Page 136 - Introduction to Marine Engineering

122 Pumps and pumping systems

Shaft-

Impeller

Impeller.

Wear ring

' Suction
Note only two stages are shown Figure 6.9 Multi-stage centrifugal pump

Alternatively an air pumping unit can be provided to individual pumps
or as a central priming system connected to several pumps.
The water ring or liquid ring primer can be arranged as an individual
unit mounted on the pump and driven by it, or as a motor driven unit
mounted separately and serving several pumps. The primer consists of an
elliptical casing in which a vaned rotor revolves. The rotor may be
separate from the hub and provide the air inlet and discharge ports as
shown in Figure 6.10. Alternatively another design has the rotor and
hub as one piece with ports on the cover. The rotor vanes revolve and
force a ring of liquid to take up the elliptical shape of the casing. The
water ring, being elliptical, advances and recedes from the central hub,
causing a pumping action to occur. The suction piping system is
connected to the air inlet ports and the suction line is thus primed by the
removal of air. The air removed from the system is discharged to
atmosphere. A reservoir of water is provided to replenish the water ring
when necessary.
When starting a centrifugal pump the suction valve is opened and the
discharge valve left shut: then the motor is started and the priming unit
will prime the suction line. Once the pump is primed the delivery valve