The Francis turbine is preferred over Pelton for
the following reason :
1. The variation in the operating head can be
more easily controlled in Francis than in Pelton.
2. The ratio of maximum and minimum operating head can be even two in case of Francis turbine.
3. The operating head can be utilized even when
the variation in the tail water level is relatively large
when compared to the total head.
4. The size of the runner, generator and power house required is small and economical if the
Francis is used instead of Pelton for the same power generation.
5. The mechanical efficiency of Pelton decreases faster with wear than Francis.
The drawbacks of the Francis compared with Pelton are listed below :
1. Water which is not clean can cause very rapid wear in high head Francis turbine. In passing
through the guide vanes and cover facings, it can quickly reduce overall efficiency of the turbine by
several percent. The effect is much more serious in turbines of small diameter than in large ones.
Particles of solid matter in the water will wear the lip of the spear, the nozzle and after several
years the runners also. The first two are easily removable, renewable and repairable. The runner repairing by welding can often be done without removing the runner from the shaft or casing.