The Multistage Impulse Turbines BY Dr. P M V Subbarao Mechanical Engineering Department I I T Delhi A concept to construct Large Capacity Turbines using with reasonable stress levels…….

Compounding of impulse turbine Consider a case of isentropic expansion from 15MPa, 5500C too 500C. Drop in enthalpy :1360kJ/kg. Increase in velocity in a nozzle: 1650 m/s. Velocity of blade (assuming u/Vai = 0.5) : 825 m/s. Speed of wheel (assuming a diameter = 2m) : 8000rpm. Compounding is done to reduce the rotational speed of the impulse turbine to practical limits. A blade tip speed of 825m/s is not possible, which is pretty high for practical uses. Compounding is achieved by using more than one set of nozzles & blades in a series, keyed to a common shaft. 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, b) velocity compounded and c) pressure and velocity compounded impulse turbines.

Strategy for Multi Staging

Pressure compounded impulse turbine

Design of Impulse Turbines BY Dr. P M V Subbarao Mechanical Engineering Department I I T Delhi

Sequence of Energy Losses Steam Thermal Power Blade kinetic Power Steam kinetic Power Nozzle Losses Stage Losses Moving Blade Losses Isentropic efficiency of Nozzle Blade Friction Factor

Irreversibilities in Turbine 3 Ideal work ws = h3 – h4s Actual work wa = h3 – h4a Internal Efficiency of a turbine T 4a 4s s

Procedure for Design of Single Stage Impulse Turbine Required Specifications: Power Generation Capacity of turbine. Number of Revolutions for minute, N. Pressure and Temperature (p3 &T3) of steam from the exit of final super heater. Condenser Pressure (p4). Calculate the velocity of steam at the exit of the nozzle, Vai. The angle of inlet absolute velocity, ai, is assumed between 14 – 20 degrees. Calculate optimum velocity ratio for maximum stage efficiency. Calculate windage losses and hence net stage Mechanical power. Optimize Stage Mechanical power for maximum relative internal efficiency of the turbine. Determine final dimension of nozzle and blades.

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 .

Enthalpy Entropy Diagram for Multistage Turbine Turbine Inlet Stage 1 h Stage 2 Stage 3 Stage 4 Stage 5 Turbine Exit s

Internal Reheating due to Irreversibilities 3 4s 4IIs 4IIIs 4Is 4Vs 4IVs 4Ia 4IIa 4IIIa 4IVa 4Va 4VIs 4VIa T s

Steam Flow Path in a Multi Stage Impulse Turbine Global available enthalpy for Power: 3 4s 4IIs 4IIIs 4Is 4Vs 4IVs 4Ia 4IIa 4IIIa 4IVa 4Va 4VIs 4VIa Internally available enthalpy for Power: 4IIss 4IIIss 4IVss Total actual stage work output per unit mass: 4Vss

Define Stage Efficiency: Global internal efficiency of turbine:

qi is always positive. Therefore, Multistage turbines will increase the possibility of recovering lost availability! The larger the number of stages, the greater is the heat recovery. The difference is called heat recovery factor, a. General value of a is 0.04 to 0.06.