1. Multi Stage Impulse Reaction Turbines
P M V Subbarao
Professor
Mechanical Engineering Department
An Immortal Concept Developed by Sir Parson……

2. Parsons Steam Turbines

3. The Theory of turbine Blading by Parson
In a Parson stage…
… the fixed blade row makes the steam go faster by forming a narrow passage
… the moving blade row changes the direction of the steam AND increases its’ speed at the same time
… to increase the steam speed, the moving blade row forms a narrow passage just like the fixed blade row.

4. Parson introduced the Reaction ?!?!?
Reaction effect is defined as …
… the moving blades increase the velocity of the steam as well as change its’ direction
… it produces a kind of jet effect
… the jet effect simply adds to the change of momentum over the moving row
… the forces are still determined using same velocity triangles
The change in velocity over the moving row can be varied by the designer
- it can be large or small
Define “degree of reaction” as an indicator of the relative increase in steam speed across the moving row
Parson’s reaction blades are 50% reaction blades
The degree of reaction in Modern blades vary typically from 30% to 70%

5. Parsons Hardware : Frugal Technology
In 1884 or four years previously, I dealt with the turbine problem in a different way.
In a Parson’s stage, the moving blades are a mirror image of the fixed blades.
Fixed blades
Moving blades

6. The Impulse-Reaction (~Parson’s) Stage
Vr1
Va1
Vr2
Va2 b1 a1 a2 b2
Fixed blades expand steam from Va2 to Va1 using a blades whose angle vary from b1 to b2.
Va0=Va2
Assuming that the gap between stator and rotor doesn’t alter the flow conditions. ?!?....
The moving blades are a mirror image of the fixed blades.

7. Vector Theory of Parson’s BladingU
Vr1
Va1
Va2 b1 a1
α1 = β2
α2 = β1

8. Fluid Dynamics of 50% Reaction
Vr1
Va1
Vr2
Va2 b1 a1 a2 b2
Va1
Vr1
Vr2
Va2

9. Thermodynamics Analysis of Parsons Blading : SSSF
Va1
Vr1
Vr2
Va2

10. Rate of Change in Whirl velocity in Parsons StageU
Vr1
Va1
Vr2
Va2 b1 a1 a2 b2
Va1
Vr1
Vr2
Va2

11. Strength of Ideal Blading
For an ideal Impulse Blade:
For an ideal Parson Blade:

12. Blade Power
Ideal Impulse Stage :
Ideal Parson Stage :

13. Blade Efficiency Available power in Impulse Stage :
Available power in 50% Reaction stage :

14. Stage Efficiency
Impulse Stage :
50% Reaction stage :

15. Maximum Efficiency of Impulse Blade

16. Maximum Efficiency of Parson Blade

17. Blade Power at Maximum Efficiency Conditions
Ideal Impulse Stage :
Ideal Parson Stage :

18. Moderate Capacity of Parson : Same Blade Velocity
So at optimum U/Va1, an impulse stage produces TWICE the power of a 50% reaction stage for same blade speed!
This means that an impulse turbine requires only half the number of stages as a 50% reaction turbine for a given application!
This fact has a major impact on the construction of the turbine
It is also responsible for some of the greatest miss understandings, since people assume that this means that impulse blading is cheaper overall - this is NOT true!
Impulse turbines have fewer stages, but they must use a different form of construction which is expensive