Difference between revisions of "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 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 positive displacement pump must not be operated against a closed valve on the discharge side of the pump, because it has no shut-off head like centrifugal pumps. A positive displacement pump operating against a closed discharge valve will continue to produce flow and the pressure in the discharge line will increase, until the line bursts or the pump is severely damaged, or both.

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 rotary pumps are pumps that move powder using the   principles of rotation. The vacuum created by the rotation of the pump   captures and draws in the powders.  

'''Advantages:'''   Rotary pumps are very efficient because they naturally remove air from   the lines, eliminating the need to bleed the air from the lines   manually.

 

'''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-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.

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

* [[Plunger Pumps]] - a reciprocating plunger pushes the powder 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.