Steam Turbine and Power Generation


Steam Turbine and Power Generation

A steam turbine is a mechanical device that converts thermal energy of the pressurized steam into useful mechanical work. It is the heart of a power plant. It has a higher thermodynamic efficiency and a lower power-to-weight ratio. It derives most of its thermodynamic efficiency because of the use of multiple stages in the expansion of the steam which results in a closer approach to the ideal reversible process. Steam turbines are one of the most versatile and oldest prime mover technologies being used to drive a generator. Power generation using steam turbines has been in use for more than 100 years. A turbo generator is the combination of a turbine directly connected to a generator for the generation of electrical power. Large steam power generators provide the majority of the electric power.

Steam turbines are ideal for very large power configurations used in power plants because of their higher efficiencies and lower costs. In a power plant, the steam turbine is attached to a generator to produce electrical power. The turbine acts as the more mechanical side of the system by providing the rotary motion for the generator, while the generator acts as the electrical side by employing the laws of electricity and magnetism to produce electrical power.

In a steam turbine rotor is the spinning component that has wheels and blades attached to it. The blade is the component that extracts energy from the steam. A typical schematic diagram of afossil fuel powered steam turbine based  power plant for electricity generation is given in Fig 1

Fossil fuel based power plant

 Fig 1 Schematic diagram for steam turbine based power generation

The energy conversion process

Steam has the following three components of energy components

  • Kinetic energy –  by virtue of its velocity
  • Pressure energy – by virtue of its pressure
  • Internal energy – by virtue of its temperature

Last two components of energy together are known as enthalpy. Total energy of steam can be represented as sum of kinetic energy and enthalpy.

Energy generation using steam turbine involves three energy conversions, extracting thermal energy from the fuel and using it to raise steam, converting the thermal energy of the steam into kinetic energy in the turbine and using a rotary generator to convert the turbine’s mechanical energy into electrical energy.

High pressure steam is fed to the turbine and passes along the machine axis through multiple rows of alternately fixed and moving blades. From the steam inlet port of the turbine towards the exhaust point, the blades and the turbine cavity are progressively larger to allow for the expansion of the steam.

The stationary blades act as nozzles in which the steam expands and emerges at an increased speed but lower pressure (Bernoulli’s conservation of energy principle which is that kinetic energy increases as pressure energy falls). As the steam impacts on the moving blades it imparts some of its kinetic energy to the moving blades.

Turbines can be of condensing, non-condensing, reheat, extraction or induction type. Condensing turbines are commonly used in power plants. These turbines exhaust steam in a partially condensed state, typically of a quality near 90 %, at a pressure well below atmospheric to a condenser. Non-condensing turbines are also known as backpressure turbines and are most widely used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly used in industries where large amounts of low pressure process steam are needed. Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion. In an extraction turbine, steam is withdrawn from one or more stages, at one or more pressures, for heating, plant process, or feed water heater needs. These turbines are also known as bleeder turbines. Extraction flows may be controlled with a valve, or left uncontrolled. Induction turbines introduce low pressure steam at an intermediate stage to produce additional power.
There are two basic steam turbine types namely impulse turbines and reaction turbines. The blades are designed to control the speed, direction and pressure of the steam as is passes through the turbine.

In the impulse design the rotor turns due to the force of the steam on the blades while the reaction design works on the principle that the rotor derives its rotational force from the steam as it leaves the blades.

To maximize turbine efficiency the steam is expanded, generating work, in a number of stages. These stages are characterized by how the energy is extracted from them and are known as either impulse or reaction turbines. Most steam turbines use a mixture of the reaction and impulse designs. Each stage behaves as either one or the other, but the overall turbine uses both. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type.
An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which is converted into shaft rotation by the bucket-like shaped rotor blades, as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage. As the steam flows through the nozzle its pressure falls from inlet pressure to the exit pressure (atmospheric pressure, or more usually, the condenser vacuum). Due to this high ratio of expansion of steam, the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades has a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the carry over velocity or leaving loss.

In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor.

The two types of turbines are shown in Fig 2.

Type of turbines

 Fig 2 Types of turbine

The diagram at Fig 3 summarizes a boiler steam turbine cycle.

Boiler turbine cycle

Fig 3 A simple boiler steam turbine cycle

The steam turbine operates on basic principles of thermodynamics using the Rankine cycle as shown in Fig 4. After leaving the boiler, superheated vapour enters the turbine at high temperature and high pressure. The high heat/pressure steam is converted into kinetic energy using a nozzle (a fixed nozzle in an impulse type turbine or the fixed blades in a reaction type turbine). Once the steam has left the nozzle it is moving at high velocity and is sent to the blades of the turbine. A force is created on the blades due to the pressure of the vapour on the blades causing them to move. A generator or other such device can be placed on the shaft, and the energy that was in the vapour can now be stored and used. The gas exits the turbine as a saturated vapour at a lower temperature and pressure than it entered with and is sent to the condenser to be cooled.

T-s diagram

Fig 4 T-s diagram of a Rankine cycle

The exhaust steam from the turbine is condensed to water in the condenser which extracts the latent heat of vaporization from the steam. This causes the volume of the steam to go to zero, reducing the pressure dramatically to near vacuum conditions thus increasing the pressure drop across the turbine enabling the maximum amount of energy to be extracted from the steam. The condensate is then pumped back into the boiler as feed-water to be used again.

The governor is a device that controls the speed of the turbine. The speed control of a turbine with a governor is necessary, since turbine is required to be run up slowly to prevent damage and the generation of AC electrical power needs precise speed control. Uncontrolled acceleration of the turbine rotor can lead to an over speed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart, often catastrophically. Modern turbines have an electronic governor that uses a sensor to monitor the turbine speed by ‘looking’ at the rotor teeth.

The steam turbine drives a generator, to convert the mechanical energy into electrical energy. The generator is a rotating field synchronous machine. The steam turbines are directly coupled to their generators. The generators must rotate at constant synchronous speeds according to the frequency of the electric power system. The most common speed is 3,000 RPM for power system with 50 Hz frequency. The energy conversion efficiency of these high capacity generators can be as high as 98 % or 99 % for a very large machine.