page_title: Draft tube
section_title: Draft tube and cavitation
hierarchical_section_title: Draft tube / Draft tube and cavitation
caption_attribution_description: English: Elbow type draft tube with rectangular cross section
context_page_description: Draft Tube is a diverging tube fitted at the exit of runner of turbine and used to utilize the kinetic energy available with water at the exit of runner. '.
  This draft tube at the end of the turbine increases the pressure of the exiting fluid at the expense of its velocity. This means that the turbine can reduce pressure to a higher extent without fear of back flow from the tail race.
  In an impulse turbine the available head is high and there is no significant effect on the efficiency if the turbine is placed a couple of meters above the tail race. But in the case of reaction turbines, if the net head is low and if the turbine is installed above the tail race, there can be appreciable loss in available pressure head to power the turbine. Also, if the pressure of the fluid in the tail race is higher than at the exit of the turbine, a back flow of liquid into the turbine can result in significant damage.
  By placing a draft tube at the exit of the turbine, the turbine pressure head is increased by decreasing the exit velocity, and both the overall efficiency and the output of the turbine can be improved.
context_section_description: Cavitation occurs when the local absolute pressure falls below the saturated vapor pressure of the water for the water temperature. The height of draft tube is an important parameter for avoiding cavitation.
  Applying Bernoulli's equation between outlet of the runner and discharge point of the draft tube neglecting any head losses in draft tube)
  z₂ = z (Height of draft tube)
  z₃ = height of tail race which is referenced as datum line (=0)
  p₂ = pressure at the outlet of the runner
  p₃ = gauge pressure
  since draft tube is a diffuser V₃ is always less than V₂ which implies p₂ is always negative
  thus height of the draft tube is an important parameter to avoid cavitation.
caption: Elbow type with rectangular cross section
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