Page 134 - Introduction to Marine Engineering

120 Pumps and pumping systems

Motor
t
half coupling Motor
support bracket

Soft
packed gland

Delivery

Main frame and casing

Figure 6.7 Single-entry centrifugal pump

Other configurations of centrifugal pumps are used for particular
duties or to meet system requirements. A vertical single stage double-
entry centrifugal pump is shown in Figure 6.8. The incoming liquid
enters the double impeller from the top and the bottom and passes into
the volute casing for discharge. A double-entry pump has a lower NPSH
required characteristic which will have advantages in poor suction
conditions. It should be noted that different impeller sizes can be fitted
into a basic pumping element. This enables various discharge head
characteristics to be provided for the same basic pump frame.
A vertical multi-stage single-entry centrifugal pump used for
deep-well cargo pumping is shown in Figure 6.9. This can be
considered as a series of centrifugal pumps arranged to supply one
another in series and thus progressively increase the discharge pressure.
The pump drive is located outside the tank and can be electric, hydraulic
or any appropriate means suitable for the location.
A diffuser is fitted to high-pressure centrifugal pumps. This is a ring
fixed to the casing, around the impeller, in which there are passages