Compounding of

steam turbines

Compounding of steam turbines is the

method in which energy from the steam is
extracted in a number of stages rather
than a single stage in a turbine. A
compounded steam turbine has multiple
stages i.e. it has more than one set of
nozzles and rotors, in series, keyed to the
shaft or fixed to the casing, so that either
the steam pressure or the jet velocity is
absorbed by the turbine in number of
stages.

Necessity
The steam produced in the boiler has
sufficiently high enthalpy when
superheated. In all turbines the blade
velocity is directly proportional to the
velocity of the steam passing over the
blade. Now, if the entire energy of the
steam is extracted in one stage, i.e. if the
steam is expanded from the boiler
pressure to the condenser pressure in a
single stage, then its velocity will be very
high. Hence the velocity of the rotor (to
which the blades are keyed) can reach to
about 30,000 rpm, which is pretty high for
practical uses because of very high
vibration. Moreover at such high speeds
the centrifugal forces are immense, which
can damage the structure. Hence,
compounding is needed.The high velocity
which is used for impulse turbine just
strikes on single ring of rotor that cause
wastage of steam ranges 10% to 12%. To
overcome the wastage of steam
compounding of steam turbine is used.

Types of steam turbines

1.Impulse: There is no change in the
pressure of the steam as it passes through
the moving blades. There is change only in
the velocity of the steam flow.

2.Reaction: There is change in both

pressure and velocity as the steam flows
through the moving blades.

Types of compounding
In an Impulse steam turbine compounding
can be achieved in the following three
ways: -

1. Velocity compounding

2. Pressure compounding
3. Pressure-Velocity Compounding

In a Reaction turbine compounding can be

achieved only by Pressure compounding.

Velocity compounding of
Impulse Turbine

Fig-1:Schematic Diagram of Curtis Stage Impulse

Turbine
The velocity compounded Impulse turbine
was first proposed by C G Curtis to solve
the problem of single stage Impulse
turbine for use of high pressure and
temperature steam.

The rings of moving blades are separated

by rings of fixed blades. The moving
blades are keyed to the turbine shaft and
the fixed blades are fixed to the casing.
The high pressure steam coming from the
boiler is expanded in the nozzle first. The
Nozzle converts the pressure energy of the
steam into kinetic energy. The total
enthalpy drop and hence the pressure drop
occurs in the nozzle. Hence, the pressure
thereafter remains constant.

This high velocity steam is directed on to

the first set (ring) of moving blades. As the
steam flows over the blades, due the
shape of the blades, it imparts some of its
momentum to the blades and loses some
velocity. Only a part of the high kinetic
energy is absorbed by these blades. The
remainder is exhausted on to the next ring
of fixed blade. The function of the fixed
blades is to redirect the steam leaving
from the first ring of moving blades to the
second ring of moving blades. There is no
change in the velocity of the steam as it
passes through the fixed blades. The
steam then enters the next ring of moving
blades; this process is repeated until
practically all the energy of the steam has
been absorbed.

A schematic diagram of the Curtis stage

impulse turbine, with two rings of moving
blades one ring of fixed blades is shown in
figure 1. The figure also shows the
changes in the pressure and the absolute
steam velocity as it passes through the
stages.

where,

Pi = pressure of steam at inlet

Vi = velocity of steam at inlet

Po = pressure of steam at outlet

Vo = velocity of steam at outlet

In the above figure there are two rings of

moving blades separated by a single of
ring of fixed blades. As discussed earlier
the entire pressure drop occurs in the
nozzle, and there are no subsequent
pressure losses in any of the following
stages. Velocity drop occurs in the moving
blades and not in fixed blades.

Velocity Diagram
As shown in the above diagram there are
two rings of moving blades separated by a
ring of fixed blades. The velocity diagram
in figure 2, shows the various components
of steam velocity and the blade velocity of
the moving blades.

where,

Va = absolute velocity of steam

Vr = relative velocity of steam

Vb = Blade velocity

θ = Nozzle angle

ø = Blade entrance angle

γ = Blade exit angle

δ = fluid exit angle

From the above figure it can be seen that

the steam, after exiting from the moving
blades, enters into the fixed blades. The
fixed blades redirect the steam into the
next set of moving blades. Hence, steam
loses its velocity in multiple stages rather
than in a single stage.

Optimum Velocity

It is the velocity of the blades at which

maximum power output can be achieved.
Hence, the optimum blade velocity for this
case is,

where n is the number of stages.

This value of optimum velocity is 1/n

times that of the single stage turbine. This
means that maximum power can be
produced at much lower blade velocities.

However, the work produced in each stage

is not the same. The ratio of work
produced in a 2 stage turbine is 3:1 as one
move from higher to lower pressure. This
ratio is 5:3:1 in three stage turbine and
changes to 7:5:3:1 in a four stage turbine.

Disadvantages of Velocity
Compounding

Due to the high steam velocity there are

high friction losses
Work produced in the low-pressure
stages is much less.
The designing and fabrication of
bsexyes which can withstand such high
velocities is difficult.

Pressure compounding of
Impulse Turbine
Fig-3:Schematic Diagram of Pressure compounded
Impulse Turbine

The pressure compounded Impulse

turbine is also called as Rateau turbine,
after its inventor. This is used to solve the
problem of high blade velocity in the
single-stage impulse turbine.

It consists of alternate rings of nozzles

and turbine blades. The nozzles are fitted
to the casing and the blades are keyed to
the turbine shaft.

In this type of compounding the steam is

expanded in a number of stages, instead
of just one (nozzle) in the velocity
compounding. It is done by the fixed
blades which act as nozzles. The steam
expands equally in all rows of fixed blade.
The steam coming from the boiler is fed to
the first set of fixed blades i.e. the nozzle
ring. The steam is partially expanded in the
nozzle ring. Hence, there is a partial
decrease in pressure of the incoming
steam. This leads to an increase in the
velocity of the steam. Therefore the
pressure decreases and velocity increases
partially in the nozzle.

This is then passed over the set of moving

blades. As the steam flows over the
moving blades nearly all its velocity is
absorbed. However, the pressure remains
constant during this process. After this it
is passed into the nozzle ring and is again
partially expanded. Then it is fed into the
next set of moving blades, and this
process is repeated until the condenser
pressure is reached.

This process has been illustrated in figure

3.
where, the symbols have the same
meaning as given above.

It is a three stage pressure compounded

impulse turbine. Each stage consists of
one ring of fixed blades, which act as
nozzles, and one ring of moving blades. As
shown in the figure pressure drop takes
place in the nozzles and is distributed in
many stages.

An important point to note here is that the

inlet steam velocities to each stage of
moving blades are essentially equal. It is
because the velocity corresponds to the
lowering of the pressure. Since, in a
pressure compounded steam turbine only
a part of the steam is expanded in each
nozzle, the steam velocity is lower than of
the previous case. It can be explained
mathematically from the following formula
i.e.

where,

V1 = absolute exit velocity of fluid

h1 = enthalpy of fluid at exit

V2 = absolute entry velocity of fluid

h2 = enthalpy of fluid at entry

One can see from the formula that only a

fraction of the enthalpy is converted into
velocity in the fixed blades. Hence, velocity
is very less as compared to the previous
case.

Velocity Diagram

Fig-4:Velocity Diagram of Pressure compounded

Impulse Turbine
The velocity diagram shown in figure 4
gives a detail about the various
components of steam velocity and Blade
velocity.

where, symbols have the same meaning

as given above.

An important point to note from the above

velocity diagram is that the fluid exit angle
(δ) is 90⁰. This indicates that the whirl
velocity of fluid at exit of all stages is zero,
which is in compliance with the optimum
velocity concept (as discussed earlier).
The ratio of work produced in different
stages is similar to as discussed for the
above type.

Disadvantages of Pressure
Compounding

The disadvantage is that since there is

pressure drop in the nozzles, it has to be
made air-tight.
They are bigger and bulkier in size 34

Pressure-Velocity
compounded Impulse
Turbine
Fig-5:Schematic Diagram of Pressure-Velocity
compounded Impulse Turbine

It is a combination of the above two types

of compounding. The total pressure drop
of the steam is divided into a number of
stages. Each stage consists of rings of
fixed and moving blades. Each set of rings
of moving blades is separated by a single
ring of fixed blades. In each stage there is
one ring of fixed blades and 3-4 rings of
moving blades. Each stage acts as a
velocity compounded impulse turbine.

The fixed blades act as nozzles. The

steam coming from the boiler is passed to
the first ring of fixed blades, where it gets
partially expanded. The pressure partially
decreases and the velocity rises
correspondingly. The velocity is absorbed
by the following rings of moving blades
until it reaches the next ring of fixed blades
and the whole process is repeated once
again.
This process is shown diagrammatically in
figure 5.

where, symbols have their usual meaning.

Pressure compounding of
Reaction Turbine

Fig-6:Schematic Diagram of Pressure compounded

Reaction Turbine
As explained earlier a reaction turbine is
one in which there is pressure and velocity
loss in the moving blades. The moving
blades have a converging steam nozzle.
Hence when the steam passes over the
fixed blades, it expands with decrease in
steam pressure and increase in kinetic
energy.

This type of turbine has a number of rings

of moving blades attached to the rotor and
an equal number of fixed blades attached
to the casing. In this type of turbine the
pressure drops take place in a number of
stages.
The steam passes over a series of
alternate fixed and moving blades. The
fixed blades act as nozzles i.e. they
change the direction of the steam and also
expand it. Then steam is passed on the
moving blades, which further expand the
steam and also absorb its velocity.

This is explained in figure 6.

where, symbols have the same meaning

as above.

Velocity Diagram
Fig-7: Velocity Diagram of Pressure Compounded
Reaction turbine

The velocity diagram given in figure 7

gives a detail about the various
components of steam velocity and blade
velocity (symbols have the same meaning
as above).

See also
Pressure compounding in turbines

References