Difference between revisions of "Positive Displacement Pumps"

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[[Category:Pumps]]{{Knoppen}}
[[Category:Pumps]]{{Knoppen}}
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[[Image:Jet_pump.jpg|thumb||right|A jet pump]]
[[Image:Jet_pump.jpg|thumb||right|A jet pump]]
 
A '''Positive Displacement Pump''' displaces a volume by physical or mechanical action.  Pumps fall into three major groups: direct lift, displacement, and gravity pumps.
A '''pump''' displaces a volume by physical or mechanical action.  Pumps fall into three major groups: direct lift, displacement, and gravity pumps.


==Types==
==Types==
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[[Image:Two_moving_spirals_scroll_pump.gif|thumb|Mechanism of a scroll pump]]
[[Image:Two_moving_spirals_scroll_pump.gif|thumb|Mechanism of a scroll pump]]


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.
A positive displacement pump causes a powder 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.
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 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.
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 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.
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 ====
==== Positive displacement types ====
[[Image:Lysholm_screw_rotors.jpg|thumb|upright|Screw pump]]
[[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:
A positive displacement pump can be further classified according to the mechanism used to move the powder:


* '''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.
* '''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 powder ring vacuum pumps.
* '''Reciprocating-type''' positive displacement: piston or diaphragm pumps.
* '''Reciprocating-type''' positive displacement: piston or diaphragm pumps.
* '''Linear-type''' positive displacement: Rope pump|rope pumps and chain pumps.
* '''Linear-type''' positive displacement: Rope pump|rope pumps and chain pumps.


===== Rotary positive displacement pumps =====
===== Rotary Positive Displacement Pumps =====
[[File:220px-Rotary_vane_pump.svg.png|thumb|Rotary vane pump]]
[[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.
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 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.
 
 
Rotary positive displacement pumps can be grouped into three main types:
* [[Gear pump]]s - a simple type of rotary pump where the liquid is pushed between two gears.
* [[Screw pump]]s - the shape of the internals of this pump usually two screws turning against each other pump the liquid.
* [[Rotary vane pump]]s - 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.


'''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.


===== Reciprocating positive displacement pumps =====
'''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 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.
===== 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.


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.
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.


Typical reciprocating pumps are:
Typical reciprocating pumps are:
* [[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 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.


 
==== Various Positive Displacement Pumps ====
==== Various positive displacement pumps ====
The positive displacement principle applies in the following types of pumps:
The positive displacement principle applies in the following types of pumps:
* [[lobe pumps]]
* [[Progressive Cavity Pumps]]
* [[Progressive cavity pumps]]
* [[Diaphragm Pumps]]
* [[gear pumps]]
* [[Screw Pumps]]
* [[piston pumps]]
* [[Peristaltic Pumps]]
* [[diaphragm pumps]]
* [[Piston Pumps]]
* [[screw pumps]]
* [[Vertical Pumps]]
* [[gear pumps]]
* [[Well Pumps]]
* [[Hydraulic pumps]]
* [[Worm Pumps]]
* [[vane pumps]]
* [[Press Pumps]]
* [[regenerative pumps]]
* [[Sump Pumps]]
* [[peristaltic pumps]]
* [[Peristaltic Pumps]]
* [[rope pumps]]
 
 
===[[Impulse 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:
* [[Hydraulic ram pumps]] - uses pressure built up internally from released gas in liquid flow. (see below)
* [[Pulser pump]]s - run with natural resources, by kinetic energy only.
* [[Airlift pump]]s - run on air inserted into pipe, pushing up the water, when bubbles move upward, or on pressure inside pipe pushing water up.
 
 
===[[Velocity pumps]]===
[[Image:Centrifugal 2.png|thumb|right|A centrifugal pump uses a spinning "impeller" which has backward-swept arms]]
 
[[Rotodynamic pumps]] (or dynamic pumps) are a type of velocity pump in which kinetic energy is added to the fluid by increasing the flow velocity. This increase in energy is converted to a gain in potential energy (pressure) when the velocity is reduced prior to or as the flow exits the pump into the discharge pipe. This conversion of kinetic energy to pressure can be explained by the First law of thermodynamics or more specifically by Bernoulli's principle.
 
Dynamic pumps can be further subdivided according to the means in which the velocity gain is achieved.
 
These types of pumps have a number of characteristics:
# Continuous energy
# Conversion of added energy to increase in kinetic energy (increase in velocity)
# Conversion of increased velocity (kinetic energy) to an increase in pressure head
 
One practical difference between dynamic and positive displacement pumps is their ability to operate under closed valve conditions. Positive displacement pumps physically displace the fluid; hence closing a valve downstream of a positive displacement pump will result in a continual build up in pressure resulting in mechanical failure of either pipeline or pump. Dynamic pumps differ in that they can be safely operated under closed valve conditions (for short periods of time).
 
 
===[[Gravity pumps]]===
 
===[[Steam pumps]]===
 
===[[Valveless pumps]]===

Latest revision as of 01:49, 23 December 2013

A jet pump

A Positive Displacement 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

A lobe pump
lobe pump internals
Mechanism of a scroll pump

A positive displacement pump causes a powder 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. 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 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 Types

Screw pump

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

  • 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 powder ring vacuum pumps.
  • Reciprocating-type positive displacement: piston or diaphragm pumps.
  • Linear-type positive displacement: Rope pump|rope pumps and chain pumps.
Rotary Positive Displacement Pumps
Rotary vane pump

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 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.

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.

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:

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