Difference between revisions of "Pumps"

Some positive displacement pumps work  using an expanding cavity on the suction side and a decreasing cavity on  the discharge side. Liquid flows into the pump as the cavity on the  suction side expands and the liquid flows out of the discharge as the  cavity collapses. The volume is constant given each cycle of operation.

Positive  displacement pumps, unlike centrifugal or roto-dynamic pumps, will in  theory produce the same flow at a given speed (RPM) no matter what the  discharge pressure. Thus, positive displacement pumps are constant flow  machines. However due to a slight increase in internal leakage as the  pressure increases, a truly constant flow rate cannot be achieved.

A relief or safety valve on  the discharge side of the positive displacement pump is therefore  necessary. The relief valve can be internal or external. The pump  manufacturer normally has the option to supply internal relief or safety  valves. The internal valve should in general only be used as a safety  precaution, an external relief valve installed in the discharge line  with a return line back to the suction line or supply tank is  recommended.

[[Image:Lysholm_screw_rotors.jpg|thumb|upright|Screw pump]]

A positive displacement pump can be further classified according to the mechanism used to move the fluid:

* '''Linear-type''' positive displacement: Rope pump|rope pumps and chain pumps.

[[File:220px-Rotary_vane_pump.svg.png|thumb|Rotary vane pump]]

Positive  displacement rotary pumps are pumps that move fluid using the  principles of rotation. The vacuum created by the rotation of the pump  captures and draws in the liquid.  

Reciprocating-type  pumps require a system of suction and discharge valves to ensure that  the fluid moves in a positive direction. Pumps in this category range  from having "simplex" one cylinder, to in some cases "quad" (four)  cylinders or more.  Most reciprocating-type pumps are "duplex" (two) or  "triplex" (three) cylinder. Furthermore, they can be either "single  acting" independent suction and discharge strokes or "double acting"  suction and discharge in both directions. The pumps can be powered by  air, steam or through a belt drive from an engine or motor. This type of  pump was used extensively in the early days of steam propulsion (19th  century) as boiler feed water pumps. Reciprocating pumps are now  typically used for pumping highly viscous fluids including concrete and  heavy oils, and special applications demanding low flow rates against  high resistance.

The positive displacement principle applies in the following types of pumps:

[[File:pulser pump.jpg|thumb|The pulser pump]]

Impulse pumps use pressure created by gas (usually air). In some impulse pumps the gas trapped in the liquid (usually water), is released and accumulated somewhere in the pump, creating a pressure which can push part of the liquid upwards. Impulse pumps include:

[[Image:Centrifugal 2.png|thumb|right|A centrifugal pump uses a spinning "impeller" which has backward-swept arms]]

Dynamic pumps can be further subdivided according to the means in which the velocity gain is achieved.