A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Steam turbines are commonly used to generate electricity and supply useful work for industrial sectors such as power, oil and gas. They convert the thermal energy of pressurized steam produced in a boiler into rotational kinetic energy. In this article, we will discuss the working principle, types, components and applications of steam turbines.
Working Principle of Steam Turbine
The basic working principle of a steam turbine relies on the thermodynamic principle that steam has high internal energy that can be extracted in a controlled manner. In a steam turbine, high-pressure steam from a boiler enters through blades called nozzles or buckets mounted on a rotor. The steam is traveling at a high velocity causing force against the blades. This transfers part of the kinetic energy of the steam to the rotor causing it to rotate. After passing through the stationary blades, the steam exits through the turbine stage at a lower pressure. Multiple stages are used to extract energy from steam at gradually lower pressures. The kinetic energy extracted from steam is converted into mechanical rotational energy of the rotor shaft which then drives an electrical generator or machinery.
Types of Steam Turbines
Based on their speed and power output, steam turbines can be categorized into different types:
- Impulse turbines: In these turbines, stationary blades or nozzles allow the high velocity steam to impulse on the rotor blades. De Laval impulse turbine is commonly used where steam passes through one stage only.
- Reaction turbines: In reaction turbines, the steam passes through multiple stages of stator and rotor blades. As the name suggests, the steam reacts with both stationary and rotating blades. Reaction turbines are more efficient and commonly used in power generation. Example - Parsons turbine.
- Compounding turbines: To improve efficiency, some turbines use multiple sets of rotors arranged in casings with multiple pressure levels. Example - Curtis turbine.
- Condensing turbines: These exhaust steam turbines operate in a partial or full vacuum to condense the steam and thus improve efficiency.
- Non-condensing turbines: When process steam is required, non-condensing turbines are used that exhaust steam at pressure above condenser pressures.
Components of a Steam Turbine
Some of the major components of a steam turbine include:
- Rotor: It is the rotating component mounted on bearings. Multiple blades are attached to extract energy from high velocity steam.
- Stationary blades/Nozzles: Fixed blades situated before rotor blades to direct and accelerate steam towards the rotor.
- Casing: It is used to house all stationary and rotating turbine components such as rotor, blades and shaft.
- Gland sealing system: Used to prevent steam leakage between stationary and rotating components.
- Lubrication system: Critical for smooth turbine operation, uses an oil pump system to lubricate bearings.
- Control systems: Used to control steam admission, turbine speed and load. System includes valves, governors and actuators.
- Condenser: Lowers exhaust steam temperature and pressure below atmospheric to maximize efficiency.
Applications of Steam Turbines
Given their unique advantages of converting heat energy to rotation, steam turbines have diverse applications in industries:
Power Generation: Steam turbines are commonly used in thermal power plants fueled by coal, natural gas, biomass etc. They convert heat from combustion into electricity through generator drivetrains. Several MW capacity turbines are used in nuclear, geothermal and solar power projects as well.
Industrial Process Drives: Medium and low power steam turbines are used to drive pumps, compressors, blowers, fans and other rotating equipment in oil & gas, petrochemical, pulp & paper industries.
Marine Propulsion: Large steam turbines are used for ship propulsion in naval vessels, cruises and bulk carriers. Combined with geared turbines, they provide efficient mechanical power for ship propellers.
District Heating: Recovered heat from electricity production or industrial processes can be used for district heating schemes supplying heat to residential and commercial buildings.
Desalination: Steam turbines are used to drive desalination plants that convert seawater into potable water through reverse osmosis systems.
Future Trends for Steam Turbines
While steam turbines have proven reliability over decades as rotating heat engines, the industry is exploring advancements:
Materials Technology: Developments in super alloys, ceramics and coatings are helping build more efficient turbine components that can withstand higher temperatures and pressures.
Condition Monitoring: Real-time monitoring of turbine performance helps detect degradation mechanisms early and schedule maintenance to improve availability.
Cogeneration and Trigeneration: Combined heat and power systems are utilizing waste heat from turbines productively for heating, cooling and other applications to maximize efficiency levels.
Nuclear Energy: Growth in nuclear power will advance steam turbine designs suitable for multipurpose applications including process heat besides electricity.
Nanotechnology: Studies are being conducted to develop nanostructured coatings on turbine blades and rotors to achieve better heat transfer properties and reduce friction losses.
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