Steam Turbine Fundamentals
Fundamentals
Energy Transfer
Coal, Natural Gas,
Nuclear, Biofuel,
Waste Fuel
1
What is a Turbine ?
-A Turbine is a device which converts the heat energy of
steam into the kinetic energy & then to rotational energy.
-The Motive Power in a steam turbine is obtained by the
rate of change in momentum of a high velocity jet of
steam impinging on a curved blade which is free to
rotate.
-The basic cycle for the steam turbine power plant is the
Rankine cycle. The modern Power plant uses the rankine
cycle modified to include superheating, regenerative feed
water heating & reheating.
RANKINE CYCLE
RANKINE CYCLE
TEMPERATURE, T
D'
D
C
WORK DONE
B
A
E
HEAT REJECTED
ENTROPY, S
PROCESS A-B:
ISENTROPIC/ADIABATIC COMPRESSION PROCESS
FEED WATER TO BOILER IS PRESSURIED TO BOILER
PROCESS B-C:
CONSTANT PRESSURE PROCESS
FEED WATER HEATED UPTO SATURATION TEMP T1 CALLED
SENSIBLE HEATING POINT C IS INTERMEDIATE POINT OF
STEAM GENERATION
PROCESS C-D:
CONSTANT PRESSURE & TEMPERATURE PROCESS
FEED WATER IS VAPOURISED CALLED LANTENT HEAT OF
VAPOURISATION POINT D IS STEAM IS DRY & SATURATED.
PROCESS D-E:
ISENTROPIC/ADIABATIC EXPANSION PROCESS
EXPANSION OF STEAM TO VACCUM
PROCESS E-A:
CONSTANT PRESSURE & TEMPERATURE PROCESS
REJECTION OF HEAT TO CONDENSOR TO CONDENSE THE STEAM.
AT POINT D, STEAM IS DRY & SATURATED.
E'
RANKINE CYCLE
(Reheat Cycle)
TEMPERATURE, T
E
D
C
WORK DONE
B
A
G
HEAT REJECTED
ENTROPY, S
F
Turbines Classification
Based on Blading Design
Impulse turbine
Steam energy is transferred to the rotor entirely by
the steam jets striking the moving blades.
Reaction turbine
Steam expands in both the stationary & moving
blades. Moving blades also act as nozzles. High axial
thrust is produced.
Combination of Impulse & Reaction turbine
Turbines Classification
Based on Inlet & Outlet Steam Condition
• Condensing turbines – Condensing turbines are most
commonly found in electrical power plants. These turbines
receive steam from a boiler and exhaust it to a condenser.
The exhausted steam is at a pressure well below
atmospheric.
• Back pressure or Non-Condensing turbines –
In a backpressure steam turbine, energy from high-pressure
inlet steam is efficiently converted into electricity, and
low
pressure exhaust steam is provided to a plant
process.
• Extraction turbines – Medium or low pressure steam
required by the process plant is extracted from the
intermediate stage of a condensing or back pressure turbine.
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.
Reaction turbine
• In the reaction turbine, the rotor blades
themselves are arranged to form
convergent nozzles.
• This type of turbine also 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 no net change in
steam velocity across the stage but with a
decrease in both pressure and temperature.
Impulse & Reaction Turbine
Impulse
-Pressure drops in nozzles and not in moving blade
Reaction
-Pressure drops in fixed blade as
well as in moving blades
-Constant blade channel area
-Varying blade channel area
-Profile type blades
-Aerofoil type blades
-Restricted round or incomplete steam admission
-All round or complete admiss.
-Diaphragm contains nozzles
-Fixed blades similar to moving blades
attached to casing serve as nozzles and
guide the steam
-Occupies less space for same power
-Occupies more space for same power
-Higher efficiency in initial stage
- higher efficiency in final stages.
-Suitable for small power requirements
-Suitable for medium or high power requ.
-Blade manufacturing is not difficult
-Blade manufacturing process is difficult.
-Velocity of steam is high
-Velocity of steam is less.
Steam turbine governing
• Steam turbine governing is the procedure of controlling the flow
rate of steam to a steam turbine so as to maintain its speed of
rotation as constant.
• The variation in load during the operation of a steam turbine can
have a significant impact on its performance.
• In a practical situation the load frequently varies from the designed
or economic load and thus there always exists a considerable
deviation from the desired performance of the turbine.
• The primary objective in the steam turbine operation is to maintain
a constant speed of rotation irrespective of the varying load. This
can be achieved by means of governing in a steam turbine.
Types of Governing
The flow rate of steam is monitored and controlled by interposing
valves between the boiler and the turbine. Depending upon the
particular method adopted for control of steam flow rate, different
types of governing methods being practiced are as follows:
1.
2.
3.
4.
5.
Throttle Governing
Nozzle Governing
Bye-pass Governing
Combination of any of the above two
Emergency Governing
Throttle Governing
• In throttle governing the pressure of steam is reduced at the turbine entry
thereby decreasing the availability of energy.
• In this method steam is passed through a restricted passage thereby
reducing its pressure across the governing valve.
• The flow rate is controlled using a partially opened steam control valve.
The reduction in pressure leads to a throttling process in which the
enthalpy of steam remains constant.
• With a reduction in the load and so torque, the turbine shaft speed
increases, as power is:
Throttle Governing
•
•
•
•
The valve is actuated by using a
centrifugal
governor
which
consists of flying balls attached
to the arm of the sleeve.
A geared mechanism connects
the turbine shaft to the rotating
shaft on which the sleeve
reciprocates axially.
With a reduction in the load the
turbine shaft speed increases.
Consequently with the increase
of centrifugal force the fly balls
fly apart.
This result in an axial movement
of the sleeve followed by the
activation of a lever, which in
turn actuates the main stop valve
to a partially opened position to
control the flow rate.
Nozzle Governing
• In throttle governing at low loads turbine efficiency is considerably
reduced due to large reduction in pressure.
• In nozzle governing the flow rate of steam is regulated by opening and
shutting of sets of nozzles rather than regulating its pressure.
• In this method groups of two, three or more nozzles form a set and each
set is controlled by a separate valve.
• With the decrease of load, the required number of nozzle sets may be shutoff.
• The actuation of individual valve closes the corresponding set of nozzle
thereby controlling the flow rate.
• In actual turbine, nozzle governing is applied only to the first stage
whereas the subsequent stages remain unaffected.
• Since no regulation to the pressure is applied, the advantage of this
method lies in the exploitation of full boiler pressure and temperature.
Nozzle Governing
Bye-pass Governing
• Occasionally the turbine
is overloaded for short
durations.
• During such operation,
bypass
valves
are
opened and fresh steam
is introduced into the
later stages of the
turbine.
• This generates more
energy to satisfy the
increased load.
• This operates in a
turbines
which
are
throttle governed.
Bye-pass Governing
Combination Governing
• Combination governing
employs usage of any
two of the above
mentioned methods of
governing.
• Generally bypass and
nozzle governing are
used simultaneously to
match the load on
turbine.
Emergency Governing
Every steam turbine is also provided with emergency governors which
come into action under the following condition:




When the speed of shaft increases beyond 110%.
Balancing of the turbine is disturbed.
Failure of the lubrication system.
Vacuum in the condenser is quite less or supply of coolant to the
condenser is inadequate.
Cogeneration
• Cogeneration, also known as combined heat and power (CHP),
refers to a group of proven technologies that operate together for
the concurrent generation of electricity and useful heat in a
process that is generally much more energy-efficient than the
separate generation of electricity and useful heat.
Types of plants
• Topping cycle plants primarily produce electricity from a
steam turbine.
• The exhausted steam is then condensed and the low
temperature heat released from this condensation is utilized
for e.g. district heating or water desalination.
• Bottoming cycle plants produce high temperature heat for
industrial processes, then a waste heat recovery boiler feeds
an electrical plant.
• Bottoming cycle plants are only used when the industrial
process requires very high temperatures such as furnaces for
glass and metal manufacturing, so they are less common.
Scope of Cogeneration
In recent years cogeneration has become an attractive and
practical proposition for a wide range of applications.
These include:
• the process industries
• pharmaceuticals,
• Paper and board,
• brewing,
• ceramics, brick, cement, food, textile, minerals etc.
Common CHP plants
The supply of high-temperature heat first drives a gas or steam
turbine-powered generator and the resulting low-temperature waste
heat is then used for water or space heating.
STEAM TURBINE CHP
Common CHP plants
GAS TURBINE CHP
Common CHP plants
GAS TURBINE & STEAM TURBINE COMBINED CYCLE
GAS TURBINE & STEAM TURBINE COMBINED CYCLE on T-s plot
Tri-generation
Tri-generation or combined cooling, heat and power (CCHP)
refers to the simultaneous generation of electricity and useful
heating and cooling from the combustion of a fuel or a solar heat
collector.
Thermal efficiency of cogeneration
For cogeneration:
For Tri-generation: