Gear pumps are self-priming and can be lifted dry, although their starting characteristics will increase if the gears are wetted. Gears need to be lubricated with pumped liquid and should not be dried 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 tolerance between the gear and the casing means that these types of pumps are prone to wear, especially when used with abrasive fluids or feeds containing entrained solids. However, some designs of gear pumps, particularly internal variants, allow for the handling of solids. The external gear pump has four bearings in the pumping medium and is tightly tolerated, making it less suitable for handling abrasive fluids. The internal gear pump is more robust, with only one bearing (sometimes two) running in the fluid. The gear pump should always be fitted with a filter on the suction side to protect it from large, potentially harmful solids.
In general, if the pump is expected to handle abrasive solids, it is recommended to select a pump with a larger capacity to be able to operate at a lower speed to reduce wear. However, it should be remembered that the volumetric efficiency of a gear pump is reduced at lower speeds and flow rates. The operating distance of the gear pump should not exceed the recommended speed.
For high temperature applications, it is important to ensure that the operating temperature range is compatible with the pump specifications. The thermal expansion of the housing and gear reduces the clearance within the pump, which also leads to increased wear and, in extreme cases, to pump failure.
Despite the best precautions, gear pumps often succumb to wear on gears, bushings and bearings over time. As the gap increases, the efficiency gradually decreases and the flow slip 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 housing, so in practice, the wear has a small effect until the critical point is reached and the performance rapidly decreases from this critical point.
The gear pump continues to pump under back pressure and will continue to pressurize the system if it is blocked by the downstream until the pump, piping or other equipment fails. Although for this reason most gear pumps are equipped with safety valves. Parker Gear Pump recommend installing safety valves elsewhere in the system to protect downstream equipment.
Internal gear pumps operating at low speeds are generally preferred for shearing sensitive liquids such as food, paint and soap. The higher speeds and lower clearances of the external gear design make them unsuitable for these applications. Internal gear pumps are also preferred when hygiene is important because they are mechanically simple and easy to disassemble, clean and reassemble.