Reaction Turbine and Impulse Turbine

The reaction turbine is quite different from the impulse turbine.

In the reaction turbine a portion of the energy of the fluid is converted into energy by the fluid’s passing through adjustable gates before entering the runner, and the remainder of the conversion takes place through the runner.

Two types of the reaction turbines:
-Radial flow, e.g Francis turbine
-Axial flow, e.g Kaplan turbine

In the impulse turbine all the mechanical energy of the liquid is converted into kinetic energy by a nozzle that forms a free jet. The energy is then taken from the jet by suitable flow through moving vanes.The vanes are partly filled with the jet open to the atmosphere throughout its travel through the runner.

Type of the impulse turbine, e.g Pelton Wheel

Pump Losses

The shaft power or energy that is supplied to the pump by the prime mover is not the same as the energy received by the liquid, some energy is dissipated as the liquid passes through the machine.

The mechanism of the loss in a pump:

1. Mechanical friction power loss due to friction between the fixed and rotating parts in the bearing and boxes
2. Leakage and circulating power loss due to a loss of liquid from the pump or circulation of the liquid in the impeller
3. Disc friction power loss due to friction between the rotating faces of the impeller and the liquid.
4. Casing power loss

CAVITATION IN PUMPS

When the liquid flows through a centrifugal or axial flow pump, the static pressure (suction pressure) at the eye of the impeller is reduced and the velocity increases. There is therefore a danger that cavitation bubbles may form at the inlet to the impeller.

- Local pitting of the impeller can result when the bubbles collapse on a metallic surface
- Noise is generated in the form of sharp cracking when cavitation takes place.

Cavitation will not occur if critical cavitation number is greater than cavitation available. Every pump has a critical cavitation number which can only be determined by testing to find the minimum value of NPSH before cavitation occurs.

STEPS TO SELECT A CENTRIFUGAL PUMP

STEPS TO SELECT A CENTRIFUGAL PUMP
Centrifugal pump selection is the process of choosing the most suitable centrifugal pump for your needs.

1. Several factors must be considered when selecting the configuration and size of the pump necessary for your system. Factors that include location, available space, maintenance requirements, reliability, are all factors in the selection process.

2. The performance requirements of your system must be specified and the pump type must be selected.

3. Alternate pumps that meet the requirements of the application also should be specified. Normally, the most suitable pump is chosen from these pumps considering economic factors.

Five steps to select a pump are:
1. Determine flow rate
2. Determine static head
3. Determine friction head
4. Calculate total head
5. Select the pump (you can use manufacturer’s catalogue)

WHAT IS TURBOMACHINE?

WHAT IS TURBOMACHINE?
Any devices that extracts energy from or imparts energy to a continuously moving stream of fluid (liquid or gas) can be called a turbomachine. The Examples of turbomachine: windmills, waterwheels, ship propeller, hydraulic and gas turbines, pump and compressor.

The work principle of turbomachine: the energy transfer being carried out by the action of one or more rotating blade rows. The dynamic action of rotating blades sets up forces between the blades and fluid while the components of these forces in the direction of blade motion give rise to the energy transfer between the blades and fluid.

The type of turbomachines fall into 2 categories depending on:

whether work is done by the fluid on the rotating member
or
whether work is done by the rotating member on the fluid

Work is done by fluid, examples:
Axial flow hydraulic turbine
Radial flow hydraulic turbine
Mixed flow hydraulic turbine
Axial flow gas turbine
Pelton wheel hydraulic turbine

Work is done on fluid, examples:
Axial flow pump
Centrifugal pump
Axial flow compressor
Centrifugal compressor
Radial flow fan

The type of turbomachines can also be defined as
the manner of fluid movement through the rotating member
If the flow is essentially axial with no radial movement of the streamlines then the machine is called an axial flow machine.
If the flow is radial, the machine is classed as a radial flow centrifugal machine