A pump is a machine that imparts energy to a liquid to increase its pressure and move it from one point to another. We used the term liquid (not fluid ) because the word pump is universally used to refer to a machine that pumps liquid whereas one that handles air, gas, or vapor, is specifically referred to as air pump, vacuum pump, compressor, blower, or fan.
In this web site the word pump is used to refer to a machinery that handles liquid. It is important to make this distinction because air, gas, or vapor is an "enemy" of a pump that impairs its performance and operation. A pump, in its universal meaning, should not be used for handling air, gas, or vapor.
Among non-electronic machineries pumps are the most widely used,next to electric motors. Pumps play an important role in our daily lives more than we probably realize it. At homes, pumps can be found in aquariums, swimming pools, washing machines, etc. Our cars need water pumps and fuel pumps to operate and run. In buildings where we work and shop, there are pumps to ensure our comfort - in water fountains, in cooling towers for centralized air- conditioning systems, in constant pressure water supply systems. And there are fire pumps, jockeys pumps, and sewage pumps, etc., protecting buildings from calamities and disasters. In agriculture there are irrigation, flood control, and deepwell water pumps to support our food supply. And of course, there are countless of other process, chemical, and specialty pumps in factories and other industries.
A typical pump has hundreds of parts. No matter how simple or complex some pumps are, their major parts are designed to perform specific functions regardless of their sizes and shapes. The major parts are:
Casing The casing (or case) contains the liquid and acts as a pressure containment vessel that directs the flow of liquid in and out of the pump. In most cases the casing includes the suction and discharge nozzles of the pump that connect it to the external piping. In some vertical pumps the casing may be referred to as bowl, and in some small pumps they may it call it housing.
Impeller The impeller is a vaned-disk (or disc) mounted on the shaft. Its function is to increase the pressure of the liquid by means of its rotating action. The impeller may be of open, semi-open, or enclosed type, and may have anywhere between two to ten vanes. It may also be of single, or double, suction design.
Shaft The shaft is usually the longest part of a pump and is made of one piece. Its function is to transmit the input power from the driver into the impeller. In a close- coupled pump, the motor has an extended length of shaft that acts as the pump shaft. In some vertical pumps, the so-called lineshaft pump, the shaft may be supplied in more than one piece of ten Feet sections depending on the pump bowl setting below the ground surface.
Seal A pump may be provided with one of two types of sealing elements - packing rings or mechanical seal. The sealing elements prevents the leakage of the pumped liquid into the atmosphere.
Bearings The functions of the bearings are to support the weight of the shaft (rotor) assembly, to carry the hydraulic loads acting on the shaft, and to keep the pump shaft aligned to the shaft of the driver.
Coupling The function of a coupling is to connect the pump shaft and the driver shaft, and to transmit the input power from the driver into the pump.
There are many other pump parts to consider but their function are collateral to that of the main parts listed above.
A pump's performance is shown in its characteristics performance curve where its capacity (in GPM) is plotted against its developed head (in FT), efficiency (in %), required input power (in BHP), and NPSHR (in FT.) The pump curve also shows its speed (in RPM) and other information such as pump size and type, impeller size, etc. [For simplicity the discussions in this web site are based on the US system of units and on 60 cycles power supply.]
The term head refers to the differential head developed by a pump expressed in feet of liquid:
H = [Pd-Ps] x 2.31 / SG
H = pump head, FT of liquid Pd = pump discharge pressure, PSIG Ps = pump suction pressure, PSIG SG = liquid specific gravity
If a pump were an ideal machine the required input power to drive the pump, called hydraulic horsepower (HHP), is calculated from:
HHP = [Q x H x SG] / 
where Q = capacity in GPM [ANSI/HI 1.1-1.5-1994, section 184.108.40.206 refers to this term as water horsepower which is a misnomer. If the specific gravity were omitted (SG=1) then the term is correctly referred to as water horsepower, WHP]
But since a pump is not an ideal machine the required input power, called brake horsepower (BHP), is calculated from: