the reaction and impulse designs: each stage behaves as either one or the other, but the overall turbine uses both. Typically, lower pressure sections are reaction type and higher pressure stages are impulse type. Impulse turbines An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which is converted into shaft rotation by the bucket-like shaped rotor blades, as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage. As the steam flows through the nozzle its pressure falls from inlet pressure to the exit pressure (atmospheric pressure or, more usually, the condenser vacuum). Due to this high ratio of expansion of steam, the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades has a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the carry over velocity or leaving loss. The law of moment of momentum states that the sum of the moments of external forces acting on a fluid which is temporarily occupying the control volume is equal to the net time change of angular momentum flux through the control volume. The swirling fluid enters the control volume at radius with tangential velocity and leaves at radius with tangential velocity . A velocity triangle paves the way for a better understanding of the relationship between the various velocities. In the adjacent figure we have: and are the absolute velocities at the inlet and outlet respectively. and are the flow velocities at the inlet and outlet respectively. and are the swirl velocities at the inlet and outlet respectively, in the moving reference. and are the relative velocities at the inlet and outlet respectively. and are the velocities of the blade at the inlet and outlet respectively. is the guide vane angle and is the blade angle. Then by the law of moment of momentum, the torque on the fluid is given by: For an impulse steam turbine: . Therefore, the tangential force on the blades is . The work done per unit time or power developed: . When ω is the angular velocity of the turbine, then the blade speed is . The power developed is then . Blade efficiency Blade