GEAR PUMP

GEAR PUMP

A gear pump uses the meshing of gears to pump fluid by displacement.[1] They are one of the most common types of pumps for hydraulic fluid power applications.
Gear pumps are also widely used in chemical installations to pump fluid with a certain viscosity. There are two main variations; external gear pumps which use two external spur gears, andinternal gear pumps which use an external and an internal spur gear. Gear pumps are positive displacement (or fixed displacement), meaning they pump a constant amount of fluid for each revolution. Some gear pumps are designed to function as either a motor or a pump.

Contents

  [hide] 
  • 1 Theory of operation
  • 3 References
  • 4 External links

Theory of operation[edit]

As the gears rotate they separate on the intake side of the pump, creating a void and suction which is filled by fluid. The fluid is carried by the gears to the discharge side of the pump, where the meshing of the gears displaces the fluid. The mechanical clearances are small— in the order of 10 μm. The tight clearances, along with the speed of rotation, effectively prevent the fluid from leaking backwards.
The rigid design of the gears and houses allow for very high pressures and the ability to pump highly viscous fluids.
Many variations exist, including; helical and herringbone gear sets (instead of spur gears), lobe shaped rotors similar to Roots blowers (commonly used as superchargers), and mechanical designs that allow the stacking of pumps. The most common variations are shown below (the drive gear is shown blue and the idler is shown purple).
Suction and pressure ports need to interface where the gears mesh (shown as dim gray lines in the internal pump images). Some internal gear pumps have an additional, crescent-shaped seal (shown above, right).
Pump formulas:
  • Flow rate in US gal/min = Fluid Density X Pump Capacity X rpm
  • Power in hp = US gal/min X (lbf/in³)/1714
Generally used in:
  • Petrochemicals: Pure or filled bitumen, pitch, diesel oil, crude oil, lube oil etc.
  • Chemicals: Sodium silicate, acids, plastics, mixed chemicals, isocyanates etc.
  • Paint and ink.
  • Resins and adhesives.
  • Pulp and paper: acid, soap, lye, black liquor, kaolin, lime, latex, sludge etc.
  • Food: Chocolate, cacao butter, fillers, sugar, vegetable fats and oils, molasses, animal food etc



External Gear Pumps

  • External Gear Pump Overview
  • How External Gear Pumps Work
  • Advantages & Disadvantages
  • Applications
  • Materials Of Construction / Configuration Options
  • Manufacturers

External Gear Pump Overview
External gear pump.External gear pumps are a popular pumping principle and are often used as lubrication pumps in machine tools, in fluid power transfer units, and as oil pumps in engines.
External gear pumps can come in single or double (two sets of gears) pump configurations with spur (shown), helical, and herringbone gears.  Helical and herringbone gears typically offer a smoother flow than spur gears, although all gear types are relatively smooth.  Large-capacity external gear pumps typically use helical or herringbone gears. Small external gear pumps usually operate at 1750 or 3450 rpm and larger models operate at speeds up to 640 rpm. External gear pumps have close tolerances and shaft support on both sides of the gears.  This allows them to run to pressures beyond 3,000 PSI / 200 BAR, making them well suited for use in hydraulics.  With four bearings in the liquid and tight tolerances, they are not well suited to handling abrasive or extreme high temperature applications.
Tighter internal clearances provide for a more reliable measure of liquid passing through a pump and for greater flow control. Because of this, external gear pumps are popular for precise transfer and metering applications involving polymers, fuels, and chemical additives.

How External Gear Pumps Work
Fluid traveling through pump.External gear pumps are similar in pumping action to internal gear pumps in that two gears come into and out of mesh to produce flow.  However, the external gear pump uses two identical gears rotating against each other -- one gear is driven by a motor and it in turn drives the other gear.  Each gear is supported by a shaft with bearings on both sides of the gear.
1.  As the gears come out of mesh, they create expanding volume on the inlet side of the pump. Liquid flows into the cavity and is trapped by the gear teeth as they rotate.
2.  Liquid travels around the interior of the casing in the pockets between the teeth and the casing -- it does not pass between the gears. 
3.  Finally, the meshing of the gears forces liquid through the outlet port under pressure.
Because the gears are supported on both sides, external gear pumps are quiet-running and are routinely used for high-pressure applications such as hydraulic applications. With no overhung bearing loads, the rotor shaft can't deflect and cause premature wear.

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
  • Fixed End Clearances

Applications
Common external gear pump applications include, but are 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

Materials Of Construction / Configuration Options
Composite external gear pump.
A composite external gear pump performs well in corrosive liquid applications.
As the following list indicates, rotary pumps can be constructed in a wide variety of materials. By precisely matching the materials of construction with the liquid, superior life cycle performance will result.
External gear pumps in particular can be engineered to handle even the most aggressive corrosive liquids.  While external gear pumps are commonly found in cast iron, newer materials are allowing these pumps to handle liquids such as sulfuric acid, sodium hypochlorite, ferric chloride, sodium hydroxide, and hundreds of other corrosive liquids.
  • Externals (head, casing, bracket) - Iron, ductile iron, steel, stainless steel, high alloys, composites (PPS, ETFE)
  • Internals (shafts) - Steel, stainless steel, high alloys, alumina ceramic
  • Internals (gears) - Steel, stainless steel, PTFE, composite (PPS)
  • Bushing - Carbon, bronze, silicon carbide, needle bearings
  • Shaft Seal - Packing, lip seal, component mechanical seal, magnetically-driven pump

Internal Gear Pumps

  • Internal Gear Pump Overview
  • How Internal Gear Pumps Work
  • Advantages & Disadvantages
  • Applications
  • Materials Of Construction / Configuration Options
  • Manufacturers

Internal Gear Pump Overview
Internal gear pump.Internal gear pumps are exceptionally versatile.  While they are often used on thin liquids such as solvents and fuel oil, they excel at efficiently pumping thick liquids such as asphalt, chocolate, and adhesives.  The useful viscosity range of an internal gear pump is from 1cPs to over 1,000,000cP.
In addition to their wide viscosity range, the pump has a wide temperature range as well, handling liquids up to 750�F / 400�C.  This is due to the single point of end clearance (the distance between the ends of the rotor gear teeth and the head of the pump).  This clearance is adjustable to accommodate high temperature, maximize efficiency for handling high viscosity liquids, and to accommodate for wear.
Rotor and idler.The internal gear pump is non-pulsing, self-priming, and can run dry for short periods.  They're also bi-rotational, meaning that the same pump can be used to load and unload vessels.  Because internal gear pumps have only two moving parts, they are reliable, simple to operate, and easy to maintain.

 How Internal Gear Pumps Work
Fluid passing through the pump.1. Liquid enters the suction port between the rotor (large exterior gear) and idler (small interior gear) teeth. The arrows indicate the direction of the pump and liquid.
2. 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.
3. The pump head is now nearly flooded, just prior to forcing the liquid out of the discharge port.  Intermeshing gears of the idler and rotor form locked pockets for the liquid which assures volume control.
4. Rotor and idler teeth mesh completely to form a seal equidistant from the discharge and suction ports. 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 requires 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:
  • 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

Materials Of Construction / Configuration Options
  • Externals (head, casing, bracket) - Cast iron, ductile iron, steel, stainless steel, Alloy 20, and higher alloys.
  • Internals (rotor, idler) - Cast iron, ductile iron, steel, stainless steel, Alloy 20, and higher alloys.
  • Bushing - Carbon graphite, bronze, silicon carbide, tungsten carbide, ceramic, colomony, and other specials materials as needed.
  • Shaft Seal - Lip seals, component mechanical seals, industry-standard cartridge mechanical seals, gas barrier seals, magnetically-driven pumps.
  • Packing - Impregnated packing, if seal not required.

Manufacturers
  • Viking Pump Inc.
  • Viking Pump (Europe) Ltd.

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