Abstract: A hydrocyclone comprising a frusto conical feed box extension, a feed box attached to the larger end of the feed box extension, the feed box having a tangential feed pipe, an overflow box positioned over the feed box and including an overflow pipe and a vent pipe, and a fish tail at the narrower end of the feed box extension, characterized in that a pressure compensating valve is positioned in the vent pipe, and operation of the valve is dependent on the pressure within the feed pipe. The pressure compensating valve comprises a valve member biased to an open position against a diaphragm located in a pressure chamber, one side of the diaphragm being in fluid communication with the feed pipe whereby variation of the pressure within the feed pipe causes the diaphragm to displace the valve member to a partially or totally closed position against an associated valve seat.