Fluid separation apparatus and system

A fluid separation apparatus includes a hydrocyclone (100) having an orifice (134) therein, a housing (202) disposed in the hydrocyclone and having a bore (204), a piston (240) disposed in the bore (204) of the housing (202), and a first annular seal (248) disposed between the housing (202) and the piston (240) and configured to restrict fluid communication between the housing (202) and the piston (240), wherein the piston (240) includes a passage (242) extending through the piston (240) and having a pin (254) coupled to a first end (240a) of the piston, the piston (240) being actuatable to move between a first position (260) where the pin (254) is clear of the orifice (134) of the hydrocyclone, and a second position where the pin (254) is disposed in the orifice (134), wherein, the piston (240) is configured such that as the piston is actuated from the first position to the second position, fluid is permitted to flow into the passage (242) of the piston (240) from the bore (204) of the housing (202).