When is a centrifugal water pump considered a lift pump?

A centrifugal water pump is considered a lift pump specifically when the suction inlet of the water source is below the pump level. This configuration necessitates the pump to lift water from a lower elevation to a higher one, which aligns with the definition of a lift pump. In this situation, the pump utilizes centrifugal force to draw the fluid upwards, overcoming the gravitational potential energy of the water.

The other options do not define a lift pump adequately. A pump with no moving parts would not be able to perform the function of lifting water, as it requires mechanical action to create flow. Operating at a high speed does not necessarily relate to whether a pump is classified as a lift pump; speed can affect performance and efficiency but is not a defining characteristic. Additionally, being submerged in water typically applies to submersible pumps, which operate differently and may not directly relate to a centrifugal pump's designation as a lift pump. Thus, the context of the pump's position relative to the water source is the key factor in categorizing it as a lift pump.