A method and arrangement for soft start up of a pump system, that includes a pump for generating a liquid flow, an electrical drive disposed to actuate the pump, and a pressure sensor disposed to measure a liquid pressure at a flow output of the pump. The rate of change of rotation speed of the pump during a start up phase is dependent on a rate of change of measured liquid pressure, wherein the rate of change of the rotation speed is a descending function of the rate of change of the measured liquid pressure.