TURBINE SPECIFIC SPEED

The specific speed value (radian/second) for a turbine is the speed of a geometrically similar turbine which would produce one unit of the specific speed of a turbine is given by the manufacturer (along with other ratings) and will always refer to the point of maximum efficiency. This allows accurate calculations to be made of the turbine's performance for a range of heads.


Well-designed efficient machines typically use the following values: Impulse turbines have the lowest ns values, typically ranging from 1 to 10, a Pelton wheel is typically around 4, Francis turbines fall in the range of 10 to 100, while kaplan turbines are at least 100 or more, all in imperial units.

Ns=n*(P^1/2)*1/H^5/4

where: n= rpm
H= Net head after turbine and waterway loss (m)
Q= water flow (m³/s)
N= Wheel speed (rpm)
P= Power (kW)
H= Water head (m)

Significance of specific speed:

For a small power under a high head a pelton wheel with its low specific speed is used to keep the rotative speed down to a suitable value, while for a large power under a low head a reaction turbine with its higher specific speed is used to raise the rotative speed to a suitable value. If the power is low, the speed can be high, or vice versa. It is also obvious that if both speed and power are high, the specific speed must be high if the head is low.