what is marine positive displacement pump

A pumps  is a device which adds the energy to a liquid or gas,causing an increase in its pressure and perhaps a movement of the fluid.

A basic pumping system consists of a suction branch, a pump and a discharge branch. the pumps employed on board ship fall into two broad classes:positive displacement pumps and centrifugal pumps.

positive displacement pumps  are those where the volume of the pump chamber is alternately increased to draw the liquid in from the suction pipe and then decreased to force the liquid out into the delivery pipe.

the liquid or gas is displaced from the suction to the discharge by the mechanical variation of the volume of a chamber or chambers.

This kind of pumps can be subdivided into two main clawes: reciprocating pumps.in which a plunger or piston is mechanically reciprocated in a liquid cylinder.

And rotary pumps,where the liquid is fored through the pump cylinder or casing by means of screws or gears.under reciprocating are the direct acting,power, and crank-fly- wheel types.

while under rotary are the gear, Vane,screw and piston types. Reciprocating and rotary positive displacememt pumps are self-priming,and can deal with liquid from a level below the pump.

The typical pumps employed in steering gear hydraulic system are rotary-piston pumps,classified either axial or radial type. The pistons in an axial pump operate along the axis of the pump drive shaft.

The cylinder block has a series of cylinder bores with pistons that move in and out.As the block rotates. each piston moves in and out of its cylinder, the length of stroke depending on the angle of the cylinder block with reference to the drive plate or swash plate according to the designs.

As each piston moves outward.oil is drawn into the cylinder bore through the valve plate.On the return stoke,the oil is forced out through the valve plate under pressure.

gear pump is a represent of the rotary pump family and is commonly used on hoard a ship.it consists of two or more meshing gears(spur,single or double helical teeth) enclosed in a close-fitted housing. these are used extensivelv for pumping fuel oil. lubricating oil and hydraulic oil.

When gears are unmeshed on the inlet side. a pressure drop occurs that helps the atmospheric pressure acting on the surface of the oil in the reservoir to force it up the pipe to fill the void,the oil is then transferred around the periphery of the pump housing.

As the gear teeth mesh again on the outlet side,they form a seal that prevents oil from backing up to the inlet, the oil in the void is then be forced out into the discharge line.