Characteristic Curves of Hydraulic Turbines

Characteristic curves  of a  hydraulic  turbine  are the  curves,  with the  help of which the exact behavior and performance of the turbine under different working conditions, can be known.  These curves  are plotted from the results  of the tests performed on the turbine under different working conditions.

The important parameters which are varied during a test on a turbine are :

1. Speed (W)

3. Discharge (Q)

4. Power (P)

5. Overall efficiency (o)

6.Gate opening.

Out of the above six parameters, three parameters namely speed (N), Head (H) and discharge (Q) are independent parameters.

Out of the three independent parameters (N, H, Q), one of the parameter is kept constant
(say  H)  and  the  variation  of the other  four  parameters with   respect  to  any one  of the remaining two  independent  variables (say N  and  Q) are plotted and  various  curves  are obtained. These curves are called characteristic curves.

The followings are the important characteristic curves of a turbine :

•    Main Characteristic Curves or Constant Head Curve.

•    Operating Characteristic Curves or Constant Speed Curve.

•    Muschel Curves or Constant Efficiency Curve.

•    Main characteristic Curves or Constant Head Curves.

Main characteristic curves are obtained by maintaining a constant head and a constant gate opening (G.O.) on the turbine. The speed of the turbine is varied by changing load on the turbine. For each value of the speed, the corresponding values of the power (P) and discharge (Q) are obtained. Then the overall efficiency (o) for each value of the speed is calculated. From these readings the values of unit  speed (Nu)  unit  power  (Pu)  and  unit discharge (Ou) are determined. Taking Nu  as abscissa, the values of Qu, Pu, P and o are plotted. By changing the gate opening, the values of Qu Pu and o and Nu are determined and taking Ns as abscissa, the values of Qu, Pu and o are plotted. Fig. below shows the main   characteristic   curves   for   a   Pelton   wheel   and   Fig.   below   shows   the   main characteristic curves for reaction (Francis and Kaplan) turbines.