Hydraulic Turbines and their types
Hydraulic
turbines are the machines which use the energy of water and convert it into
mechanical energy. These are also called hydraulic motors or prime movers. The mechanical
energy developed by a turbine is used in running an electric generator which is
directly coupled to the shaft of the turbine. The electric generator thus
develops electric power, which is known as hydroelectric power. Since the
generation of hydro-electric power is relatively cheaper than the power
generated by other sources such as coal, oil, etc., now-a-days a number of
hydro-electric and multipurpose projects have been undertaken in our country in
order to harness more and more power from the available water potential.
Selection of turbine
plays a crucial role in electricity generation. A turbine must be installed (or
chosen) according to the operating conditions of dam.
Hydraulic turbines
may be classified according to several considerations.
According to
the action of water flowing through the turbine runners the turbines may be
classified as impulse turbine and the reaction turbine.
The turbines
may also be classified according to the main direction of flow of water in the
runner as
1. Tangential
flow turbines: In tangential flow turbine the water flow along the tangent to
the path of rotation of the runner, Pelton wheel is a tangential flow turbine.
2. Radial
flow: The water flows along the radial
direction and remains wholly and mainly in plane normal to the axis of
rotation, as it passes through the runner. A radial flow turbine may be either
inward radial flow type turbine or outward radial flow type turbine. Thomson
turbine, Girard radial turbine, Fourneyron turbine are the examples of radial
flow turbines.
3. Axial
flow turbine: In an axial flow turbine the flow of water through the runner is
wholly and mainly along the direction parallel to the axis of rotation of the
runner. Jonval turbine, Girard axial flow turbine, Propeller turbine, Kaplan
turbine etc, are some of the examples of axial flow turbines.
On the basis
of the head and the quantity of water required, the turbines may be classified
as
1. High
head turbine: high head turbines are those which are capable of working under
very heads ranging from several hundred meters to few thousand meters. These turbines
thus require less amount of water. In general impulse turbine are high head
turbines. In particular Pelton wheel has so far been used under a highest head
of 1770m.
2. Medium
head: medium head turbines are those which are capable of working under medium
heads from 60m to 250m. these turbines require relatively large quantity of
water. Modern Francis turbine may be classified as medium head turbine.
3. Low
head turbine: Low head turbines are those which are capable of working under
the heads less than 60m. These turbines thus require a large quantity of water.
Kaplan and other Propeller turbines may be classified as low head turbine.
The
turbines may also be classified according to their specific speed. The specific
speed of turbine is defined as the speed of geometrically similar turbine that
would develop kilowatt power when working under a head of one meter. On the
basis of the specific speed the various turbines may be considered in the three
groups.
- Specific speed varying from 8.5 to
30 – Pelton wheel with single jet and upto 43 (50) for wheel with double
jet.
- Specific speed varying from 50 to 340 - Francis turbine
- Specific speed varying from 255 to
860 – Kaplan and other propeller turbine
