Internal Gear Pumps: What Are The Limitations?
The Internal Gear Pumps is self-priming and can dry up, although it will improve its oil absorption characteristics if the gear is wet. Gears need to be lubricated by the pumped liquid and should not idle for long periods of time. Some gear pump designs can operate in either direction, so, for example, the same pump can be used to load and unload containers.
The tight tolerances between gears and housings mean that these types of pumps are prone to wear, especially when used with abrasive liquids or feeds containing entrained solids. External gear pumps have four bearings in the medium being pumped and tight tolerances, so they are not as good as internal gear designs for handling abrasive fluids. For these applications, internal gear pumps are more robust, with only one bearing (sometimes two) running in the fluid. Gear pumps should always have a filter on the suction side to prevent them from accumulating large amounts of solids that may be damaged.
Generally, if you want the pump to be able to handle abrasive solids, it is recommended to choose a pump with a larger capacity so that it can run at a lower speed to reduce wear. However, it should be kept in mind that the volumetric efficiency of a gear pump decreases at lower speeds and flows. Gear pumps should not run faster than the recommended speed.
For high temperature applications, it is important to ensure that the operating temperature range is compatible with the specifications of the pump. Thermal expansion of the casing and gear reduces the clearance in the pump, which can also lead to increased wear and, in extreme cases, pump failure.
Despite the best precautions, gear pumps often wear out gears, housings and bearings over time. As the gap increases, the efficiency gradually decreases and the slip in the flow direction increases: the pumped fluid leaks from the discharge port back to the suction side. The flow slip is proportional to the cube of the gap between the gear teeth and the sleeve, so in practice, the effect of wear is small until it reaches a critical point, from which the performance drops rapidly.
Gear pumps continue to withstand back pressure, and if blocked downstream, they will continue to pressurize the system until the pump, piping, or other equipment fails. Although most gear pumps are equipped with safety valves for this reason, it is always recommended to install safety valves elsewhere in the system to protect downstream equipment.
The lower speeds, larger clearances, and larger internal volumes of internal gear pumps make them more suitable for shear-sensitive liquids than external gear designs, such as food, paint, and soap. When hygiene is important, internal gear pumps are also preferred because of their mechanical simplicity and the fact that they are easy to disassemble, clean and reassemble.
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