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Pumps and pumping - Procedure for ship service systems
Marine pumps fall into two broad categories either displacement or rotodynamic categories.
Displacement : The liquid or gas is displaced from the suction to the discharge by the mechanical variation of the volume of a chamber or chambers.
Displacement pumps can be subdivided into two categories : reciprocating pumps , in which a piston or plunger is is mechanically reciprocating
in a liquid cylinder , and rotary pumps , where the liquid is forced through the pump cylinder or casing by means of screws or gears.
Rotodynamic : The centrifugal pump operates on a rotodynamic principle in that flow through the pump is induced by the centrifugal force imparted to the liquid by rotation of an impeller. The axial flow pump relies on rotation and the screw propeller form the rotor.
The centrifugal pump is now used for most applications and systems on ships.
In the machinery space it provides a much more reliable service than the steam
reciprocating pumps that were still being installed in the 'fifties as auxiliary
boiler feed and fuel pumps for example. These reciprocating pumps required
regular maintenance and, if neglected,
they needed constant attention to keep
them functioning. The general use of the centrifugal pump helped to make the
unmanned machinery space viable.
Summarized below some of the basic procedure of marine pumps and pumping system onboard :
- Axial flow pumps
An axial flow pump is one in which a screw propeller is used to create an increase in pressure by causing an axial acceleration of liquid within its blades. The incidental rotation imparted to the liquid is converted into straight axial movement by suitably shaped outlet guide vanes.....
- centrifugal pumps
Rotation of a centrifugal pump impeller causes the liquid it contains to move outwards from the centre to beyond the circumference of the impeller. The revolving liquid is impelled by centrifugal effect.....
- Centrifugal pump cavitation
Because of their self-priming ability, positive displacement pumps are widely used for lubricating oil duties. This practice is completely satisfactory in installations where the pump speed is variable but when the pump is driven by a constant speed a.c. motor it is necessary to arrange a bypass which can be closed in to boost flow. ....
- Gear pumps
Diesel engine and gearbox lubrication systems are normally supplied by gear
pumps which are independently driven for large slow speed engines and
stand-by duties but usually shaft driven for medium and high speed engines.
Gear pumps are also used for fuel and oil transfer, boiler combustion systems
and other duties.....
- General pumping system characteristics
A pump divides its pipe system into two distinct parts, each with different
characteristics. These are the suction and discharge sides. On the suction side
the drop in pressure that can be produced by a pump is limited to that of an
almost perfect vacuum. On the discharge side there is theoretically, no limit to
the height through which a liquid can be raised.....
- General purpose pumps
Single entry general purpose pumps are used for salt and fresh water circulating and also for bilge and ballast duties. The impeller is suspended from the shaft with no bottom support. ....
- Lobe pumps
Lobe pumps as manufactured by Stothert and Pitt have inner and outer
elements which rotate in a renewable liner fitted in the pump body. The inner
rotor is eccentric to the outer and is fitted to a shaft located by bearings in the
pump covers....
- Marine pumps construction
Marine pumps are usually installed with the shaft vertical and the motor above the pump. This positions the pump as low as possible for the best NPSH, takes up the least horizontal space and leaves the electric motor safer from gland or other leakage.....
- Pumps erosion
A pump handling liquids which contain abrasives, will suffer erosion on all internal surfaces, including bearings and shaft seals. The sea-water circulating pumps of ships operating in waters that contain large quantities of silt and sand
require frequent renewal of shaft seals or packing, also of shaft sleeves in way of the gland and bearings.....
- Rotary displacement pump
Positive displacement rotary pumps rely on fine clearances between moving
parts for their efficient operation. Excessive wear or erosion of parts, due to
friction contact or the presence of abrasives, is avoided by employing this type
of pump for specialized rather than general duties......
- Screw pumps
Both double-screw pumps, in which the screws are driven in phase by timing
gears , and triple screw pumps , in which the centre
screw is driven and the outer screws idle are used at sea especially for pumping
high viscosity liquids such as oil and some liquid cargoes.....
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