In a power generating plant, the steam turbine is a favorite primary mover. The capability of the steam turbine can be from 5 megawatts – 2000 megawatts.
Working Principle of Steam Turbine-
A practical turbine normally has two important parts nozzles and blades.
The nozzle is a tool installed at a turbine’s steam inlet. The high-temperature, high-pressure steam with negligible kinetic energy is extended, loses pressure and therefore receives enough kinetic energy to use the nozzles to conduct mechanical work.
The turbine blades are also called deflectors. This is because when it hits the blades, the dynamics steam gets deflected. Expanding steam’s mechanical power is obtained from turbine blades.
Pressurized steam acts on the piston in a reciprocating steam engine, causing mechanical piston movement. Ideally, in a reciprocating system, no dynamic steam action is used. But the dynamic action of suddenly extended steam is primarily used for mechanical work in the case of a steam turbine. The steam in the nozzles extends in a steam turbine, thereby gaining kinetic energy and dropping its pressure. During its expansion the steam gets kinetic energy from its internal enthalpy.
The turbine blades obstruct the steam momentum and thus force the stream to modify its flow direction. In other words, steam momentum creates a force on the blades of the turbine. We can conclude that the momentum of steam expansion is a steam turbine’s driving force.
Steam expansion and momentum change may occur once in a single stage or multiple times in various stages based on turbine kinds.
At high pressure and temperature, the steam enters the turbine. The steam exhausts at much reduced pressure and temperature after performing the required job of rotating the rotor. The steam may enter the turbine at 1800 Pa and 1000oF respectively, and the exhausting steam pressure and temperature may be 1 Pa and 100oF respectively.
Types of Steam Turbine-
There are mainly 2 types of Steam Turbine. These are
- Impulse Turbine
- Reaction Turbine
If there is only one supply of steam expansion in a turbine and the steam pressure stays uniform throughout the cycle after expansion through the nozzles, the turbine is referred to as a single-stage impulse turbine. In high-pressure impulse turbine, high-temperature steam from the nozzle head extends and forms a steam jet that strikes the moving blades straight, causing the turbine rotor to rotate.
There is another form of turbine in which throughout the process the steam is extended. Here, when it moves through the turbine blades, steam expansion takes place. Steam enthalpy transforms into kinetic energy during expansion, thereby rotating the turbine rotor with propeller action. This type of turbine is said reaction turbine. There are two sets of blades in this type of turbine. One set of fixed blades connected to the turbine’s stationary parts and another set of moving blades connected to the turbine’s rotor parts. Steam expansion occurs in the space of fixed and moving blades.
A steam turbine has the following benefits over a diesel engine-
- A steam turbine’s size is much smaller than a diesel engine’s size.The size of a steam turbine of 30 megawatts is the same as a 5 megawatt diesel engine.
- The steam turbine is much simpler in construction than a diesel engine.The three vital elements of a steam turbine are the rotor shaft, blades, steam control valve.
- If the rotating sections of the system are properly mounted and aligned, a steam turbine suffers from less vibration than that of the diesel engine.
- A steam turbine’s speed is much greater than a diesel engine’s. A steam turbine used in an electrical generating station has a standard speed of 3600 RPM in the USA and 3000 RPM in the UK, whereas a diesel engine used for the same purpose has a highest standard speed of 200 RPM.
- Steam turbine control is much simpler than diesel engine control. For this purpose, a control valve is used. The valve is attached to the steam inlet line. The steam flow to the turbine is regulated by this control valve. Before the control valve, there is one stop valve mounted. The function of stop valve in case of an abnormality is to block the entire flow of steam to the turbine. The valve for the stop is an emergency valve.
Also Read – What is Electric Power?
Also Read – What is electricity?