Compounding of impulse turbine Compounding is done to reduce the rotational speed of the impulse turbine to practical limits. Compounding is achieved by using more than one set of nozzles, blades, rotors, in a series, keyed to a common shaft; so that either the steam pressure or the jet velocity is absorbed by the turbine in stages. Three main types of compounded impulse turbines are: a) Pressure compounded Steam Turbine : The Rateau Design b) velocity compounded Steam Turbine : The Curtis Design c) pressure and velocity compounded Impulse turbines : The Rateau-curtis Design.

Multistage Impulse Turbine : GE Product

Pressure compounded impulse turbine

Impulse Turbines with pressure stages Multistage turbines with pressure stages have found a wide field of usage in industry as prime movers (~ 10 MW). The number pressure stages vary from 4 to 5. The distribution of enthalpy drop in a large number of pressure stages enables the attainment of lower velocities for the steam flowing through the system of moving blades. As a result more advantageous values of blade speed ratio and blade friction factor are obtained .

Selection of Number of Stages

Impulse Turbines with pressure stages Total enthalpy drop available for mechanical power

Variation of Diameter along a stages Stage Z

The Curtis Design

A System of Velocity Triangles for Curtis Turbine 1Vr1 1Va1 1Vr2 1Va2 1 1  2 U 2Vr1 2Va1 2Vr2 2Va2 1 1 2 2 U 3Vr1 3Va1 3Vr2 3Va2 1 1 2 2

The Curtis Impulse Turbine Total enthalpy drop available for mechanical power

Curtis Turbine With 2 Rotors Total power with similar blading U 1Vr1 1Va1 1Vr2 1Va2 1 1  2

Efficiency of two rotor Curtis Turbine

Efficiency of two rotor Curtis Turbine

The most powerful steam turbine-generator in the world at the time of it's construction:1903 Built in 1903, the 5,000-kilowatt Curtis steam turbine-generator was the most powerful in the world. It stood just 25 feet high, much shorter than the 60 feet reciprocating engine-generator of a similar capacity

Efficiency of Multi Rotor Curtis Turbine For a three rotor Curtis Turbine: For a n-rotor Curtis Turbine:

The Curtis-Rateau Design

Compound Impulse-Reaction turbine The shape of the blade improves considerably. The blade sizes varyies at a uniform rate, thus contributing to more economic designs. As a result of enthalpy drop occurring in the moving blades, there is a considerable amount of pressure is exerted on the rotor. This is transmitted to thrust bearing. To void large axial thrust it is usual to allow: Low degree of reaction in high pressure stages. In large steam turbines (>300 MW), it is now usual to allow 60 – 70% of degree of reaction in low pressure stages.

Customization of DoR Irreversible Flow Through A Stage Steam Thermal Power Blade kinetic Power Steam kinetic Power Nozzle Losses Stage Losses Moving Blade Losses Isentropic efficiency of Nozzle Blade Friction Factor

Losses in Nozzles Losses of kinetic energy of steam while flowing through nozzles or guide blade passages are caused because of Energy losses of steam before entering the nozzles, Frictional resistance of the nozzles walls, Viscous friction between steam molecules, Deflection of the flow, Growth of boundary layer, Turbulence in the Wake and Losses at the roof and floor of the nozzles. These losses are accounted by the velocity coefficient, .

Losses in Moving Blades Losses in moving blades are caused due to various factors. The total losses in moving blades are accounted for by the load coefficient, ψ. These total losses are comprised of the following: Losses due to trailing edge wake. Impingement losses. Losses due to leakage of steam through the annular space between stator and the shrouding. Friction losses. Losses due to the turning of the steam jet in the blades Losses due to shrouding.

Stage with General Value of Degree of Reaction First law for fixed blades: First law for relative flow through moving blades:

True Available Enthalpy