The pump has to be installed reaching directly into the liquid with its liquid intake piece. It can not be blocked by a solid material so it should - contrary to other machines - float with the intake at least but it can float in liquid completely as well.
A liquid pump needs power and a pipe connected to a vent or intake of another machine or it won't run. If the pressure in the pipe is too high (vent is blocked by solid material or pressure in room or liquid intake of next machine is not functioning due to subsequent pipe system problems) the pump will stop working.
A pump always only works at full speed or not at all. It can be switched off by setting its mode to "disabled" but for this the pump needs to be physically reachable by the Duplicants. Otherwise switching off power to the pump with a switch achieves the same effect.