Mechanics of Fluids

and CP = P/ϱω 3 D 5 in place of Q, H and P respectively. Typical pump
characteristics are shown in Fig. 13.36.
A curve of H against Q which has a peak is termed an unstable characteristic.
This is because the slope of the H−Q curve for an external system
is normally always positive (see the dashed line in Fig. 13.31). If part of
the pump characteristic also has a positive slope there is thus the possibility
that, at the point of intersection, the pump characteristic could have a
greater slope than the other curve. Any slight increase of Q, for example,
would then result in the pump head rising more than the system head, and the
excess head at the pump would cause Q to increase still further. Moreover,
the two curves could intersect again at a second, higher, value of Q; if the
two intersections were fairly close together the pump would tend to hunt
or surge from one to the other. Such instability is undesirable and so an
unstable part of the characteristic should be outside the normal operating
range of the pump.
For pumps handling liquids, the increase of total head is little different
from the increase of piezometric head (i.e. the manometric head). For fans,
however, kinetic energy often forms a substantial part of the total energy
increase. Characteristic curves for fans are therefore normally based on the
total head (static plus kinetic). The operating conditions of a fan are set by
the intersection of its total head characteristic with the curve of the total
head required by the external system (i.e. the ducting and so on, to which
the fan is connected). The latter head, it should be remembered, consists not
Rotodynamic pumps 645
Fig. 13.36