Difference between revisions of "Positive Displacement Pumps"

[[Image:Jet_pump.jpg|thumb||right|A jet pump]]

A '''pump''' displaces a volume by physical or mechanical action.  Pumps fall into three major groups: direct lift, displacement, and gravity pumps.

==Types==

===[[Positive Displacement Pumps]]===

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

==== Positive Displacement Pump behavior and safety ====

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.

==== Positive Displacement Types ====

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

* '''Reciprocating-type''' positive displacement: piston or diaphragm pumps.

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

===== Rotary Positive Displacement Pumps =====

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

'''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 powders will  cause  erosion.  Rotary pumps that experience such erosion eventually  show  signs of  enlarged clearances, which allow powder to slip through  and  reduce the  efficiency of the pump.

===== Reciprocating Positive Displacement Pumps =====

Reciprocating-type    pumps require a system of suction and discharge valves to ensure  that    the powder 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 powders  including concrete and  heavy oils, and special applications demanding  low flow rates against  high resistance.

Typical reciprocating pumps are:

*  [[Diaphragm Pumps]] - similar to plunger pumps, where the plunger    pressurizes hydraulic oil which is used to flex a diaphragm in the    pumping cylinder.  

==== Various Positive Displacement Pumps ====

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

* [[Progressive Cavity Pumps]]

* [[Diaphragm Pumps]]