Positive Displacement Pumps

Internal Gear Pumps

(with crescent)

·          Only two moving parts

·          Only one stuffing box

·          Positive suction, non-pulsating discharge

·          Ideal for high viscosity liquids

·          Constant and even discharge regardless of various pressure conditions

·          Available with built in safety relief valve

·          Can operate equally well in either direction

·          Can be made to operate with one direction of flow with either rotation

·          Low NPSH required

·          Easy to maintain

·          Low speeds usually required

·          Medium pressure limitations

·          One bearing runs in product pumped

·          Overhung load on shaft bearing

External Gear Pumps

(Spur, helical, herringbone)

·          High Speed

·          Medium pressure

·          No overhung bearing loads

·          Relatively quiet

·          Design lends itself to use of a wide variety of materials

·          Four bushings in liquid area

·          Four stuffing boxes

·          No solids allowed

Multiple Screw Pumps

(Two or three)

·          High speed

·          High pressures

·          Quiet

·          Sometimes considered as an “industry standard” fuel pump

·          Good efficiency

·          Medium high capacity

·          No solids allowed

·          Expensive

·          Not good for very thin liquids

·          Two-screw pumps require timing gears

Single Screw Pumps

(Progressive cavity)

·          High solids allowed

·          Dry materials

·          High viscosity

·          Versatile (Same pump without changing clearance can be used with such liquids as water, wine and syrup)

·          Good for sludge or slurry

·          Cannot run dry

·          Not for high temperatures (Stator swells)

·          Hard to maintain

·          Expensive parts

·          More space required

·          Stuffing box exposed to vacuum

Sliding Vane Pumps

(Unbalanced)

·          Medium capacity

·          Medium speed

·          Thin liquids

·          Sometimes preferred for solvents, LPG

·          Can be made to provide variable capacity

·          Can run dry for short periods

·          Most have two stuffing boxes

·          Not best for viscous liquids

·          Pressure medium

Sliding Vane Pumps

(Balanced)

·          Medium capacity

·          Medium speed

·          Thin liquids

·          Balanced radial bearing loads

·          One seal or stuffing box

·          Can run dry for short periods

·          Develops good vacuum

·          Complex casing

·          Not suitable for high viscosity

·          Not good with abrasives

Flexible Vane Pumps

(Flexible fingers on vanes)

·          Handle some solids

·          High vacuum

·          Relatively inexpensive

·          Low discharge pressure

·          Cannot run dry

·          Low temperatures only

·          Liquid must be compatible with elastomer in rotor

Internal Gear Pumps

(With crescent)

·          Easiest to clean

·          Good on high viscosity

·          Better suction than lobe type

·          No timing gears required

·          Smaller space

·          Can run dry for short periods

·          Can be repaired in field

·          Will crush solids

·          Suction not as good as flexible vane

·          Bushing in product

Lobe Pumps

(Single, multiple and circumferential piston)

·          Pass medium solids

·          Rates high acceptance

·          Little galling possibility

·          Timing gears required

·          More space required

·          Factory service required

·          Requires two seals

External Vane Pumps

·          Pass large solids

·          Little galling possibility

·          Simple

·          Expensive

·          Low rate of field acceptance

·          Low speed

Flexible Vane Pumps

(Flexible fingers on vanes)

·          Pass medium solids

·          High vacuum

·          Relatively inexpensive

·          Easy field maintenance

·          Low discharge pressure

·          Cannot run dry

·          Low temperature only