The term "suction mode" refers to a suction state with negative inlet pressure at which the pumped medium is supplied from a point located below the pump's suction port and which is in contact with the atmosphere (open system). This means that the pressure applied at the pump's suction port is less than or equal to atmospheric pressure. Centrifugal pumps always require sufficient inlet pressure in order to ensure that pump output is in accordance with the performance curve.
The available NPSH is crucial to proper operation. This is a system parameter that must be greater than the pump's required NPSH value plus a safety margin of at least 0.5 m:
NPSHA > NPSHR + 0.5 m safety margin