The present invention provides a method (100) of operating a centrifugal separator (1). The centrifugal separator (1) comprises a centrifuge bowl (10) arranged to rotate around an axis of rotation (X) and in which the separation of a liquid mixture takes place, a stationary frame (2) which defines a surrounding space (3) in which said centrifuge bowl (10) is arranged, a drive member (4) configured to rotate the centrifuge bowl (10) in relation to the frame (2) around the axis of rotation (X), wherein the centrifuge bowl (10) further comprises an inlet (11) for receiving the liquid mixture to be separated, at least one liquid outlet (12) for discharging a separated liquid phase and an intermittent discharge system (30) for discharging a separated sludge phase from the centrifuge bowl. The method (100) comprises the steps of a) supplying (101) a liquid feed mixture to be separated to the inlet (11) of the centrifuge bowl (10), b) separating (102) the liquid feed mixture into at least one separated liquid phase and a separated sludge phase, and c) supplying hydraulic fluid to the intermittent discharge system (30) to initiate discharge (104) of a separated sludge phase from the centrifuge bowl (10), wherein the amount of supplied hydraulic fluid is determined by the magnitude of a generated trigger signal Tgen and further wherein the magnitude of the generated trigger signal Tgen is dependent on the air pressure around the centrifuge bowl (10).