Part 1 - Dry-run protection
The term dry-run, strictly speaking, is when the pump is being operated with no liquid within the pump casing. The result is rapid overheating of the pump interior caused by friction between stationary and moving parts. Dry-running almost always results in pump failure if not detected and remedied quickly. Similar features of overheating can also happen when the pump is insufficiently cooled, such as shut-off valve, improper priming, air entering the pump from the suction system, vaporizing of the chemical under severe cavitation, etc. While these causes are not technically dry-running, it is often described as having the features of dry-running.
The common observable feature of dry-running and other failure modes that result in the pump over-heating is either no flow output or significantly reduced output. From this reasoning, we can easily detect such operating conditions by monitoring the output flow capacity of the pump by sending digital flow meter readings to the control station or the PLC.
In many industries, digital flow meters are not commonly installed due to their relatively high costs. However, since the flow output (loading) of the pump is low, the amount of energy required to rotate the pump is also reduced. We can use this feature to indirectly detect potential dry-running and similar failure modes by monitoring the loading of the motor. This is commonly done by measuring the current being drawn by the motor, which is how most dry-run detection devices on the market works. We measure the loading (current) of the motor during normal operation and we set a lower current value as the lower limit to trigger a warning or the immediate shut-off of the pump.
Part 2 - Pump cooling and minimum flow
In the previous section, we know that the dry-run and dry-run-like failure of the pump is due to insufficient cooling of the pump. In a centrifugal pump, the cooling of the pump is either done by the pumped liquid itself, or in the case of some mechanically sealed pumps, by applying external cooling to the mechanical seal. If the cooling is done completely by the pumped liquid, such as in sealless pumps such as magnetic drive pumps and canned motor pumps, then a minimum flow must be maintained to prevent overheating. The recommended minimum flow value is usually shown when making the pump selection. Please note that this minimum flow value is usually derived based on tests conducted using water at ambient temperature. If your liquid medium has poor thermal conductivity, or you are pumping the liquid at elevated temperatures, or you are running the pump at higher speeds using a VFD, then the required minimum flow will likely be higher. Please consult with the pump manufacturer or their authorized representatives for such applications.
Part 3 - Overload and decoupling protection
While dry-running and dry-run-like symptoms can be detected with a drop in motor loading (current), other pump failure modes can result in increased motor loading. A common cause for increased loading could be abnormally low temperatures in the winter, increasing the S.G. and viscosity of the chemical.
Other causes could mean the pump is already damaged and should be stopped immediately to prevent further damage. For magnetic drive pumps, increases in loading could mean that there are factors obstructing the rotation of the impeller. Examples could range from crystallization of the chemical, chemical coating of the chemical, excessive wear/roughening of the rotating parts, abnormal contact between rotating parts with the pump structure, etc. The step to check for obstruction on a pump with a fan-cooled induction motor is to shut down the pump and use a thin wire to manually rotate the cooling fan. If the fan cannot be rotated or it takes great effort to rotate the fan, there is excessive contact or friction within the pump. Do not continue to run the pump and schedule for a service agent to dismantle and check the pump. Canned motor pumps may not have an external cooling fan and are generally not customer serviceable. If your canned motor pump is experiencing increased loading, please contact the manufacturer or its service agents for assistance.
For magnetic drive pumps, rotational energy from the motor is transmitted to the pump via a magnetic coupling. Since the magnetic couplings are not in physical contact, if the rotor of the pump (impeller and inner magnet) is jammed and could not move, it is possible that the external (drive-side) magnet could still rotate. This phenomenon is called decoupling. In addition to the pump rotor being rendered immobile, other factors could also lead to decoupling.
Operating the pump at the extremes of the pump curve. At the extremes, either at low flow or at very high flow, the starting torque could exceed the designed safety margin of the magnetic coupling. Once the magnets decouple, they cannot recouple during operation due to the high rotation speeds. In such a scenario, it is advisable to perform a soft start of the pump either through a VFD or using a soft starter.
For chemicals with increased viscosity during winter, it is possible to restart the pump with a shut-off discharge valve. This lowers the loading of the motor, allowing the pump to start moving and overcome the inertia. Once the pump is up to speed, slowly open the discharge valve to gradually increase the load of the pump. In this case, make sure not to overload the motor.