Performance of Water Turbines
- Water turbines are designed to work under given head, discharge and output at certain speed. However, the turbine may be needed to work under such operating conditions which are different than designed values.
- Therefore, the performance testing of turbines becomes very essential to evaluate its characteristics.
- Also, it is essential to know the variation of its performance on all dependent parameters.
- Since the manufacturing of actual machine is highly costly and to the predict the performance of its actual machine, the performance tests are carried out on its prototypes and models.
- The important parameters on which the performance of turbine depends are head (H), discharge rate (Q), power (P) and efficiency (n). Various characteristic curves are discussed below.
Operating Characteristics (Constant Speed Characteristics) : Operating characteristics of the turbine are obtained by testing of turbines under constant speed. Since the head cannot be changed under actual working conditions, therefore, the tests on turbine are conducted under a constant head . In order to maintain constant speed, the gate openings are adjusted when the load on turbine changes , which is achieved by governing mechanism. The corresponding power input and output are measured and the efficiency is calculated.
From the data obtained, following curves are plotted.
(a) Load Vs efficiency
(b) Discharge Vs power
(c) Discharge Vs efficiency
(a) Load Vs efficiency :
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| Figure A |
Figure A shows the load Vs efficiency curve for various turbines. Load is represented as the percentage of full load for which a turbine is designed.
In case of Kaplan and Pelton turbines the maximum efficiency is obtained at comparatively low loads compared to Francis and propeller turbines and it remains almost constant with further increase in load. Thus, these turbines can be operated at high efficiency for wide range of load conditions. Whereas, the Francis and propeller only give high efficiency at 70% or higher loads of full loads.
(b) Discharge (Q) Vs power (P) and Discharge Vs Efficiency curves :
Power Vs discharge and efficiency Vs discharge curves are shown in Figure B at constant head and speed of turbine.
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| Figure B |
It could be seen that the efficiency of turbine increases with the increase in discharge parabolically. The power output varies linearly with discharge. It could be noted that power output is zero for certain value of discharge, Q0. It is due to the fact that initially the power developed by the turbine is lost in overcoming the friction losses and the inertia load of rotating parts of a turbine.