Steam turbines
Nimesh Gajjar
What exactly is the turbine?
Turbine is an engine that
converts energy of fluid
into mechanical energy
The steam turbine is
steam driven rotary
engine.
Physical Principles: Steam Turbines:
• High Pressure Steam
expands through a
governor valve and a
nozzle.
• Experiences an increase
in velocity and
momentum
• Pushes the impeller to
drive the turbine.
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Principle & Types of Steam Turbine.
• Types
 Impulse Turbine.
 Reaction Turbine.
• Principle: when steam is allowed to expand through
a narrow orifice, it assumes kinetic energy at the
expense of its enthalpy (heat energy). This kinetic
energy of steam is changed to mechanical
(rotational) energy through the impact (impulse) or
reaction of steam against the blades.
Construction of steam turbines
Construction of steam turbines
Classification of steam turbines
a) way of energy
conversion
- impulse turbines
- reaction turbines
Classification of steam turbines
b) flow direction
- axial
- radial
c) number of
stages
- single stage
- multi-stage
Classification of steam turbines
d) rotational speed
- regular
- low-speed
- high-speed
e) inlet steam pressure
- high pressure (p>6,5MPa)
- intermediate
pressure(2,5MPa
<p<6,5MPa)
- low-pressure (p<2,5MPa)
Classification of steam turbines
f) way of energy
utilisation
- condensing
- extraction
- back-pressure
Turbine Types
condenser
Straight-flow condensing
process
Back-pressure
Turbine Types
One section
Two sections
Types of Turbine
Reheat turbine
Types of Turbine
Extraction turbine
Classification of steam turbines
g) application
- power station
- industrial
- transport
How does the steam turbine work?
•
•
Impulse stage – whole
pressure drop in nozzle
(whole enthalpy drop is
changed into kinetic energy
in the nozzle)
Reaction stage – pressure
drop both in stationary
blades and in rotary blades
(enthalpy drop changed
into kinetic energy both in
stationary blades and in the
moving blades in rotor)
Single Stage Impulse Turbine
IMPLUSE STAGE
•
•
•
Nozzles
Blades
Velocity
•
It is usually called De-Laval turbine
The steam is fed through one or several
convergent-divergent nozzles
The nozzles do not extend completely
around the circumference of the rotor,
so that only part of the blades are
impinged upon by the steam.
Pressure drop occurs in the nozzle and
not in the blades.
Maximum velocity and hence kinetic
energy of the steam occurs at the
nozzle exit
Velocity change occurs in the rotor
blades where the steam gives up its
energy to the rotor blades.
Pressure
•
•
Impulse Stage
Compounded Steam Turbines
• Compounded steam turbine means multistage turbine.
• Compounding is needed when large enthalpy drop is
available.
• Since optimum blade speed is related to the exit nozzle speed.
It will be higher as the enthalpy drop is higher.
• The blade speed is limited by the centrifugal force as well as
needs of bulky reduction gear
• Compounding can be achieved either by velocity
compounded turbine or pressure compounded turbine.
Velocity Compounded Impulse Turbine
• The velocity compounded turbine was first proposed
by C.G Curtis.
• It is composed of one stage of nozzles, as the single
stage turbine, followed by two rows of moving
blades instead of one.
• These two rows are separated by one row of fixed
blades which has the function of redirecting the
steam leaving the first row of the moving blades to
the second row of moving blades.
Velocity Compounded Impulse Turbine
(Contd.)
Velocity-compounded Stage
Pressure Compounding Impulse
Turbine (Contd.)
Two Stages Pressure Compounding
Turbine
Three Stages Pressure Compounding Turbine
Pressure-compounded Stage
Pressure-velocity Compounded Turbine
Reaction Principle
• Reaction effect results from issuing a fluid at very high
velocity from a nozzle. This results in a reaction which
moves the nozzle in the opposite direction.
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• Pure reaction happens if the flow is accelerated from zero
velocity to its exist velocity in the moving blades.
• Since this is not the case in turbines, thus there are no pure
reaction turbine but it is usually a mix between impulse and
reaction. Accordingly the term reaction turbine does not
mean a full reaction turbine but a partially impulse and
partially reaction.
Reaction Turbine
Reaction Turbine
•
•
•
•
Reaction turbine has been invented by
C.A. Parson
Turbine with 50% reaction is the
turbine where 50% of the enthalpy
drop happens in the stator and the
other 50% occurs in the rotor. It is
important to mention that this does
not mean equal pressure drops.
Pressure drop is usually higher for the
fixed blades and greater for the high
pressure conditions, where the
pressure drop per unit of enthalpy
drop is higher at the high pressure
The rotor blades of a reaction turbine
are not symmetrical as in the impulse
turbine, they are similar to those of
the stator but curved in the opposite
direction.
Compounding
Velocity Compounded Impulse Turbine
(Contd.)
Pressure Compounding Impulse
Turbine (Contd.)
Two Stages Pressure Compounding
Turbine
Three Stages Pressure Compounding Turbine
STEAM TURBINE
Compounding
Types of turbine
• Impulse turbine
– Steam expands in nozzles only
– Steam pressure at the outlet side of the blade
equals that at inlet side
• Impulse – Reaction turbine
– Steam expands in nozzles as well as it passes
through blades
– Steam pressure at the outlet side of the blade may
be less than that at inlet side.
Impulse Stage
Velocity-compounded Stage
Pressure-compounded Stage
Pressure-velocity Compounded Turbine
Reaction Turbine
Impulse Turbine
• A turbine that is driven by high
velocity jets of water or steam from
a nozzle directed on to vanes or
buckets attached to a wheel. The
resulting impulse (as described by
Newton's second law of motion)
spins the turbine and removes
kinetic energy from the fluid flow.
Reaction Turbine
• A type of turbine that develops
torque by reacting to the
pressure or weight of a fluid; the
operation of reaction turbines is
described by Newton's third law
of motion (action and reaction
are equal and opposite).
Comparison of Impulse & Reaction Turbine
Comparison Between Velocity and Pressure
Compounding Impulse Turbines
Velocity Compounding
Pressure Compounding
Not equal velocity drop for each stage
Equal velocity drop for each stage
No pressure drop per stage
Not equal pressure drop per stage
Non equal power per stage
Equal power per stage
High friction losses due to high velocities
Low friction losses due to reduced steam
velocity
Not recommended for more than two
stages
Recommended for multistage
No problem with steam leak
Larger steam leak
Suitable for small turbines as well as only Suitable for large turbines
for the first stage in large turbine
Velocity Diagram
F
T
Optimum Velocity
Impulse Stage
Optimum Velocity
Velocity-compounded Stage
where n is the number of stages
If n = 2, P(1) : P(2) = 3:1.
If n = 3, P(1) : P(2) : P(3) = 5:3:1.
Therefore, n > 2 is not economically justified.
Optimum Velocity
Reaction Stage
Maximum Efficiency
Impulse Stage
Reaction Stage
Comparison of Stages
Impulse Stage
Comparison of Stages
Two-stage velocity-compounded Impulse Stage
Comparison of Stages
Reaction Stage