STEAM TURBINE - WORKING PRINCIPLE AND TYPES OF STEAM TURBINES

STEAM TURBINE WORKING PRINCIPLE AND TYPES OF STEAM TURBINES

WHAT IS STEAM TURBINE ?

• The steam turbine is one kind of heat engine machine in which steam's heat energy is converted to mechanical work.The construction of steam turbine is very simple.There is no piston rod,flywheel or slide valves attached to the turbine.So maintenance is quite easy.It consists of a rotor and a set of rotating blades which are attached to a shaft and the shaft is placed in the middle of the rotor.An electric generator known as steam turbine generator is connected to the rotor shaft.The turbine generator collects the mechanical energy from the shaft and converts it into electrical energy. Steam turbine generator also improves the turbine efficiency.

WORKING PRINCIPLE OF STEAM TURBINE

• Working principle of steam turbine depends on the dynamic action of steam.A high-velocity steam is coming from the nozzles and it strikes the rotating blades which are fitted on a disc mounted on a shaft.This high-velocity steam produces dynamic pressure on the blades in which blades and shaft both start to rotate in the same direction.Basically,in a steam turbine pressure energy of steam extracts and then it converted into kinetic energy by allowing the steam to flow through thew nozzles.The conversion of kinetic energy does mechanical work to the rotor blades and the rotor is connected to a steam turbine generator which acts as a mediator.Turbine generator collects mechanical energy from the rotor and converts into electrical energy.Since the construction of steam turbine is simple, its vibration is much less than the other engine for same rotating speed.Though different types of governing system are used to improve turbine speed.

TYPES OF STEAM TURBINE

• Based on operating principle the steam turbine is classified as two types

1. Impulse turbine
2. Reaction turbine

Impulse Turbine

• In principle, the impulse steam turbine consists of a casing containing stationary steam nozzles and a rotor with moving or rotating buckets.

• When the steam passes through the stationary nozzles and is directed at high velocity against the rotor buckets. The rotor buckets starts to rotate at high speed.

Events take place in the nozzle

• The steam pressure decreases.
• The enthalpy of the steam decreases.
• The steam velocity increases
• The volume of the steam increases.

In nozzles, the pressure energy of the steam is converted into kinetic energy. They are two types of nozzles used in the steam turbine. They are

1. Convergent nozzles
2. Convergent -divergent nozzles

The Convergent Nozzles

• They are used for smaller pressure drops where the minimum exit pressure is 0.577 x the inlet pressure (the critical pressure for nozzles.) If the exit pressure is less than 0.577 x inlet pressure, eddy-currents are developed and finally, the exit velocity will be less than calculated.

The Convergent-divergent Nozzles

• The convergent-divergent nozzles prevent eddy currents and the calculated velocity will be obtained even at large pressure drops.

• The purpose of the bucket or moving blade on the rotor is to convert the kinetic energy of the steam into mechanical energy. If all kinetic energy is converted the steam exit velocity will be 0 m/s. This is not possible but it shows that the rotor blades must bring the steam exit velocity near 0 m/s.

The Impulse Principle

• steam at high pressure enters through a stationary nozzle of a steam turbine, as a result the pressure of the steam is decrease and an increase in steam velocity.  As a result of increased steam velocity steam pass through the nozzle in the form of a high-speed jet. This high-velocity steam hit the properly shaped turbine blade, as a result, the steam flow direction is changed. The effect of this change in direction of the steam flow will produce an impulse force. This force cause the blade move, thereby the rotor will start to rotate.

• The force applied to the blade is developed by causing the steam to change the direction of flow (Newton’s 2nd Law – change of momentum). The change of momentum produces the impulse force.

Impulse Turbine Working:

• In the impulse turbine pressure drops and the velocity increases as the steam passes through the nozzles. When the steam passes through the moving blades the velocity drops but the pressure remains the same.

• The fact that the pressure does not drop across the moving blades is the distinguishing feature of the impulse turbine. The pressure at the inlet of the moving blades is same as the pressure at the outlet of moving blades.

Reaction Turbine Principle:

• In the case of reaction turbine, the moving blades of a turbine are shaped in such a way that the steam expands and drops in pressure as it passes through them. As a result of pressure decrease in the moving blade, a reaction force will be produced. This force will make the blades to rotate.

Reaction Turbine Working:

• A reaction turbine has rows of fixed blades alternating with rows of moving blades. The steam expands first in the stationary or fixed blades where it gains some velocity as it drops in pressure. Then enters the moving blades where its direction of flow is changed thus producing an impulse force on the moving blades. In addition, however, the steam upon passing through the moving blades, again expands and further drops in pressure giving a reaction force to the blades.

• This sequence is repeated as the steam passes through additional rows of fixed and moving blades.

• Note that the steam pressure drops across both the fixed and the moving blades while the absolute velocity rises in the fixed blades and drops in the moving blades.

• The distinguishing feature of the reaction turbine is the fact that the pressure does drop across the moving blades. In other words, there is a pressure difference between the inlet to the moving blades and the outlet from the moving blades.