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Centrifugal Pumps & Pumping Elements for General Marine Duties

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.

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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 shaft.

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.

Centrifugal pump
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.

Single entry centrifugal pump
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.

Multi stage centrifugal pump
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 when necessary.

Water-ring primer
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.

sketch of a self priming centrifugal pump
Sketch of a self priming centrifugal pump

Sketch of a central priming system: The water ring exhausters maintain the vacuum in the tank with preset limits. Opening of the SDNR valves or cocks for a pump will cause priming to take place. To prevent water entering the vacuum tank after priming has taken place, air release valves are fitted which automatically close.
Central priming system for centrifugal pumps
Central priming system for centrifugal pumps


The function of the central priming system is to provide priming for all centrifugal pumps located in the engine room. The advantages of a central priming system are:
  1. Saving in power as each pump does not require its own exhauster
  2. reduced capital cost
  3. simplified maintenance
  4. automatic.

What type of pump would be preferable to use as an emergency bilge pump?

: A centrifugal pump with a self-priming device, as centrifugal pumps can pump high volumes of water (60 kg/s), and they also require little attention. Some emergency bilge pumps have their motor encased in an air bell so the pump can be operated when submerged.

What is cavitation? When and where might it occur?

: Cavitation is very high pressure hammer blows caused by pressure regions occurring in the fluid flow at points where high local velocity exist. If vaporisation occurs due to low pressure areas, then bubbles occur, these expand as they move with the flow and collapse when they reach a high pressure region caused by high pressure hammer blows. This can lead to pitting, noise, vibration and fall in pump efficiency.

To prevent this, some centrifugal pumps have inducers fitted to the pump impeller and their purpose is to ensure there is sufficient supply of fluid at the impeller, avoiding cavitation at impeller suction. They will also allow the pump to operate with a lower net positive supply head.



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    centrifugal-pump
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