Page 128 - Introduction to Marine Engineering

Pumps and pumping systems 115

Pump types
There are three main classes of pump in marine use: displacement, axial
flow and centrifugal. A number of different arrangements are possible
for displacement and centrifugal pumps to meet particular system
characteristics.

Displacement
The displacement pumping action is achieved by the reduction or
increase in volume of a space causing the liquid (or gas) to be physically
moved. The method employed is either a piston in a cylinder using a
reciprocating motion, or a rotating unit using vanes, gears or screws.
A reciprocating displacement pump is shown diagrammatically in
Figure 6.3, to demonstrate the operating principle. The pump is

A Piston
T moving
I upwards
Air vessel
Suction
valve closed Discharge
valve open
Lf-, , -S
Liquid
discharge

Suction Discharge
valve open valve
closed
Figure 6.3 Diagrammatic reciprocating displacement pump

double-acting, that is liquid is admitted to either side of the piston where
it is alternately drawn in and discharged. As the piston moves upwards,
suction takes place below the piston and liquid is drawn in, the valve
arrangement ensuring that the discharge valve cannot open on the
suction stroke. Above the piston, liquid is discharged and the suction
valve remains closed. As the piston travels down, the operations of
suction and discharge occur now on opposite sides.
An air vessel is usually fitted in the discharge pipework to dampen out
the pressure variations during discharge. As the discharge pressure rises