Another type of pump you find aboard ship is the rotary pump. Rotary pumps are capable of pumping more fluid than reciprocating pumps of the same weight. A number of types are included in this classification, among which are the gear pump, the screw pump, and the moving vane pump. Unlike the centrifugal pump, the rotary pump is a positive-displacement pump. This means that for each revolution of the pump, a fixed volume of fluid is moved regardless of the resistance against which the pump is pushing. As you can see, any blockage in the system could quickly cause damage to the pump or a rupture of the system. You, as a pump operator, must always be sure that the system is properly aligned so a complete flow path exists for fluid flow. Also, because of their positive displacement feature, rotary pumps require a relief valve to protect the pump and piping system. The relief valve lifts at a preset pressure and returns the system liquid either to the suction side of the pump or back to the supply tank or sump.
Rotary pumps are also different from centrifugal pumps in that they are essentially self-priming. As we saw in our discussion of centrifugal pumps, the pump is located below the liquid being pumped; gravity creates a static pressure head which keeps the pump primed. A rotary pump operates within limits with the pump located above the source of supply. A good example of the principle that makes rotary pumps self-priming is the simple drinking straw. As you suck on the straw, you lower the air pressure inside the straw. Atmospheric pressure on the surface of the liquid surrounding the straw is therefore greater and forces the liquid up the straw. The same conditions basically exist for the gear and screw pump to prime itself.
The illustration shows a gear pump located above the tank. The tank must be vented to allow air into the tank to provide atmospheric pressure on the surface of the liquid. To lower the pressure on the suction side of the pump, the clearances between the pump parts must be close enough to pump air. When the pump starts, the air is pumped through the discharge side of the pump and creates the low-pressure area on the suction side, which allows the atmospheric pressure to force the liquid up the pipe to the pump. To operate properly, the piping leading to the pump must have no leaks or it will draw in air and can lose its prime. The suction piping should be at least one size larger than the pump suction connection.
Rotary pumps are useful for pumping oil and other heavy viscous liquids. In the engine room, rotary pumps are used for handling lube oil and fuel oil and are suitable for handling liquids over a wide range of viscosities. Rotary pumps are designed with very small clearances between rotating parts and stationary parts to minimize leakage (slippage) from the discharge side back to the suction side. Rotary pumps are designed to operate at relatively slow speeds to maintain these clearances; operation at higher speeds causes erosion and excessive wear, which result in increased clearances with a subsequent decrease in pumping capacity.
Classification of rotary pumps is generally based on the types of rotating element. In the following paragraphs, the main features of some common types of rotary pumps are described.
The simple gear pump (Illustration) has two spur gears that mesh together and revolve in opposite directions. One is the driving gear, and the other is the driven gear. Clearances between the gear teeth (outside diameter of the gear) and the casing and between the end face and the casing are only a few thousandths of an inch. As the gears turn, the gears unmesh and liquid flows into the pockets that are vacated by the meshing gear teeth. This creates the suction that draws the liquid into the pump. The liquid is then carried along in the pockets formed by the gear teeth and the casing. On the discharge side, the liquid is displaced by the meshing of the gears and forced out through the discharge side of the pump. Operating a rotary gear pump far below its rated speed when handling low viscosity liquids, will cause its volumetric efficiency percent of volume being actually being pumped to decrease to where the pump no longer develops flow.
Use of gear pumps include but are not limited to diesel engine fuel pumps, heavy oil fuel pumps, lube oil supply and transfer pumps, dirty oil pumps, and centrifuge supply pumps.
Several different types of screw pumps exist. The differences between the various types are the number of intermeshing screws and the pitch of the screws. Top left shows a double-screw, low-pitch pump; and top right shows a triple-screw, high-pitch pump.
Screw pumps are used aboard ship to pump fuel and lube oil and to supply pressure to the hydraulic system. In the double-screw pump, one rotor is driven by the drive shaft and the other by a set of timing gears. In the triple-screw pump, a central rotor meshes with two idler rotors.
In the screw pump, liquid is trapped and forced through the pump by the action of rotating screws. As the rotor turns, the liquid flows in between the threads at the outer end of each pair of screws. The threads carry the liquid along within the housing to the center of the pump where it is discharged.
One of the disadvantages of screw-type positive displacement rotary pumps is that they have performance characteristics sensitive to viscosity change.
Most screw pumps are now equipped with mechanical seals. If the mechanical seal fails, the stuffing box has the capability of accepting two rings of conventional packing for emergency use.