Page 135 - Introduction to Marine Engineering

Pumps and pumping systems 121

Pumping element
removal gear

Coupling spacer

Shaft coupling
Soft packed gland
Motor support

mpeller

Figure 6.8 Double-entry centrifugal pump

formed by vanes. The passages widen out in the direction of liquid flow
and act to convert the kinetic energy of the liquid into pressure energy.
Hydraulic balance arrangements are also usual. Some of the
high-pressure discharge liquid is directed against a drum or piston
arrangement to balance the discharge liquid pressure on the impeller
and thus maintain it in an equilibrium position.
Centrifugal pumps, while being suitable for most general marine
duties, are not self priming and require some means of removing air
from the suction pipeline and filling it with liquid. Where the liquid to be
pumped is at a level higher than the pump, opening an air cock near the
pump suction will enable the air to be forced out as the pipeline fills up
under the action of gravity. If the pump is below sea water level, and sea
water priming is permissible in the system, then opening a sea water
injection valve and the air cock on the pump will effect priming.