My answer to Why is priming necessary for centrifugal pump and not in reciprocating pump?
Answer by Arûn Râinâ:
Centrifugal pumps are a sub-class of dynamic axis-symmetric work-absorbing turbo-machinery.Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber (casing), from where it exits.
Common uses include water, sewage, petroleum and petrochemical pumping. The reverse function of the centrifugal pump is a water turbine converting potential energy of water pressure into mechanical rotational energy.
A centrifugal pump converts rotational energy, often from a motor, to energy in a moving fluid. A portion of the energy goes into kinetic energy of the fluid. Fluid enters axially through eye of the casing, is caught up in the impeller blades, and is whirled tangentially and radially outward until it leaves through all circumferential parts of the impeller into the diffuser part of the casing. The fluid gains both velocity and pressure while passing through the impeller.
Priming is the process in which the impeller of a centrifugal pump will get fully sub merged in liquid without any air trap inside.This is especially required when there is a first start up.But it is advisable to start the pump only after primping.
For Centrifugal Pump, priming is required only when the elevation of liquid in the suction tank/pond is less then the elevation of the pump. Centrifugal pump can not "suck" / "pull" the liquid but it push the liquid. Liquid enter in the suction of centrifugal pump by the energy it posses (atmospheric pressure in the suction tank).
A "self-priming" centrifugal pump overcomes the problem of air binding by mixing air with water to create a fluid with pumping properties much like those of regular water. The pump then gets rid of the air and moves water only, just like a standard centrifugal pump
In order for a centrifugal pump, or self priming, pump to attain its initial prime the casing must first be manually primed or filled with water. Afterwards, unless it is run dry or drained, a sufficient amount of water should remain in the pump to ensure quick priming the next time it is needed.
A reciprocating pump is a class of positive-displacement pumps which includes the piston pump, plunger pump and diaphragm pump. It is often used where a relatively small quantity of liquid is to be handled and where delivery pressure is quite large.
In Centrifugal pumps we require priming while in the Reciprocating pumps when the piston or diaphragm moves backward the volume of cavity increases and pressure inside it drops, this opens the inlet valve and liquid is taken in until the differential pressure between the supply side and the chamber is zero. Then the piston or diaphragm moves forward decreasing the volume of the chamber and pressure inside the chamber increase, this opens the outlet valve and the liquid from inside the chamber is pushed outside in the discharge line until the differential pressure is zero. This cycle repeats and an intermittent discharge of fluid is obtained.
Reciprocating and rotary pumps are self-priming. This is an important consideration where a prime cannot be maintained on the pump. Centrifugal pumps are not inherently self-priming, although some manufacturers do specially design self-priming units. External priming sources, such as an eductor or vacuum pump can also be employed.