positive displacement pump

 

What is a positive displacement pump?
Positive-displacement pumps are another category of pumps. Types of positive-displacement pumps are reciprocating, metering, and rotary pumps. Positive-displacement pumps operate by forcing a fixed volume of fluid from the inlet pressure section of the pump into the discharge zone of the pump. These pumps generally tend to be larger than equal-capacity dynamic pumps. Positive-displacement pumps frequently are used in hydraulic systems at pressures ranging up to 5000 psi. A principal advantage of hydraulic power is the high power density (power per unit weight) that can be achieved. They also provide a fixed displacement per revolution and, within mechanical limitations, infinite pressure to move fluids.
Description of positive displacement pump:
Positive Displacement Pumps has 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.
Different positive displacement pumps:
·                   rotary lobe pump
·                   progressing cavity pump
·                   rotary gear pump
·                   piston pump
·                   diaphragm pump
·                   screw pump
·                   gear pump
·                   vane pump
·                   regenerative (peripheral) pump
·                   peristaltic
Positive Displacement Pumps, unlike a Centrifugal or Roto-dynamic Pumps, will produce the same flow at a given speed (RPM) no matter the discharge pressure.
·                   Positive Displacement Pumps are "constant flow machines"
Why Positive Displacement over Centrifugal?
  • If liquid viscosity is too high
  • Constant flow is needed over varying differential pressure
  • Suction lift or high vapor pressure liquid involved
Common Application Parameters for Positive Displacement Pumps
  • Most require some modification to run at higher viscosity
  • Reduced speed
  • Increased clearances
  • Larger ports
  • Most require some modification to run at higher temperatures
  • Change in materials
  • Increased clearances
  • Most need to be slowed down with shear sensitive liquids
  • All needs overpressure protection
Applying Positive Displacement Pumps
A successful application results when the following are known and understood:
  • Characteristics of the liquid pumped
  • Piping and system design
  • Pump design

 

Positive Displacement Pump Design
  • Individual pump principal and design is important in applying a pump
  • Understanding a pump principal and design helps in determining application limits
 
Precaution needed while using positive displacement pump:
        
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 until the pressure in the discharge line are increased 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 absolute necessary. The relief valve can be internal or external. The pump manufacturer has normally 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.
 
 


Types of positive displacement pump:
Reciprocating Pumps
In a reciprocating pump, a volume of liquid is drawn into the cylinder through the suction valve on the intake stroke and is discharged under positive pressure through the outlet valves on the discharge stroke. The discharge from a reciprocating pump is pulsating and changes changes only when the speed of the pump is changed. This is because the intake is always a constant volume. Often an air chamber is connected on the discharge side of the pump to provide a more even flow by evening out the pressure surges. Reciprocating pumps are often used for sludge and slurry.
 One construction style of a reciprocating pump is the direct-acting steam pump. These consist of a steam cylinder end in line with a liquid cylinder end, with a straight rod conection between the steam piston and the pump piston or plunger. These pistons are double acting which means that each side pumps on every stroke.
 Another construction style is the power pump which convert rotary motion to low speed reciprocating motio using a speed reducing gear. The power pump can be either single or double-acting. A single-acting design discharges liquid only on one side of the piston or plunger. Only one suction and one discharge stroke per revolution of the crankshaft can occur. The double-acting design takes suction and discharges on both sides of the piston resulting in two suctions and discharges per crankshaft revolution. Power pumps are generally very efficient and can develop high pressures. These pumps do however tend to be expensive.


Metering Pumps
Metering pumps provide precision control of very low flow rates. Flow rates are generally less than 1/2 gallon per minute. They are usually used to control additives to the main flow stream. They are also called proportioning orcontrolled-volume pumps. Metering pumps are available in either a diaphragm or packed plunger style, and are designed for clean service and dirty liquid can easily clog the valves and nozzle connections.


Rotary Pumps
A rotary pump traps fluid in its closed casing and discharges a smooth flow. They can handle almost any liquid that does not contain hard and abrasive solids, including viscous liquids. They are also simple in design and efficient in handling flow conditions that are usually considered to low for economic application of centrifuges. Types of rotary pumps include cam-and-piston, internal-gear, lobular, screw, and vane pumps. Gear pumps are found in home heating systems in which the burners are fired by oil. Rotary pumps find wide use for viscous liquids. When pumping highly viscous fluids, rotary pumps must be operated at reduced speeds because at higher speeds the liquid cannot flow into the casing fast enough to fill it. Unlike a centrifugal pump, the rotary design will deliver a capacity that is not greatly affected by pressure variations on either the suction or discharge ends. In services where large changes in pressure are anticipated, the rotary design should be considered.


Moyno Pumps
Browse through the Moyno technical bulletins to see how the rotor turns inside the casing. This is called a "progressing cavity". This pump handles solids beautifully. It is said that they can pump strawberries with little damage to each berry.

 

 

 

 
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