HYDRAULIC TURBINES

MECHANICAL TIPS AND STUDY MATERIAL

HYDRAULIC TURBINES

These are the devices that convert the hydraulic/water energy into mechanical energy to generate mechanical work. These devices are reverse of pumps which we have studied in previous article where mechanical energy is utilized to generate water energy. The water energy may be in the form of potential or kinetic energy. Potential energy is static water source like dam or large reservoir and kinetic energy can be found in running water.

CLASSIFICATION OF HYDRAULIC TURBINES

      Direction of fluid flow

      Specific speed

      Water action on the blades

1.      Direction of fluid flow:

      Axial flow

      Radial flow

      Tangential flow

      Mixed flow

Axial flow water turbines:

In such turbines, the direction of flow of water is parallel to the axis of rotation of runner. Such type of turbines is propeller and Kaplan turbine.

Radial flow turbines:

As the name implies, the water enters the turbine in radial direction and the flow of water is perpendicular to the axis of rotation of runner. Flows are of two types namely 1) inward flow 2) outward flow.

In inward flow, water enters the turbine from periphery and directed towards the runner and leaves from inner periphery. Whereas in Outward flow type machines the water enters from inner periphery, goes pass the runner and exits through outer periphery. It is just the reverse case of inward flow type machine.

Tangential flow turbines:

Such flow is categorized by flow of water in tangential direction to the rotation of runner. Consider the figure of Pelton wheel as shown in the above figure which represents the tangential action of water jet on the turbine blades.

Mixed flow type turbines:

Mixed flow type is combination of radial and axial flow actions of water. In such flow type, the water enters the turbine radially inwards (outer periphery) and passes over the runner and then it exits axially (parallel to axis of rotation of runner). Its application is Deriaz turbine.

2.  SPECIFIC SPEED

      Low specific speed

      Medium specific speed

      High specific speed

Low specific speed turbines employ large head and low discharge. The head may be in range of 200 meter – 1800 meter. Example is pelton wheel.  Medium specific speed turbines have head 50 meter – 200 meter. These have moderate head and discharge. Example is Francis turbine. High specific speed turbines have small head but with large discharge. The head may maximum go up to 50 m. Examples is Kaplan turbine.

3. ACTION OF WATER ON THE BLADES OF TURBINE:

      Impulse

      Reaction

Impulse action of water:

In such machines only kinetic energy is available at the inlet of turbine for generating power or transforming energy and pressure at the inlet and outlet is the same. Given below is the impulse action of water on the blades.

Reaction type turbines:

In addition to kinetic energy present at the inlet of machine, the fluid also has potential energy for power generation and energy transformation. Example is Parson’s turbine.

The various types of efficiencies encountered in hydraulic turbines are:

      Mechanical efficiency

      Hydraulic efficiency

      Volumetric efficiency

      Overall efficiency