A rotor of a modern steam turbine
A steam turbine is a mechanical device that extracts thermal energy
from pressurized steam, and converts it into rotary motion.
Types
Industrial steam turbines employ both impulse and reaction. Their
capacities vary from 0.5 MW to 1000 MW.
Steam Supply and Exhaust Conditions
These types include condensing, non condensing, reheat, extraction and
induction.
process steam are available.
Condensing turbines are most commonly found in electrical 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.
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.
Extracting type turbines are common in all applications. In an extracting
type turbine, steam is released from various stages of the turbine, and used
for industrial process needs or sent to boiler feedwater heaters to improve
overall cycle efficiency. 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.
Casing or Shaft Arrangements
These arrangements include single casing, tandem compound and cross
compound turbines. Single casing units are the most basic style where a single
casing and shaft are coupled to a generator. Tandem compound are used where two
or more casings are directly coupled together to drive a single generator. A
cross compound turbine arrangement features two or more shafts not in line
driving two or more generators that often operate at different speeds. A cross
compound turbine is typically used for many large applications.
Principle of Operation and Design
An ideal steam turbine is considered to be an isentropic process, or
constant entropy process, in which the entropy of the steam entering the
turbine is equal to the entropy of the steam leaving the turbine. No steam
turbine is truly “isentropic”, however, with typical isentropic efficiencies
ranging from 20%-90% based on the application of the turbine. The interior of a
turbine comprises several sets of blades, or “buckets” as they are more
commonly referred to. One set of stationary blades is connected to the casing
and one set of rotating blades is connected to the shaft. The sets intermesh
with certain minimum clearances, with the size and configuration of sets
varying to efficiently exploit the expansion of steam at each stage.
Schematic diagram outlining the difference between an
impulse and a reaction turbine
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 impulse or reaction
turbines. Most modern steam turbines are a combination of the reaction and
impulse design. Typically, higher pressure sections are impulse type and lower
pressure stages are reaction type.
Impulse Turbines
An impulse turbine has fixed nozzles that orient the steam flow into
high speed jets. These jets contain significant kinetic energy, which the rotor
blades, shaped like buckets, convert into shaft rotation 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 steam chest
pressure to condenser pressure (or atmosphere pressure). Due to this relatively
higher ratio of expansion of steam in the nozzle the steam leaves the nozzle
with a very high velocity. The steam leaving the moving blades is 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".
Reaction Turbines
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.
Speed regulation
The control of a turbine with a governor is essential, as turbines need to
be run up slowly, to prevent damage while some applications (such as the
generation of alternating current electricity) require precise speed control.
Uncontrolled acceleration of the turbine rotor can lead to an overspeed 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 spectacularly. Turbines are expensive to make, requiring precision
manufacture and special quality materials.
