Types of turbines

The most common types of turbines are summarized as follows;

Pelton wheel turbine:

A Pelton wheel turbine is the most common type of impulse turbine. The Pelton wheel turbine consists of a rotor at the periphery of which are mounted equally spaced double hemispherical of double ellipsoidal buckets. The direction flow of water is tangential. Water is transferred from a high head source through the penstock, which is fitted with a nozzle, through which the water flows out as a high speed jet. All the available potential energy is thus converted into the kinetic energy before the jet strikes the buckets of the runner. The pressure all over the wheel is constant and equal to atmosphere so that energy transfer occurs due to purely impulse action. The pelton turbine is provided with casing to prevent the splashing of the water and to discharge water tail race. Pelton turbine is suitable for high head plants.

Francis turbine:

The modern Francis turbine is an inward mixed flow reaction turbine (earlier it is purely radial) i.e. water under pressure enters the runner from the guide vanes towards the center in radial direction and discharges out of the runner axially. The Francis turbine operates under medium heads and also requires the medium quantity of water. It is employed in medium head power plants. This type of turbine covers wide range of heads. Water is brought down to the turbine through the penstock and directed to a number of stationary orifices fixed all around the circumference of the runner. These stationary orifices are called guide vanes or wicket gates. The runner of turbine consists of number of curved blades which are 12-22 nos. depending upon the specific speed.

In Francis turbine the pressure at inlet is more than pressure at the outlet. This means that the water in the turbine must flow in a closed conduit. Unlike Pelton type the water strikes only a few of the runner buckets at a time in the Francis turbine the runner is always full of water. The moment of the runner is affected by the change of both the potential and kinetic energies of the water. After doing the work water is discharged to the tailrace through a closed tube of gradually enlarging section, which is called draft tube. It is not allowed water to fall freely to tailrace level as in Pelton turbine. The free end of the draft tube is submerged in deep in tail water making, thus the entire water passage right from the headrace to tailrace totally enclosed.

Propeller and Kaplan turbine:

The working principle of the general arrangements of the Propeller/Kaplan turbines and Francis turbine are more or less similar. The scroll case, water tight arrangements, stay rings and draft tubes are installed in the same way. They also work in the principle of reactive force change in the turbine unit. The major difference is that the way of the working water, in Francis turbine the flow is mixed one whereas in Propeller and Kaplan turbine the flow are axial ones. The runner of these turbines, therefore, resembles a propeller and consists of a central hub on periphery of which curved vanes are mounted. The vanes acts as cantilevers supported only at the hub. The number of blades for propeller/Kaplan turbine varies from 3-8 depending upon the specific speed range. The hub is semi conical hollow body outer surface of which becomes the boundary for water passage. Inside the hub is the shaft of the turbine. In case of the Kaplan turbines the hub houses the mechanical arrangement to automatically turn the blades depending upon the load.

All the features of Kaplan and propeller turbine are same. The only difference is the possibility of blades adjustments. Kaplan turbine has the possibility of the adjustment of the blades according to the change in discharge whereas the propeller has the fixed blades. Due to facility of the movable blade the efficiency of the Kaplan remains high over large range of load from 50% under load to 50% overload. To be more familiar with this one has to refer the efficiency curves.

Deriaz turbine:

It is the diagonal type of turbine in which flow over the runner is at an angle of 45 degree to the axis. It has the blade arrangement included. At the same time the flow diagonal or mixed as in Francis turbines. It can thus be said to be a cross between the two turbines and can be used for heads up to 200m. The number of blades varies from 10-12 which are mounted on spherical flanges.

Bulb turbine:

It is the reaction turbine designed when the head is extremely low and discharge is high. It is the axial flow turbine paced horizontally. This type of turbine is perfectly suitable for tidal plant. In bulb turbine the generator is house in enclosed bulb-shaped casing, which is installed right in the middle of the flow passage (remains always submerged under water). The bulb itself is a water tight structure and the bulb together with propeller turbine from an integral part. The outer surface of the bulb is stream lined so as to form the boundary of the water passage from the head pond. The bulb is streamline shaped so the loss of head is minimum. The draft tube is a straight conical flaring tube. The bulb could be either upstream or downstream of the runner. The main advantage of the bulb turbine is its compact arrangements of the components and less danger of cavitation.