Method for controlling of a pump unit and a pump unit for pumping liquid or suspension

A method for controlling a pump unit for pumping liquid or suspension, the pump unit comprising

• a centrifugal pump, and
• an inducer in a proximity upstream of an inlet of the centrifugal pump,
• the centrifugal pump comprising a pump housing forming a flow channel inside the pump housing and an impeller configured to be rotated in the flow channel around a central axis by a shaft,
• the inducer comprising an inducer rotor having a sleeve-shaped rotor body, the inducer rotor configured to be rotated around the central axis in the flow channel, the rotor body including a number of blades extending inwards from the rotor body,
• the inducer rotor being separate to the impeller and a rotation speed of the inducer rotor independently controlled in relation to a rotation speed of the impeller, the inducer comprising an electric motor that is an annular motor encircling and connected to the inducer rotor, the method comprising
• controlling the pump unit based on a determination of a combination of the inducer and the centrifugal pump at least on parameters of total volumetric flow rate and pressure difference over the pump unit, and controlling the pump unit based on a surface level on a suction side of the pump unit, and an NPSHaA at an inlet before the pump unit is measured, calculated or otherwise determined, and when necessary, the output power of the inducer is increased or decreased to affect a NPSHaB at the flow channel before the impeller, so that during operation of the pump unit the NPSHaB is greater than a NPSHr of the centrifugal pump.