STATE EXAM 01. AUXILIARY MACHINERY

STATE EXAM

1. Auxiliary machinery (pumps, centrifugal separator, fresh water generator)

1. 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.

Gear Pumps and Screw Pumps

These usually consist of two gears, one drive gear and the other driven, although some pumps may have one drive and two driven gears. As there is direct meshing of the gears the fluid being pumped must be a lubricant. This type of pump is simple and maintenance is minimal, even more so if bearing bushes are fitted as opposed to rolling element bearings.

Gear pumps are relatively trouble free and most problems tend to be with the relief valve which may get dirt trapped underneath the seat if the valve lifts. Another problem is that pressure often gets regulated by the relief valve and the spring can eventually weaken and possibly break.

Unlike a gear pump the screws do not mesh. Instead they are driven by synchronizing gears. It is important to ensure the screws are correctly assembled after overhaul. If the synchronizing gears are not marked, they should be marked before removing the screws.

Centrifugal Pumps

These pumps can only handle fluids of a relatively low viscosity. The discharge pressure is much lower than a positive displacement pump, however, the discharge rate is much higher. For higher discharge pressures, such as boiler feed pumps, multi-stage pumps are required. Centrifugal pumps cannot pump air, therefore, a priming pump is required unless there is a positive suction head.

Impellers can be single or double entry. With double entry the axial thrust is balanced out. Water enters the impeller axially at low velocity and leaves the impeller radially at high velocity.

The volute casing has an increasing cross sectional area which converts the kinetic energy of the water into pressure energy.

The discharge rate is controlled by throttling the discharge valve.

There are very few moving parts so maintenance is mainly concerned with inspecting the impeller and checking seal ring clearances. Impellers are subject to corrosion and erosion due to cavitation, particularly pumps that are used for sea water. The clearances should always be checked against the manufacturers recommendations.