What is Dry Running and How to Protect a Borehole Pump Against It?

Submersible borehole pumps have high performance and are designed for continuous pumping of clean water from boreholes, wells, and collection tanks. A necessary condition for their proper functioning is that the borehole pumps remain submerged below the dynamic water level throughout their operation. Submersible pumps draw in water, which they then push to the point of use. The flow of surrounding water also ensures the cooling of the pump and prevents the motor from overheating.

However, if there is not enough clean water in the borehole, the deep well pump continues to run but without a sufficient amount of liquid. This condition is called dry running and can lead to the complete destruction of the pump.

In the event of a water shortage, the following undesirable effects may occur:

• The pump draws contaminated groundwater, which may contain an excessive amount of solid particles. Borehole pumps are designed for pumping clean water: a higher content of abrasive particles and sandy material in the water increases pump wear, reduces efficiency, and significantly shortens the lifespan of the borehole pump. Contaminated water is further pushed to the points of use, but it may also be hazardous to health. The water borehole must be cleaned for repeated water pumping by a submersible pump.

• The submersible water pump draws in air or another gas from the surrounding soil. Bubbles form in the water, which significantly reduces the pump's performance, and their presence can lead to damage and complete destruction of the pump.

• There is insufficient cooling of the submersible borehole pump. The flow of water around the borehole pump ensures its cooling. If there is no water in the source, the pump cannot cool itself. There is a sharp increase in the temperature of the submersible pump, which can lead to critical bearing wear and burning and complete destruction of the motor. 

How to Protect a Submersible Pump Against Dry Running?  

Damage to a borehole pump due to dry running is one of the most common causes of pump failures, yet the investment in proper protection is negligible compared to the cost of a new pump. There are several technologies that ensure your pump turns off in time in case of low water levels. Borehole pumps have a specific design, and therefore not all methods of protection against dry running can be used in this case.

In general, protection against dry running for a submersible pump can be addressed in several ways, but when choosing a specific method, it is necessary to consider the conditions of the borehole where the pump protection will be installed.

So how can you ensure protection against dry running?

• Purchase a submersible borehole pump that already has built-in protection against dry running.

• Install level monitoring probes in the water borehole/well.

• Mount switches on the submersible pump.

Borehole Pumps with Built-in Dry Running Protection

Submersible pumps with a frequency converter, such as Grundfos SQ/SQE borehole pumps, have built-in protection against dry running.

Submersible borehole pumps monitor pressure using a control unit. If the water drops below the suction level, the pressure drops sharply. The frequency converter immediately detects the change and stops the motor before it can be damaged. Grundfos SQ/SQE borehole pumps then automatically restart after five minutes, but if there is still not enough water, they shut off again within five seconds to prevent motor overheating. The entire process repeats until there is a sufficient amount of water in the source.

Switches for Submersible Pumps

Switching units for pump protection against dry running are mounted directly on the submersible pump. This is an effective form of protection for standalone pumps that are not controlled by a control unit.

Float switches offer a simple solution for monitoring water levels. The switch is connected to the pump in the basic position, and when the level drops, it flips, turning off the pump. When the level rises again, the pump starts. The disadvantage of float switches is that they require sufficient space around the pump for the float to move. In the case of narrow boreholes, their installation is not possible.

Combined switches monitor sufficient pressure and flow of the submersible pump. When a tap is opened (water is drawn), the pressure drops, and the combined switch responds by turning on the pump. Once water drawing is stopped, the switch detects the cessation of flow and pressure increase and turns off the pump. If there is not enough water in the borehole, the pump will not start, as the pressure and flow are insufficient for pumping.