Centrifugal Pumps

Description

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.

How it works

Like most pumps, 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.  The doughnut-shaped diffuser, or scroll, section of the casing decelerates the flow and further increases the pressure.

Priming Centrifugal Pumps

Most centrifugal pumps are not self-priming. In other words, the pump casing must be filled with liquid before the pump is started, or the pump will not be able to function.  If the pump casing becomes filled with vapors or gases, the pump impeller becomes gas-bound and incapable of pumping.

Also, to ensure that a centrifugal pump remains primed and does not become gas-bound. Most centrifugal pumps are located below the level of the source from which the pump is to take its suction.  The same effect can be gained by supplying liquid to the pump suction under pressure supplied by another pump placed in the suction line.

Self priming Centrifugal Pumps

In normal conditions, common centrifugal pumps are unable to evacuate the air from an inlet line.  This line is leading to a fluid level whose geodetic altitude is below that of the pump.  Self-priming pumps have to be capable of evacuating air (see Venting) from the pump suction line without any external auxiliary devices.

Also, Centrifugal pumps which are not designed with an internal or external self-priming stage.  They can only start to pump the fluid after the pump has initially been primed with the fluid.  In addition, a suction-side swing check valve or a vent valve must be fitted to prevent any siphon action.  This will ensure that the fluid remains in the casing when the pump has been stopped.

In Conclusion, self-priming centrifugal pumps with a separation chamber for the fluid pumped.  It also has entrained air bubbles are pumped into the separation chamber by the impeller action.