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Rotary Positive Displacement Pump Guide

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    Rotary Positive Displacement Pumps are generally categorised into four categories: Internal Gear, External Gear, Vane and Rotary Lobe.

    Rotary Positive Displacement Pumps

    Application Requirements for a Rotary Positive Displacement Pump

    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

    Internal Gear PumpsGear Pump Operating diagram

    • 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 Operating Principle

    Liquid enters the suction port between the rotor and idler teeth. The liquid then 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. The intermeshing of gears form locked pockets for the liquid which assures volume control. This seal forces the liquid out of the discharge port.

    Advantages & Disadvantages of Internal Gear Pumps

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

    Applications for Internal Gear Pumps

    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 extremely high-temperature applications.
    • Precise transfer and metering applications involving polymers, fuels, and chemical additives.

    External Gear Pump Operating Principle

    External Gear Pumps operate similarly to Internal Gear Pumps as they only have two moving components. The medium is driven through the pump by filling the cavities between the teeth of both gears inside the pump head. The fluid is then carried around the external areas of the gears until the casing is completely filled with the fluid and is discharged out of the pump outlet under pressure. No fluid passes between the gears.

    Advantages & Disadvantages of External Gear Pumps

    Advantages

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

    Disadvantages

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

    Applications for External Gear Pumps

    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 a 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

    Lobe Pump Operating PrincipleOperating Principle of a Lobe Pump

    Inside a Lobe Pump, there are two lobes which counter-rotate without touching each other to create suction. As the lobes rotate past the suction port, the suction increases the volume of products between the lobes which draws fluid into the pump casing. The lobes drive a predetermined amount of fluid through the pump and out through the discharge port.

    The pump shaft support bearings are located in the gearbox and the pressure is generally limited to 30 Bar.

    When operating with high viscosity fluids, the speed of the pump will need to be reduced, this is typically reduced by 25% of the rated speed or lower.

    Advantages & Disadvantages of a Lobe Pump

    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.Vane Pump Operating Principle

    Vane Pump Operating Principle

    Vanes are mounted inside the pump’s casing into slots on the rotor. As the rotor turns, the vanes move in and out of their slots, as they do, they make contact with the wall of the pump which traps fluids between the vane and the rotor. The rotor continues turning and as each area of trapped liquid rotates past the discharge port, the fluid is forced out.

    Advantages & Disadvantages of a Vane Pump

    Advantages

    • External vane pumps can handle large solids.
    • Flexible vane pumps can only handle small solids but create a 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 Technologies

    Comparing the four rotary technologies:

    E = Excellent, G = Good, A = Average, P = Poor

    Abrasives Thin Liquids Viscous Solids Dry Prime Different Pressure
    Internal Gear G G E P A G
    External Gear P G G P A E
    Lobe G A E E A G
    Vane P E A P G A

    Rotary Positive Displacement Pumps Vs Centrifugal Pumps

    E = Excellent, G = Good, A = Average, P = Poor

    Rotary Positive Displacement Pumps Centrifugal Pump
    Max Viscosity, cSt 1,320,000 550
    Max Capacity, m³/hr 750 40’000 +
    Efficiency E A
    Energy Costs E A
    Self-Priming Yes No
    Flow Control E P
    Life Cycle Costs G G
    Initial Costs A E
    in Pump FAQs
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