These gear pumps are rotary displacement or rotary positive displacement pumps. Two toothed wheels shown, mesh together and are a close fit in casing. Initially the air or gas is trapped between each pair of consecutive teeth and the same is dragged along the casing from suction to discharge side till no more air is left on the suction side. Liquid from the tank  will thus rise up into suction line under atmospheric pressure, subsequently this liquid will now be trapped between each pair of two consecutive teeth and dragged along the casing into the discharge side and pumping of liquid will commence. The working principle just explained is what makes the pump a self priming pump. Further if liquid level on suction side is at a higher level, the liquid will flow into the suction side on its own at first instant itself.

Usually the gear pumps are electric motor driven through a chain or wheel drive. Control of flow rate is achieved by a bypass valve or controlling speed of prime  mover.

A number of such pumps in series can be used to develop high pressure. Such pumps are efficient and smooth running.

Applications

Gear pumps are used for duties as a lube oil pump, boiler fuel oil pump, fuel oil transfer pump, main engine driven lube oil pump. As a main engine driven lube oil pump it will have a set of suction and discharge valve to give same side discharge at all times irrespective of ahead or astern movement of the main engine.

References

“Basic Marine Engineering” by J.K.DHAR