Abstract: A centrifugal separator for separating contaminants from contaminated oil has an outer casing and a rotor within that casing mounted for rotation about a vertical axis. Oil under pressure enters the rotor at one end and at the other end exits through jets into a chamber within the casing which is normally filled with air. As the jets of oil impinge upon the inner wall of the casing, a small amount of air becomes entrained in the oil and as the oil flows to a sump, the air is gradually removed from the interior of the casing. In order to prevent a build-up of oil within the casing, which could interfere with rotation of the rotor, air under pressure is supplied to the interior of the casing by means of a float-operated valve member so that while the valve member is normally closed, any increase in the oil level will cause the float to rise with that oil level and thereby open the valve to allow additional air to enter into the interior of the casing.