The theoretical velocity of jet for an effective head (H) on the turbine is given by

Concept:

Heads on a Turbine

• There are two types of head on a hydraulic turbine,
  1. Gross head
  2. Net head
• The gross head of an impulse turbine is the elevation difference between head water level (head race water level) and the jet of the nozzle center as shown in figure (a).
• In the case of a reaction turbine, the gross head is the difference between head race and tailrace water level as shown in figure (b).
• The net or effective head is the head of water available at the point of entry to the turbine.
• The difference between the gross and net heads is evidently equal to  the loss of head due to friction and other causes in the conveyance system

Net head on turbine (H) = Gross head - 

Explanation:

• Since at the exit of the jet and at the level of Net head available, only atmospheric pressure will be there so using Bernoulli's theorem (Take reference level as the level of the jet)
• The theoretical velocity of jet for an effective head (H) on the turbine is given by,