- Steam Turbine Definition: A steam turbine is a device that converts high-pressure steam into mechanical energy to generate electricity.
- Advantages: Steam turbines are smaller, simpler, and operate at higher speeds with less vibration compared to diesel engines.
- Working Principle: Steam turbines use the dynamic action of expanded steam to produce mechanical movement.
- Impulse and Reaction Turbines: Impulse turbines expand steam in a nozzle and strike the blades, while reaction turbines expand steam continuously through fixed and moving blades.
- Components: Essential parts include nozzles that expand steam and blades that extract mechanical energy from the steam.
A steam turbine converts high-pressure steam into rotating shaft work, which a generator then turns into electricity. The advantages of a steam turbine over a diesel engine are as follows.
- A steam turbine is much smaller than a diesel engine of similar power. As a rough size comparison, a 30-megawatt steam turbine can occupy about the same space as a 5-megawatt diesel set.
- The rotating steam path is simpler than a diesel’s many reciprocating parts. The steam path is a rotor shaft, blades and a steam control valve, plus casings, seals and bearings around them.
- A steam turbine vibrates less than a diesel engine when the rotating parts are installed and aligned correctly.
- Steam-turbine speed can be much higher than diesel speed. A two-pole machine in an electrical generating station runs at 3600 RPM on 60 Hz (USA) and 3000 RPM on 50 Hz (UK). Large four-pole nuclear and thermal units run at 1800 or 1500 RPM. Diesel generating sets span slow-speed two-stroke machines near 100 to 200 RPM up to high-speed four-stroke sets near 1500 RPM.
- Controlling a steam turbine is simpler than controlling a diesel engine. A control valve at the steam inlet regulates steam flow to the turbine. A stop valve before the control valve blocks all steam flow to the turbine in emergencies.
Steam enters the turbine at high pressure and temperature. After turning the rotor, the steam exits at a much lower pressure and temperature. A common older US fossil rating is about 1800 psi and 1000°F at the throttle, with exhaust to a condenser near 1 psia and a temperature near 100°F. Utility machines range from a few megawatts to well over 1,000 MW.
Working Principle of Steam Turbine
In a reciprocating steam engine, pressurised steam pushes a piston. That piston force does the work. A steam turbine instead uses the dynamic action of expanding steam.
In a steam turbine the steam expands in the nozzles, so it gains kinetic energy and loses pressure. That kinetic energy comes from the steam’s internal enthalpy. The blades intercept the steam’s momentum and turn the flow. The change of momentum puts a force on the blades. That momentum change is the driving force of a steam turbine.
Expansion and the change of direction can happen once in a single stage or several times in successive stages, depending on the turbine type.
If steam expands once in the nozzles and then stays at nearly uniform pressure while it crosses the moving blades, the machine is a single-stage impulse turbine. High-pressure, high-temperature steam leaves the nozzle as a jet and strikes the moving blades, which turn the rotor.
In a reaction turbine, steam keeps expanding as it passes the blades. Enthalpy turns into kinetic energy in the blade passages, and the rotor turns with a propeller-like action. Large utility turbines often combine impulse stages at the high-pressure end with reaction stages later, so the two types can share one shaft.
A reaction turbine has two blade sets. Fixed blades sit on the stationary casing. Moving blades sit on the rotor. Steam expands in the passages formed by both sets.
A working turbine still needs nozzles and blades. The nozzle sits at the steam inlet. High-temperature, high-pressure steam arrives with little kinetic energy, expands, loses pressure and leaves with enough speed to do work on the blades.
Turbine blades are also called deflectors: they turn the steam when it strikes them. That turn extracts mechanical energy from the expanding steam.
