When does suction head exist in a pumping system?

Suction head in a pumping system occurs when the source of supply is above the centerline of the pump. When water is positioned above the pump, the force of gravity assists in drawing the liquid into the pump. This gravitational pull creates a positive suction head, which enhances the pump's ability to move water effectively.

In this scenario, the pump can easily initiate flow due to the pressure differential created by the elevation difference between the supply source and the pump itself. A higher position of the water source relative to the pump helps ensure that there is enough hydrostatic head to facilitate the flow into the pump, which is critical for the efficient operation of any pumping system.

When examining the other options, being underwater or having the supply source below the pump would not provide the necessary suction head because the pump would be fighting against the weight of the water column, leading to difficulty in achieving adequate flow. Also, while reduced pressure in the pump can influence other aspects of the operation, it does not directly create suction head, as suction head is fundamentally related to the elevation of the water source.