ABSTRACT
The energy losses that occur within a pump or turbine are attributable to volumetric, mechanical, and hydraulic losses.
Volumetric loss arises because of the small clearances that must be provided between the rotating element and the casing. A slight leakage
from the high pressure side to the low pressure side is unavoidable. Thus the impeller passages of a pump are handling more water than is actually delivered, whilst the runner passages of a turbine are handling less than is available. The amount of leakage depends on the pressure difference and the clearance sectional area. The volumetric efficiency 7]v of a pump and reaction turbine respectively is given b y :
It is not possible to scale down clearances to any extent so that this is a major reason for the lower overall efficiency of small units. With large and medium size units (Q > 0*3 cumecs) rjv is rarely less than 98 per cent.
