REACTION TURBINE TOPICS• Degree of reaction• Velocity diagrams• Blade efficiency and its derivation• Calculation of blade height • Requirement of an ideal working

fluid• Methods of attachment of blades to

turbine rotor• Losses in steam turbine• Labyrinth packing and governing of

steam turbine• Blade materials

DEGREE OF REACTION

This is defined as the ratio of isentropic heat drop in the moving blade to the sum of the isentropic heat drop in the fixed blade and the moving blade i.e. in a stage.

When the degree of reaction is half i.e. 0.5, it is called parson’s reaction turbine. In parson’s reaction turbine, it is assumed that the mean diameter of fixed as well as moving blades is same and the blade height is uniform throughout all the stages, thus the velocity triangle abd and adf will be similar.

VELOCITY DIAGRAM

Blade Efficiency

bww vVV )( 1

2

)(2

1

mV

VVVm bwwb

21)(2

V

VVV bww

Blade Efficiency - It is the ratio of work done on blade to the energy supply to the blade.Energy supplied to the J/s

W.D= m J/S

K.E at inlet per second=

K.E at exit per second=

W.D on blade = -

2

2mV

2

21mV

2

21mV

2

2mV

2

222

21

2

mV

mVmV

b

2

21

2

V

VVb

Condition for maximum efficiency in impulse turbine 2

21

2

V

VVb

Blade efficiency is minimum when V1 is minimum. From the combined velocity triangleFor max. efficiency steam should leave the turbine at right angle to their motion.

Also for impulse turbine and then

2max

2

V

VV bwb

wb VV2

1 cos5.0 VVb

2max

2

V

VV bwb

2

5.02

V

VV ww

2

22 cos

V

V

2max cosb

1rr VV

d = Diameter of Runnerh = Height of the Blades

Total Area A = π(d+h)*h {if thickness t = h ;if t ≠ h then A= π(d+h)*t}

Volume of steam flow=Area * volume = π(d+h)*h*Vf1

Volume of 1KG of steam at a given pressure Vg = π(d+h)*h*Vf1

Vgd+h = dm

dm = Mean blade Diameter

Calculation of height of the blades of Reaction turbine

Requirements of the “ideal” working fluid• High chemical and thermal stability - Maintain its stability at the

highest temperatures, non-flammable, non-corrosive and non-toxic.• Should have a high molecular weight and self- lubricity for the

smooth rotation of the turbine and the pump .• Low latent heat - Capacity to absorb and reject heat easily, lowering

heat loss.• High specific heat for enlarging heat capacity.• Fluid’s critical point (pressure and temperature) should be above the turbine’s operating temperature.• Required operating pressure should not put danger of explosion or rupture.• Fluid volume in its gaseous state should be small enough to avoid

the need for costly, over-sized turbines.

METHODS OF ATTACHMENT OF BALADES TO THE TURBINE

LOSSES IN STEAM TURBINE

Some of the losses which occur within any turbine are as follow :

1 Losses in regulating valves : The steam generated in the boiler has to flow through stop regulating valves

before entering the steam turbine. At these valve, the steam gets throttled and pressure of the steam at entry to the turbine is less than the boiler pressure. Although throttling occur at constant enthalpy, the enthalpy drop available for work is reduced. The magnitude of losses due to the throttling may be of the order of 5 to 10% of the pressure of live steam.

2 Steam friction losses in nozzle: It was out earlier when analyzing the steam flow through nozzle, that there is a

friction between the steam and wall of the nozzle and internal friction between the fluid layer itself due to turbulence and eddies formation. The friction result in less available enthalpy drop and consequently the velocity of the steam leaving the nozzle is less. The effect of friction is taken into consideration by nozzle efficiency which was defined as the ratio of actual enthalpy drop to isentropic enthalpy drop.

3 Steam friction losses in blade passage:While plotting the velocity triangles for impulse turbine ,mention was made of the fact that the steam experiences resistance to it motion when flowing through the blade passage. The effect of this resistance is considered by taking a factor K defined as blade velocity coefficient. The losses in blade passages mainly due to:

a) Energy losses which occur when the steam meets the leading edges of the blade passage. b) Friction losses in the blade passage. c) Leaving or wake losses due to the formation of eddies as soon as steam leaves the blade passage. 4 Residual velocity losses:The steam leaves the turbine with the certain absolute velocity, the equivalent kinetic energy of which is wasted.

Exhaust losses = (Ca2)2/2gcJ Kcal/Kg

If H is the isentropic heat drop, than percentage loss due to leaving velocity is by :

= (Ca2)2/2gcJ. H * 100 In the single stage impulse wheel these loss may be as high as 10-12%. However in multistage turbine the loss is much less because of a part of this utilized when the steam passes to the next stage. When this kinetic energy is pass to the next stage.

5 Loss due to leakage : The steam leakage occur at the following points : a) Between the shaft and the bearing. b) Around the tips of the reaction turbine blades. The leakage loss is 1-2% and is reduced by the use of seal strips and labyrinth packings.

6 Loss due to mechanical friction in the bearing : The friction in the bearings can be kept to the minimum by a proper lubrication system.

7 Loss due to radiation Usually the turbines are heavily insulted to reduce the heat loss to surroundings by radiation and as such these losses are negligible.

LABYRINTH PACKING AND GOVERNING OF A STEAM TURBINE

Steam turbine governing is the procedure of controlling the flow rate of steam into a steam turbine so as to maintain the speed of rotation as constant. Labyrinth packing is one of the most widely used packing; a contactless device placed between two or mare parts that move relative to one another. It consists of a row of narrow slits altering with expansion chamber. Such a packing is most frequently used to seal the space between a rotating shaft and a stationary housing. Labyrinth packing prevents leakage of a liquid or a gas, facilitates the use of high rotational speeds, and operates reliably at high temperature.

Blade Materials• Among the Materials that have been found

to be Suitable for Use in Blades are Steels , Titanium Alloys and Nickel Base Alloys

• All the three types of alloys which are mainly used, have varying Proportions of Chromium and Aluminum to improve the strength and corrosion at High Temperatures.

• Different Alloy Compositions have been Developed which have a good Stability to withstand the thermal Stresses.

• Alloys have been Modified to give Good Erosion-Corrosion Characteristics to the Blades.

• Manufacturing of Blades Determine the Strength, Uniformity of Micro-Structure.

13

Factors To Be Considered in the Selection of Blade Materials

• High Operating temperatures. Material should posses good strength at high temperatures.

• Centrifugal tensile stresses due to rotational speeds as high as 30000 rpm.

• Bending stresses due to equivalent impulse load of the gases acting on the cantilever blade at a certain distance from fixing.

• It must have high creep strength.• Hot erosive and corrosive effects due to the high temperature

combustion products . For example- CO2 and CO with water vapor.

• Varying temperature and should have structural stability when exposed to varying temperatures.

14