Rotary Positive Displacement Pump Guide

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Rotary Positive Displacement Pumps are generally used for the following:

  • - Accurate & pulseless flow at high differential pressures
  • - Fluids too viscous for centrifugal pumps
  • - Higher differential pressures than centrifugal pumps
  • - High mechanical efficiency across different pressures and varying flow
  • - Efficient suction lift applications
  • - Operating at different points on their curves

They are general categorised into 4 categories:

  • - Internal Gear
  • - External Gear
  • - Vane
  • - Rotary Lobe

Internal Gear Pumps:

  • - Useful for fluids ranging from 1 – 1,000,000 cPs
  • - Temperatures up to 400˚C
  • - Non pulsing flow
  • - Self priming
  • - Short run dry capability
  • - Bi-directional
  • - 2 moving parts – relatively simple and easy to maintain
internal gear pump operation
  • - Liquid enters the suction port between the rotor and idler teeth.
  • - Liquid travels through the pump between the teeth of the "gear-within-a-gear" principle. The crescent shape divides the liquid and acts as a seal between the suction and discharge ports.
  • - Intermeshing of gears form locked pockets for the liquid which assures volume control.
  • - This seal forces the liquid out of the discharge port.

Advantages:

  • - Only two moving parts
  • - Only one stuffing box
  • - Non-pulsating discharge
  • - Excellent for high-viscosity liquids
  • - Constant and even discharge regardless of pressure conditions
  • - Operates well in either direction
  • - Can be made to operate with one direction of flow with either rotation
  • - Low NPSH required
  • - Single adjustable end clearance
  • - Easy to maintain
  • - Flexible design offers application customization

Disadvantages:

  • - Usually require moderate speeds
  • - Medium pressure limitations
  • - One bearing runs in the product pumped
  • - Overhung load on shaft bearing

Applications:Common internal gear pump applications include, but are not limited to:

  • - Lubricating fluids
  • - All varieties of fuel oil and lube oil
  • - Resins and Polymers
  • - Alcohols and solvents
  • - Asphalt, Bitumen, and Tar
  • - Polyurethane foam (Isocyanate and polyol)
  • - Food products such as corn syrup, chocolate, and peanut butter
  • - Paint, inks, and pigments
  • - Soaps and surfactants
  • - Glycol

External Gear Pumps

  • - Shaft support on both sides of the gears – robust & reliable
  • - Pressures over to 200 bar
  • - Wide speed range, up to 3450 rpm
  • - Often used as lubrication pumps in machine tools, in fluid power transfer units, and as oil pumps in engines.
  • - Not well suited to handling abrasive or extreme high temperature applications.
  • - Precise transfer and metering applications involving polymers, fuels, and chemical additives.

Operation:

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Similar to Internal Gear. Fluid however passed around the outside of the gears, not between.

Advantages:

  • - High speed
  • - High pressure
  • - No overhung bearing loads
  • - Relatively quiet operation
  • - Design accommodates wide variety of materials

Disadvantages:

  • - Four bushings in liquid area
  • - No solids allowed
  • - Not good with volatile fluids
  • - Need to be constantly flooded and lubricated

Applications: Common external gear pump applications include, but not limited to:

  • - Various fuel oils and lube oils
  • - Chemical additive and polymer metering
  • - Chemical mixing and blending (double pump)
  • - Industrial and mobile hydraulic applications (log splitters, lifts, etc.)
  • - Acids and caustic (stainless steel or composite construction)
  • - Low volume transfer or application.

Lobe Pumps

  • - Used in wide variety of industries, pulp and paper, chemical, food, beverage, pharmaceutical, and biotechnology.
  • - Offer superb sanitary qualities (CIP & SIP friendly), high efficiency, reliability & corrosion resistance.
  • - Variety of lobe options including single, bi-wing (shown), tri-lobe and multi-lobe.
  • - Lobes are non-contact & have large pumping chambers – can handle large solids e.g. cherries or olives without damage.
  • - Not self-priming

Operation:

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  • - Similar to External Gear, fluid passes around the outside of the lobes.
  • - Pump shaft support bearings are located in the gearbox – pressure is limited to 30 bar generally.
  • - High-viscosity liquids require reduced speeds. Reductions of 25% of rated speed and lower are common.

Advantages:

  • - Pass medium solids
  • - No metal-to-metal contact
  • - Superior CIP/SIP capabilities
  • - Long term dry run (with lubrication to seals)
  • - Non-pulsating discharge

Disadvantages:

  • - Requires timing gears
  • - Requires two seals
  • - Reduced lift with thin liquids

Vane Pumps

  • - Excel at handling low viscosity liquids such as LP gas (propane), ammonia, solvents, alcohol, fuel oils, gasoline, and refrigerants.
  • - No internal metal-to-metal contact and self-compensate for wear, enabling them to maintain peak performance.
  • - Though efficiency drops quickly, they can be used up to 500 cPs
  • - Various vane configurations: sliding vane (shown), flexible vane, swinging vane, rolling vane & external vane.
  • - Dry priming
  • - Temperatures up to 260˚C
  • - Differential pressures up to 15 Bar.

Operation:

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Combination of Internal & External Gear

Advantages:

  • - External vane pumps can handle large solids.
  • - Flexible vane pumps can only handle small solids but create good vacuum.
  • - Sliding vane pumps can run dry for short periods of time and handle small amounts of vapour.
  • - Handles thin liquids at relatively higher pressures
  • - Compensates for wear through vane extension
  • - Sometimes preferred for solvents, LPG
  • - Can run dry for short periods
  • - Develops good vacuum

Disadvantages:

  • - Can have two stuffing boxes & have complex housings and many parts
  • - Not suitable for high pressures
  • - Not suitable for high viscosity
  • - Not good with abrasive fluids

Comparing the 4 technologies

AbrasivesThin LiquidsViscousSolidsDry PrimeDiff. Pressure
Internal GearGGEPAG
External GearPGGPAE
LobeGAEEAG
VanePEAPGA

  • - E = Excellent
  • - G = Good
  • - A = Average
  • - P = Poor

Rotary Positive Displacement Pumps vs Centrifugal

Rotary PD PumpsCentrifugal Pumps
Max Viscosity, cSt1,320,000550
Max Capacity, m³/hr75040’000 +
EfficiencyEA
Energy CostsEA
Self-PrimingYesNo
Flow ControlEP
Life Cycle CostsGG
Initial CostsAE

  • - E = Excellent
  • - G = Good
  • - A = Average
  • - P = Poor
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