STEAM TURBINE -Operation principles
Operation principles:
In principle, the impulse steam Turbine consists of a casing containing stationary steam nozzles and rotor with moving or rotating buckets.
The steam passes through the stationary nozzles and is directed at high velocity against the rotor buckets causing the rotor to rotate at high speed.
The following events take place in the nozzles:
- The steam pressure decreases
- The enthalpy of steam decreases
- The steam velocity increases.
- The volume of the steam increases
There is a conversion of heat energy to kinetic energy as the heat energy from the decreases in steam enthalpy is converted into kinetic energy by the increased steam velocity.
The nozzle may be convergent nozzles or they may be convergent-divergent nozzles convergent nozzle are used for smaller pressure drops where the minimum exit pressure 0.577 x the inlet pressure of the exit pressure less than 0.577 x on let pressure eddy – currents are developed and the exit velocity will be less than calculated.
The convergent-divergent nozzles prevent eddy current and the calculated velocity will be obtained even at large pressure drops.
The purpose of the bucket or moving blade the rotor it to convert the kinetic energy of the steam into mechanical energy if all kinetic energy is converted to the steam exit velocity will be 0m/s this is not possible but it was shown that the rotor blades must bring the steam exit velocity near ‘0’/
The impulse principle :
If steam at high pressure is allowed to expand through a stationary nozzle the result will be a drop in the steam pressure and increase in steam velocity on the fact the steam will issue from the nozzle on the form of a high-speed jet. If high-velocity steam is applied to a properly shaped turbine blade it will change in direction due to the shape of the blade the effect of the change in direction of steam. Flow will be to produce an impulse force on the blade causing it to move of the blade is attached to the rotor of a turbine then the rotor will revolve.
Force applied to the blade is developed by causing the steam to change the direction of flow (Newton 2nd law) change of momentum the change of momentum products the impulse force.
In an actual impulse turbine, there are a no.of stationary nozzles and the moving blades are arranged completely around the rotor periphery note that the pressure drops and the velocity increases as the steam passes through the nozzles then as 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 to the moving blades is the same as the pressure at the outlet from the moving blades.
If there is no escape opening or nozzle for the steam then the pressure will be expanded and drops in pressure as it passes through them then a reaction will be produced which gives a force to the blades this reaction effect can be illustrated by considering a container filled with high-pressure steam.
If there is no escape opening or nozzle for the steam then the pressure will be the same an all walls of the container and the container will remain at rest if however the container has an escape opening or nozzle the steam will expand through the opening and drop in pressure, therefore, there will be an unbalanced pressure on the wall opposite to the opening and a reaction force & will be produced causing the container to move reaction effect.
A reaction turbine has rows of fixed blades with rows of moving blades the steam expands first in the stationary or fixed blades where it gains some velocity as it drops on pressure it then enters the moving blade where its direction of flow is charged 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 on the fixed blades and drops on the moving blades.
The distinguishing feature of the reaction turbine is the fact that the pressure does drop across the moving blades on order words there is a pressure difference between the inlet to the moving blades and the outlet from the moving blades.