At its simplest a pump is a device or a machine that is used to move a fluid or a gas from one point to another by imparting energy generally via rotary or reciprocating mechanical action.There are two broad groups in to which most pumps can be placed, these are Positive displacement and Centrifugal.
Positive Displacement Pumps
Positive displacement pumps basically push or force product through the machine.
Positive displacement pumps can produce extremely high pressures and are capable of handling highly viscous and sheer sensitive materials from such things as chocolate, to sausage mince, to sewerage sludge to molasses.
Positive displacement pumps are generally either rotary or reciprocating style.
Reciprocating pumps include piston, plunger and diaphragm style.
Rotary include Lobe pumps, Helical rotor pumps, screw pumps and Cam pumps.
Generally positive displacement pumps are capable of providing extreme accuracy and repeatability per cycle of operation and tend to be highly efficient.
A positive displacement pump must be sized and controlled carefully, it is possible for a positive displacement pump to operate to destruction if it is run against a shut head, such as in a closed discharge valve situation.
Centrifugal Pumps
Centrifugal pumps are rotary pumps.
Centrifugal pumps form by far the greater population of pumping machines used throughout the world.
Centrifugal pumps impart Kinetic energy to the pumped fluid, fluid flows to the centre or the eye of the impeller and travels along the impeller vanes to exit at the pump discharge nozzle.
Impeller speed, shape and size will govern the pump capacity.
There are three main impeller styles, these are:
1. Radial Flow
The fluid exits the impeller at 90 degrees to entry, full performance is achieved from the centrifugal force exerted by the impeller.
Radial flow centrifugal pumps tend to be used for lower flow applications with high discharge pressure requirements pumping clean fluids.
Radial flow impellers have quite fine tolerances and as such are not suitable for use with fluids with solids entrained.
Radial flow impellers are used in applications such as high pressure boiler feed.
2. Mixed Flow
The fluid exits the impeller radially and axially.
Centrifugal force and the action of the impeller vane combine to produce performance.
Mixed flow impellers are generally used where reasonable flows need to be combined with reasonable discharge pressures.
Mixed flow impellers are generally designed with large tolerances and as a general rule are capable of passing substantial sized solids.
Mixed flow impellers are used in applications from moving large volumes of clean water, to moving high density slurries in the process industry, to handling waste products in Sewerage plants and Abbatoirs.
3. Axial Flow
The fluid exits the impeller axially and relies totally on the action of the impeller vane to produce pressure.
Axial flow impellers are used to move large quantities of water at very low pressure.
Axial flow pumps are widely used in water harvesting, Aquaculture and ballast exchange.
Axial flow pumps are often referred to as Propeller pumps due to the design and operation of the impeller and its similarity to a ships propeller.
Whereas Centrifugal pumps should not be operated for long periods against a shut head they will not continue to build pressure in the same way as a positive displacement pump
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