Reciprocating Pumps
STATE EXAM
Pumps There are many types of pumps installed on board ship and these may be direct drive, belt drive or driven through a gearbox. Positive Displacement Pumps Positive displacement pumps are capable of pumping air and can generate high pressures. These pumps are therefore fitted with a relief valve on the discharge side before the discharge valve. It is important that the discharge valve is fully open and discharge pressure is often regulated with the relief valve. Positive displacement pumps can be categorised as follows: – Reciprocating. – Rotary – screw, gear, vane, rotor (mono). – Rotary / Reciprocating. Reciprocating Pumps These are usually motor driven and have a reduction gear to the crankshaft drive wheel. These pumps are most often used as bilge pumps, where a large amount of foreign matter in water may clog a centrifugal pump. Perhaps only one or two reciprocating pumps will be found on board. Despite their high efficiency – they have the highest efficiency of all positive displacement pumps due to their low leakage – maintenance is high due to the large number of moving parts. The discharge pressure is also pulsating, so an air reservoir must be fitted on the discharge side to dampen out the pulsations. The discharge rate is quite small but high discharge pressures can be generated and the flow is regulated by throttling the suction valve. As with all positive displacement pumps it is self priming. Maintenance entails lubricating the bearings which can be rolling element bearings with a grease nipple or, on larger pumps, plain bearings which may have oil cups that need to be filled regularly when the pump is in use. Valves also need overhauling, although this mainly consists of cleaning.
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