Hydroelectric turbine working | Francis water turbine specific speed

Hydroelectric Turbine classification:

Water turbines are used to convert the energy of falling water into mechanical energy.
Classification based on direction of flow:
  • Axial flow turbine: In this turbine, flow is parallel to the axis of rotation of the runner. ex: Propeller and kaplan turbines.
  • Tangential flow turbine: In this, flow is tangential to the path of rotation of the runner. Ex: Pelton turbine.
  • Radial flow turbine: In this flow, is in radial direction. Flow remains in the plane normal to the axis of rotation of the runner. Ex: Francis turbine.
  • Mixed flow turbine: In this flow, water enters the runner at the outer periphery in the radial direction and leaves it at the centre in axial direction. Ex: Modern francis turbine.
Classification based on action of water:
1. Impulse turbine: In this turbine, all the available energy of water is converted into kinetic energy or velocity head by passing through a nozzle provided at the end of penstock. 
Ex: Pelton wheel turbine.
2. Reaction turbine: In this turbine, all the available energy of water remains mostly as pressure energy. As the water flows through the runner, the pressure energy gradually decreases. Therefore, the runner must be enclosed in an air tight casing. 
Ex: Francis, Propeller, Kaplan turbines.

Specific speed of hydroelectric turbine:

It is defined as hypothetical speed of the turbine at which it produces 1 H.P of power under head of 1 meter. It is an important parameter in the design of turbines.
Specific speed, NS = N√P / H5/4 r.p.m
Where N is the Actual rotation speed in rpm.
H is the head in meters
P is the power in HP = 735.5 watts
NS = (1.165)N√P / H5/4 r.p.m
Where P is the power expressed in KW

Selection of turbine:

It depends on head, specific speed, efficiency, part load operation.

Selection of turbine based on Specific speed: 
Specific speed Medium of Specific speed Turbine 
12 - 70 Low speed Pelton wheel
80 - 420 Medium speed Francis
310 - 1000 High speed Propeller and Kaplan

Selection of Turbine based on Head:
Head Head in meters Turbine used
Low head less than 15m Propeller or kaplan
Medium head 15 - 70m Kaplan or Francis
High head 71 - 250m Francis or pelton
Very High head 250m and above Pelton wheel

Francis turbine working:

Parts of a francis turbine is described below.
Spiral casing: The purpose of the casing is to provide an even distribution of water around the circumference of the turbine runner.
The cross-sectional area of the casing is gradually decreased to keep the velocity of water constant throughout its path.

Speed ring or Stay ring: The purpose of ring is to direct the water from the spiral casing to the guide vanes or wicket gates.
Guide vanes or Wicket gates: The function is to regulate the quantity of the water supplied to the runner and to direct water on to the runner at an angle appropriate to the design. The guide vanes are operated by a governor.
Runner: It consists of a series of curved vanes around the circumference to change the direction of flow of water when it passes through the runner. The runner is keyed to a shaft.
Draft tube: It is a pipe or passage of gradually increasing cross-sectional area which connects the runner exit to the tail race. The lower end must be submerged below the level of water in the tail race.
Function of Draft tube:

  • It permits a negative or suction head to be established at the runner exit, thus making it possible to install the turbine above the tail race level without loss of head.
  • It converts a large proportion of velocity energy rejected from the runner into useful pressure energy i.e, it acts as a recuperative of pressure energy.

Governing of turbine:

The operation of regulation of speed of turbine runner is known as Governing of turbine. It is done automatically by means of Governor.
Runaway speed: This is the maximum speed at which a turbine wheel would run under the worst conditions of operation with all gates open so as to allow all possible water inflow under maximum head.