Some reasons for a pump running without water, often referred to as "dry running," are the pump's discharge becoming blocked to flow and the pump's suction being blocked or starved of flow. Poor operation practices are the main reasons why centrifugal pumps run without water.
Operating practices that result in a pump operating dry include failing to vent the pump before startup, emptying the tank completely for introduction of a new product, and running a tank dry at the end of an operation cycle. These practices deplete the pump of its suction liquid, forcing it to operate without lubrication or a way of cooling its internal bearings.
Sometimes pumps run without water when the control rods for the float switch are faulty and give a false reading to the operator. The operator may think the liquid level is adequate, when in fact the tank is almost empty.