gate opening and blade angle are adjusted simultaneously by governing mechanism. Its efficiency is about 90% at all loads. The cavitation problem in Kaplan turbine is more serious than in Francis tur­bines.

The special feature of Kaplan turbine is that its run­ner is capable of reverse operation as a pump and is, there­fore, ideally suited to pumped storage plants.

4. Propeller Turbine:

It is an axial flow reaction type turbine and has got no provision for changing the runner blade angle while turbine is in motion. In such a turbine the blades are cast integrally with the hub. Though its construction is simple and easy but its efficiency falls sharply at reduced loads and, therefore, such a turbine is kept fully loaded for efficient operation. Its efficiency is about 92% at full load and drops to 65% at half full-load.