Water pumps in the engine room

Gear pump are suitable for oil handling

Horizontal end suction pump are common

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Pumps are the biggest group of machinery on board a ship. They are installed in practically every machinery that needs cooling, lubrication, and power transmission, combustion. They are also used to transfer fluid between tanks, for fire fighting, and washing. 

Generally, they can be divided into water pumps and oil pumps. 

The maintenance of pumps is usually assigned to the junior engineers, as they are comparatively simple to maintain. More senior engineers may handle however more complicated pumps like the variable stroke pumps of hydraulic steering gears, fuel injection pumps of diesel engines.

Water pumps are normally called centrifugal pumps. The impeller inside the pump casing is rotated to a high speed of about 1450-rpm, flinging the water out by centrifugal force from the center towards the edges. By means of specially shaped volute housing or by means of diffuser plates of turbine pumps, the high velocity water stream is converted to a slower speed, pressurized water at the discharge of the pump.

Oil pumps are usually positive displacement pumps. Because the moving parts have very close fit, oil is needed to provide a lubricating film between the surfaces. 

Some common positive displacement pumps in use on board ships are gear pumps, screw pumps, piston and plunger pumps, vane pumps, and lobe pumps. All of these pumps have pressure relief valves installed at the discharge. 

The pressures generated can become very high especially during the winter months, when the oil is insufficiently heated.

Critical pumps are usually installed in pairs, so that if one fails, a standby unit can be started. Some pumps are designed to be used for general purpose, i.e. for ballasting, fire fighting, bilge, while others are dedicated for one function only, as in engine cooling water pumps. 

The cooling water pumps work in conjunction with coolers or heat exchangers to maintain the temperatures in the engines.

More information on shaft seals like mechanical seals and gland packing.