Page 130 - Introduction to Marine Engineering

Pumps and pumping systems 117

OUTLET
INLET

(a)

Discharge
Screw

Bearing

Driving
shaft

Screw
(b) 4 Suction
Figure 6.4 Rotary displacement pumps: (a) rotary vane displacement pump, (b) screw
displacement pump

Axial-flow pump
An axial-flow pump uses a screw propeller to axially accelerate the
liquid. The outlet passages and guide vanes are arranged to convert the
velocity increase of the liquid into a pressure.
A reversible axial flow pump is shown in Figure 6.5. The pump casing
is split either horizontally or vertically to provide access to the propeller.
A mechanical seal prevents leakage where the shaft leaves the casing. A
thrust bearing of the tilting pad type is fitted on the drive shaft. The
prime mover may be an electric motor or a steam turbine.
The axial flow pump is used where large quantities of water at a low
head are required, for example in condenser circulating. The efficiency