page_title: Steam turbine
section_title: Impulse turbines
hierarchical_section_title: Steam turbine / Principle of operation and design / Impulse turbines
caption_attribution_description: English: Blade Profile
context_page_description: A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884.
  The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency from the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible expansion process. Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator—about 85% of all electricity generation in the United States in the year 2014 was by use of steam turbines.
context_section_description: 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 .
caption: Velocity triangle
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