A base unit and a modular centrifugal separator are disclosed. The base unit includes a stationary frame, a rotatable member, and a drive unit. The rotatable member delimits an inner space being configured for receiving at least one part of an exchangeable separation insert therein. The rotatable member is provided with a first opening at a first axial end configured for a first fluid connection of the exchangeable separation insert to extend through the first opening. The rotatable member includes a second opening at a second axial end configured for a second fluid connection of the exchangeable separation insert to extend through the second opening.