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Centrifugal pumps for general marine duties
Centrifugal pump principles and working procedure
A pump is a machine used to raise liquids from a low point to a high
point. In a centrifugal pump liquid enters the centre or eye of the impeller and
flows radially out between the vanes, its velocity being increased by the
impeller rotation. A diffuser or volute is then used to convert most of the
kinetic energy in the liquid into pressure.
The arrangement of a centrifugal pump is shown
diagrammatically in figure below
A vertical, single stage, single entry, centrifugal pump for general
marine duties is shown in Figure here. The main frame and casing,
together with a motor support bracket, house the pumping element
assembly. The pumping element is made up of a top cover, a pump
shaft, an impeller, a bearing bush and a sealing arrangement around the
The sealing arrangement may be a packed gland or a mechanical
seal and the bearing lubrication system will vary according to the type of
seal. Replaceable wear rings are fitted to the impeller and the casing.
Fig: Centrifugal pumping operation
The motor support bracket has two large apertures to provide access to
the pumping element, and a coupling spacer is fitted between the motor
and pump shaft to enable the removal of the pumping element without
disturbing the motor.
Fig: Single entry centrifugal pump
A vertical multi-stage single-entry centrifugal pump used for
deep-well cargo pumping is shown in Figure below. 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.
Fig: Multi stage centrifugal pump
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
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.
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 down. 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
Fig: Water-ring primer
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
can be slowly opened and the quantity of liquid can be regulated by
opening or closing the delivery valve. When stopping the pump the
delivery valve is closed and the motor stopped.
Regular maintenance on the machine will involve attention to
lubrication of the shaft bearing and ensuring that the shaft seal or gland
is not leaking liquid. Unsatisfactory operation or loss of performance
may require minor or major overhauls. Common faults, such as no
discharge, may be a result of valves in the system being shut, suction
strainers blocked or other faults occurring in the priming system. Air
leaks in the suction piping, a choked impeller or too tight a shaft gland
can all lead to poor performance.
When dismantling the pump to remove the pumping element any
priming pipes or cooling water supply pipes must be disconnected.
Modern pumps have a coupling spacer which can be removed to enable
the pumping element to be withdrawn without disturbing the motor: the
impeller and shaft can then be readily separated for examination. The shaft bearing bush together with the casing and impeller wear rings
should be examined for wear.
Marine machineries - Useful tags
Marine diesel engines
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- Mud boxes, Suction pipes, Steam traps, Expansion pieces & Drains for Machinery space piping system The mud box is a coarse strainer with a straight tailpipe down to the bilge..Suction pipes in tanks should be arranged with a bell mouth or foot..An expansion piece is fitted in a pipeline which is subject to considerable temperature variations..Drains are provided in pipelines and usually have small cocks to open
or close them.....
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- Centrifugal pump principles and working procedure
In a centrifugal pump liquid enters the centre or eye of the impeller and flows radially out between the vanes, its velocity being increased by the
- How a pneumatically control valve works ?
Many pneumatic devices use a nozzle and flapper system to give a
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- Domestic water systems for general cargo ship Domestic water systems usually comprise a fresh water system for
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. Both use a basically similar arrangement of an automatic
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The bilge main is arranged to drain any watertight compartment other
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The number of pumps and their capacity depend upon the size, type
and service of the vessel.....
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