In what situation might a pump need to be primed?

A pump needs to be primed when there is air in the suction line because air can create a barrier that prevents the proper flow of fluid. Pumps rely on a continuous column of liquid to function effectively; they need to establish this column to create the necessary pressure differential to draw fluid into the pump. If air is present in the suction line, it can lead to a loss of prime, meaning that the pump cannot generate suction to move the liquid.

In scenarios where the pump is submerged, it is typically already filled with fluid, eliminating the need for priming. Starting a pump for the first time may require specific procedures, but the necessity for priming mainly arises from air-related issues, not the newness of the pump. Lastly, if the discharge line is closed, it would not necessarily cause an air problem in the suction line, but it would prevent flow through the system. Thus, the presence of air in the suction line is the critical reason indicating that priming is essential.