Difference between revisions of "Positive Displacement Pumps"

A positive displacement pump causes a fluid to move by trapping a fixed amount of it and then forcing (displacing) that trapped volume into the discharge pipe.

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.

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

* '''Rotary-type''' positive displacement: internal gear, screw, shuttle block, flexible vane or sliding vane, circumferential piston, helical twisted roots (e.g. the Wendelkolben pump) or liquid ring vacuum pumps.

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.  

'''Drawbacks:''' Positive displacement rotary  pumps also have their weaknesses. Because of the nature of the pump,  the clearance between the rotating pump and the outer edge must be very  close, requiring that the pumps rotate at a slow, steady speed. If  rotary pumps are operated at high speeds, the fluids will cause erosion.  Rotary pumps that experience such erosion eventually show signs of  enlarged clearances, which allow liquid to slip through and reduce the  efficiency of the pump.

* [[Gear Pumps]] - a simple type of rotary pump where the liquid is pushed between two gears.

* [[Screw Pumps]] - the shape of the internals of this pump usually two screws turning against each other pump the liquid.

* [[Rotary Vane Pumps]] - similar to scroll compressors, consisting of a cylindrical rotor encased in a similarly shaped housing. As the rotor turns, the vanes trap fluid between the rotor and the casing, drawing the fluid through the pump.

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.

These positive displacement pumps have  an expanding cavity on the suction side and a decreasing cavity on the  discharge side. Liquid flows into the pumps 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.

* [[Plunger Pumps]] - a reciprocating plunger pushes the fluid through one or two open valves, closed by suction on the way back.

*  [[Diaphragm Pumps]] - similar to plunger pumps, where the plunger  pressurizes hydraulic oil which is used to flex a diaphragm in the  pumping cylinder. Diaphragm valves are used to pump hazardous and toxic  fluids.

* [[Piston Displacement Pumps]] - usually simple devices  for pumping small amounts of liquid or gel manually. An example is the  common hand soap pump.