A machine used to raise liquids from a low level to a higher one, or to provide liquid with an increase in energy enabling it to flow or build up a pressure.
There are various types of pumps used in industrial and marine application including:
It develops its pressure as a result of centrifugal force. Liquid enters the center of the impeller and flows radially out between vanes. Its velocity being increased by the impeller rotation. It escapes through a discharge pipe at the circumference of the casing. Centrifugal pumps may be classified in several ways. For example, they may be either single-stage or multi-stage. A single-stage pump has only one impeller; a multi-stage pump has two or more impellers housed together in one casing. In a multi-stage pump, each impeller usually acts separately, discharging to the suction of the next- stage impeller. Centrifugal pumps are also classified as horizontal or vertical ones, depending on the position of the pump shaft. Impellers used in centrifugal pumps may be classified as single-suction or double-suction, depending on the way the liquid enters the eye of the impeller. The single-suction impeller allows liquid to enter the eye from one side only; the double-suction impeller allows liquid to enter the eye from both sides. The double suction arrangement has the advantage of balancing the end thrust in one direction with the end thrust in the other direction.
Axial flow pump
A centrifugal pump which uses a screw propeller to accelerate the liquid axially. It is effectively a propeller in a tube. A large volume flow of liquid at low pressure is produced. Pumps of this type operate at comparatively high speed usually directly connected to their driver. They are used where large volumes at relatively low pressures are required. They run as fresh or seawater cooling pumps, general service, boiler feed, ballast, firefighting or as lubricating oil pumps.
A pump operating by the reduction or increase in volume of a space by a mechanical action which physically moves the liquid or gas. In positive displacement pumps one or two chambers are filled and emptied. These include reciprocating, screw, gear and water ring type pumps. They do not require a priming device and may themselves be used as one. They cause a reduction and increase in volume of space and cause the liquid or gas to be physically moved.
A pump in which a plunger or piston is mechanically reciprocated in a liquid cylinder. The reciprocating pump has positive pressure characteristics and is used principally to handle small volumes at relatively high pressures. Due to its reciprocating motion and the inertia effect of the parts, speeds are relatively low. This type of pump is self-priming and the delivered capacity is practically constant regardless of discharge pressure.
These are useful for pumping oil and other liquids of high viscosity. 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 low speeds to maintain these clearances. The operation at higher speeds causes erosion and excessive wear which result in increased clearances with a subsequent decrease in pumping capacity. Classification of the rotary pumps is generally based on the types of rotating element.
The simple gear pump 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, they 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.
Rotary vane pumps
The rotary vane pump has a cylindrically-bored housing with a suction inlet on one side and a discharge outlet on the other side. A rotor (smaller in diameter than the cylinder) is driven about an axis that is placed above the center line of the cylinder to provide minimum clearance between the rotor and cylinder at the top and maximum clearance at the bottom. The rotor carries vanes (which move in and out as the rotor rotates) to maintain sealed spaces between the rotor and the cylinder wall. The vanes trap liquid on the suction side and carry it to the discharge side, where contraction of the space expels liquid through the discharge line. The vanes slide on slots in the rotor. Vane pumps are used for lube oil service and transfer, tank stripping, bilge, and in general, for handling lighter viscous liquids.
There are several different types of screw pumps. The differences between the various types are the number of intermeshing screws and the screw pitch. 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. 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.