Steam Turbines

A turbine is a rotating engine that exerts energy from a fluid flow and converts it into useful work.

Force

Steam out Steam in

There are two basic types of turbines according to the mode of steam

• Impulse turbine
• Reaction turbine

Impulse turbine
It runs by impulse of steam. Nozzle directs steam into curved blades, that causes them to rotate. The
energy to rotate the impulse turbine is derived from the kinetic energy of the steam flowing through
the nozzle. In doing so the velocity of the steam reduces when it passes over the blades. If high
velocity steam is blown on to a curved blade then the steam direction will be changed as it passes
across the blade. If the blade were free, it would move off in the direction of the force. If then a
number of blades were fixed round the circumference of a disk and the disk were free to rotate on a
shaft, steam blown across the blades in the manner would cause the disk to rotate. This is the
principal of steam turbine.

1) Velocity compounding

Source Wikimedia By Subikkumar - Own work, CC BY-SA 3.0,

https://commons.wikimedia.org/w/index.php?curid=19425431
 2) Pressure compounding

Source Wikimedia By Subikkumar - Own work, CC BY-SA 3.0,

https://commons.wikimedia.org/w/index.php?curid=19425431

Pressure- Velocity Compounding

Source Wikimedia By Subikkumar - Own work, CC BY-SA 3.0,

https://commons.wikimedia.org/w/index.php?curid=19425431
Reaction turbine:
Working principle of reaction turbine

Source: https://www.mecholic.com/2015/10/comparison-between-impulse-and-reaction-
turbine.html?m=0

Impulse Turbine Reaction Turbine

Steam expands in the nozzle and pressure Steam expands partially in the nozzle and further
remains constant as it flows though blades expansion takes place in the rotor blades
Relative velocity of steam passing over the blade Relative velocity of steam passing over the blades
remains constant in the absence of friction increases as the steam expands
Blades are symmetrical Blades are having aerofoil section
The pressure is same at inlet and outlet of the Pressure at inlet and outlet of the blades are
blades different
For the same power developed, number of stages For the same power developed, number of stages
required are less required are more
The blade efficiency curve is less flat The blade efficiency curve is more flat
The steam velocity is high-speed of turbine is Speed of turbine is low
high
Efficiency
• To maximise efficiency of steam turbine, the steam is expanded, generating work in a
number of stages,
• Multiple stage turbines are highly efficient
• Most steam turbines use a mixture of impulse and reaction design
• Higher pressure sections are impulse type and lower pressure sections are reaction type

Velocity diagram-Impulse Turbine

Let

𝑎𝑖 = absolute velocity of steam i.e. the steam as it leaves the nozzle

𝛼 = steam delivery angle

u = mean blade speed

𝑟𝑖 = velocity of steam relative to the blade

ϴ = angle between 𝑟𝑖 and the direction

Φ = angle of blade at exit

𝑟𝑖
𝑎𝑖
𝛼 u ϴ

u
u

u
Φ
𝑟𝑒
𝑎𝑒

Fig 1 Fig 2

Velocity diagrams for above’s are as follows-Note: Not scaled

u u
 𝑎𝑖 ϴ 𝑎𝑒
𝑟𝑖 Φ

𝑟𝑒

𝑤𝑖 𝑤𝑒

Fig 3 Fig 4

fe
fi
f=fi-fe
w

Fig 5- Super imposition of Fig 3 and Fig 4

Where

Horizontal components of absolute and inlet and exit are called Whirl velocity, w. fi and fe are
velocities of flow at inlet and outlet.

Note: If there is no friction in the blades then

If there is no friction in the blades then

𝑟𝑖 = 𝑟𝑒

Work done on the blades

The component of absolute velocities in the direction of motion of the blades is the effective part of
velocities in producing motion in the blades such as whirl velocities, w.

F= mass X change of velocity.

𝑤 = (𝑤𝑖+ 𝑤𝑒 )
F= m X (𝑤𝑖+ 𝑤𝑒 )

Therefore, the force in direction to the rotation 𝐹 = 𝑚(𝑤𝑖+ 𝑤𝑒 ) Newtons

where m is in kg/s and velocities are in m/s.

u is the mean velocity of the blades

Work i = Force X Distance

P= m (wi+we) u (Watts)

𝑎𝑖2
Kinetic energy of the steam supplied = , J/kg steam /s, W/kg steam
2
Blade or Diagram efficiency= Work done by blade per kg steam/Energy supplied per steam

𝑢(𝑤𝑖 +𝑤𝑒 )
Blade or Diagram Efficiency =
𝑎𝑖2 /2

The axial component mf is the axial thrust on the wheel, that must be taken up by the bearings in
which the shaft is mounted

Therefore, Axial Thrust = mf

Reaction Turbine
In a reaction turbine a stage is made up of a row of fixed blades followed by a row of moving blades.
Steam acceleration normally takes place in both fixed and moving blades as both are nozzle shaped,
ref the diagram provided in the previous chapter. Degree of reaction can be defined as the extent of
enthalpy drop that occurs in the moving blades.

Velocity diagram for reaction turbine stage:

𝑎𝑒 𝑎𝑖 𝑟𝑖

𝑟𝑒

Power = MU (change of velocity of whirl)

End Thrust = m change in velocity of flow

Specific enthalpy drop in stage = h J/kg

𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒 𝑖𝑛 𝑠𝑡𝑎𝑔𝑒 𝑚𝑢 (𝑤𝑖 + 𝑤𝑒 ) 𝑢 (𝑤𝑖 + 𝑤𝑒 )
𝑆𝑡𝑎𝑔𝑒 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = = =
𝐸𝑛𝑡ℎ𝑎𝑙𝑝𝑦 𝑑𝑟𝑜𝑝 𝑖𝑛 𝑠𝑡𝑎𝑔𝑒 𝑚ℎ ℎ
Velocity Diagram-Impulse Turbine: Tutorials

Construction:

One of the methods of solving turbine problems is by constructing accurate velocity diagrams such
as follows:

Example

The nozzles of a simple impulse turbine are inclined at an angle of 200 to the direction of moving
blades. The steam leaves the nozzles at 375 m/s. The blade speed is 165 m/s.

a) Find suitable inlet and outlet angles for the blades assuming that there is no axial thrust. The
velocity of the steam in passing over the blade ring being reduced by 15%.
b) Determine the power developed per when the steam flow is 1kg/s
c) Kinetic energy of the steam finally leaving the wheel.

Solution

As there is no axial velocity, that means that there is no change of flow velocity, the peak of both
inlet and outlet triangles are to be at the same level.

For the construction of velocity diagram, using above method, you will need a complete set of
geometry box and a normal ruler.

Refer the following diagram-Note it is not scaled

A u = 165 m/s B E
0
20

F
D C

You need to choose a correct scale using your ruler eg 1 cm of your ruler = 10 m/s etc

1. To draw blade speed u: Draw horizontal line AB = blade speed u= 165 m/s
2. To draw steam velocity at outlet of nozzle, a I : Construct a line AC such that angle BAC = 200
and AC = a I = 375 m/s
3. Join BC to complete the inlet triangle. BC=r I = relative velocity of the steam at inlet to the
blade. From the diagram r I = 228 m/s.
4. 15% loss of velocity: Calculate 15% off 228. Mark point F such that CF is 15% of BC
 5. With centre B, draw an arc FD. Note that being no axial thrust, C and D are at the same level
6. Join BD. BD=relative velocity at exit= re
7. Join AD, AD=absolute velocity at exit = ae
8. ABD is the outlet triangle

Measured values from the diagram

Relative velocity of steam at inlet r I = 228 m/s

Blade inlet angle = EBC = 340

Relative velocity at exit r e = 195 m/s

Absolute velocity at exit a e = 132.5 m/s

Blade outlet angle =ABD = 410

Whirl velocity = 320 m/s

a) Inlet and outlet angles are 340 and 410 respectively

b) Power developed for steam flow of 1 kg/s= muw= 1X165X320 = 52.8 kW
𝑎𝑒2
c) Kinetic energy of the steam finally leaving the wheel = = 132.52/2 = 8.7 kW/kg
2

Further Tutorials

1) Steam with a velocity of 600 m/s enters an impulse turbine at an angle of 250to the plane of
rotation of the blades. The mean blade speed is 255m/s. The exit angle from the blades is 300. There
is a 10% loss in relative velocity due to friction in blades, Determine

a) entry angle of the blades [Ans 41.50]

b) work done [Ans 150.5 kW/kg]
c) the diagram efficiency [Ans 0.836]
d) the end thrust [Ans -90N/kg/s]

2) In a single stage impulse turbine, the mean blade speed is 250 m/s and the nozzle angle is 200. The
enthalpy drop is 550 kJ/kg and nozzle efficiency is 0.85. The blade outlet angle is 300 and the power
developed is 30kW. Steam consumption is taken as 360 kg/hr. Calculate inlet angle and the axial
thrust on the blading. (Ans Axial thrust 7.5N).

3) In a simple impulse turbine, the diameter of the ring (blade) is 2.5m, the blade speed is 300rpm,
nozzle angle is 200. Ratio of blade velocity to steam velocity is 0.4. Losses= 8%. Blade exit angle is 30
less than inlet. Steam flow is 36000kg/h. Calculate

a) power developed (Ans 3800kW)

b) blade efficiency (Ans 78.7)
 c) Steam consumption kg/kW/h (Ans 9.47)

You can solve these problems by DRAWING [ using geometry box] or mathematically ( Ref: Eastop
and McConkey, Applied Thermodynamics for Engineering Technologists, 1978)

Reaction Turbine

Ex 1

At a stage in a reaction turbine, the mean blade diameter is 1 m and the turbine runs at a speed of
50rev/s. The blades are designed for 50% reaction with exit angles 300 and inlet angles 500. The
turbine is supplied with steam, the steam flow rate being 600000 kg/hr and the stage efficiency is
85%. Calculate

a) the power output of the stage [Ans 11.6 kW]

b) enthalpy drop in the stage [Ans 82 kJ/kg]
c) the percentage increase in relative velocity in the moving blades due to expansion in these
blades [Ans [52%]