Abstract: A method and related system are disclosed for controlling the operation of a fixed rate pumped hydrocyclone liquid separation unit to maintain a desired back-pressure therein. A pressure control valve and a liquid level control valve are connected to a separated liquid discharge conduit extending from at least one hydrocyclone. The pressure control valve operates in response to the pressure of the liquid introduced into the hydrocyclone and the liquid level control valve operates in response to the liquid level within a liquid holding vessel that supplies the liquid through a fixed rate pump to the hydrocyclone.