The external gear pump belongs to the class of rotating positive displacement pumps. They can manage low viscosity liquids like alcohols, solvents or liquid gases as well as medium and high viscosity liquids like polymer melts, gum base or rubber. Liquids containing solids are not really suitable for gear pumps unless the particles are really small.
A gear pump comprises a housing with two covers. The powered gear wheel and driven gear wheel are borne in four friction bearings. The protruding drive shaft is sealed. The meshing gear wheels are enclosed by the housing. The amount of clearances between the tips of the teeth and the housing is extremely small and precisely defined There are ports in the housing. One port is at the suction side (inlet) and the other at the pressure side of the pump (discharge). One gear wheel is normally driven by a motor connected to the drive shaft, which protrudes from the housing. When the gear wheels are rotating, a chamber, which is filled with the medium being pumped, is formed between two teeth and the housing. The medium is thus conveyed from the suction side to the pressure side of the pump. The medium being pumped is displaced from between the gap between the teeth at the point at which the teeth mesh. For this reason, the gear pump is known as a displacement pump.

 

 

External gear pumps are self-priming and can dry-lift although their priming characteristics improve if the gears are wetted. The gears need to be lubricated by the pumped fluid and should not be run dry for prolonged periods. Some gear pump designs can be run in either direction so the same pump can be used to load and unload a vessel, for example.
The close tolerances between the gears and casing mean that these types of pump are susceptible to wear particularly when used with abrasive fluids or feeds containing entrained solids. External gear pumps have four bearings in the pumped medium, and tight tolerances, so are less suited to handling abrasive fluids. For these applications, internal gear pumps are more robust having only one bearing (sometimes two) running in the fluid. A gear pump should always have a strainer installed on the suction side to protect it from large, potentially damaging, solids.
Generally, if the pump is expected to handle abrasive solids it is advisable to select a pump with a higher capacity so it can be operated at lower speeds to reduce wear. However, it should be borne in mind that the volumetric efficiency of a gear pump is reduced at lower speeds and flow rates. A gear pump should not be operated too far from its recommended speed.
For high temperature applications, it is important to ensure that the operating temperature range is compatible with the pump specification. Thermal expansion of the casing and gears reduces clearances within a pump and this can also lead to increased wear, and in extreme cases, pump failure.
Despite the best precautions, gear pumps generally succumb to wear of the gears, casing and bearings over time. As clearances increase, there is a gradual reduction in efficiency and increase in flow slip: leakage of the pumped fluid from the discharge back to the suction side. Flow slip is proportional to the cube of the clearances between the cog teeth and casing so, in practice, wear has a small effect until a critical point is reached, from which performance degrades rapidly.
Gear pumps continue to pump against a back pressure and, if subjected to a downstream blockage will continue to pressurize the system until the pump, pipework or other equipment fails. Although most gear pumps are equipped with relief valves for this reason, it is always advisable to fit relief valves elsewhere in the system to protect downstream equipment.
The high speeds and tight clearances of external gear pumps make them unsuitable for shear-sensitive liquids such as foodstuffs, paint and soaps. Internal gear pumps, operating at lower speed, are generally preferred for these applications.