Pump

I

Introduction

Pump, device used to raise, transfer, or compress liquids and gases. Four general classes of pumps for liquids are described below. In all of them, steps are taken to prevent cavitation (the formation of a vacuum), which would reduce the flow and damage the structure of the pump. Pumps used for gases and vapours are usually known as compressors (see Air Compressor). The study of fluids in motion is called fluid dynamics (see Fluid Mechanics).

II

Reciprocating Pumps

Reciprocating pumps consist of a piston moving back and forth in a cylinder that has valves to regulate the flow of liquid into and out of the cylinder. These pumps may be single or double acting. In the single acting pump, the pumping action takes place on only one side of the piston, as in the case of the common lift pump, in which the piston is moved up and down by hand. In the double acting pump, the pumping action takes place on both sides of the piston, as in the electrical or steam-driven boiler feed pump, in which water is supplied to a steam boiler under high pressure. These pumps can be single-stage or multi-staged. Multi-staged reciprocating pumps have multiple cylinders in series. See Steam Engine.

III

Centrifugal Pumps

Also known as rotary pumps, centrifugal pumps have a rotating impeller, also known as a blade, that is immersed in the liquid. Liquid enters the pump near the axis of the impeller, and the rotating impeller sweeps the liquid out towards the ends of the impeller blades at high pressure. The impeller also gives the liquid a relatively high velocity that can be converted into pressure in a stationary part of the pump, known as the diffuser. In high-pressure pumps, a number of impellers may be used in series, and the diffusers following each impeller may contain guide vanes to gradually reduce the liquid velocity. For lower-pressure pumps, the diffuser is generally a spiral passage, known as a volute, with its cross-sectional area increasing gradually to reduce the velocity efficiently. The impeller must be primed before it can begin operation—that is, the impeller must be surrounded by liquid when the pump is started. This can be done by placing a checkvalve in the suction line, which holds the liquid in the pump when the impeller is not rotating. If this valve leaks, the pump may need to be primed by the introduction of liquid from an outside source such as the discharge reservoir. A centrifugal pump generally has a valve in the discharge line to control the flow and pressure.

For low flows and high pressures, the action of the impeller is largely radial. For higher flows and lower discharge pressures, the direction of the flow within the pump is more nearly parallel to the axis of the shaft, and the pump is said to have an axial flow. The impeller in this case acts as a propeller. The transition from one set of flow conditions to the other is gradual, and for intermediate conditions, the device is called a mixed-flow pump.

IV

Jet Pumps

Jet pumps use a relatively small stream of liquid or vapour, moving at high velocity, to move a larger flow of fluid. As the high-velocity stream passes through the fluid, it carries some of the fluid out of the pump; at the same time, the high-velocity stream creates a vacuum that pulls fluid into the pump. Jet pumps are often used to inject water into a steam boiler. Jet pumps have also been used to propel boats, particularly in shallow water where a conventional propeller might be damaged. See Jet Propulsion.