Double seat flow control valve

A valve unit, comprising:

• a body including first and second valve seats and defining first and second product flow paths, a leakage flow path, and a connecting passageway between said first and said second product flow paths;
• first and second valve disks movable between respective open and closed positions, said first and said second valve disks cooperating with said first and said second valve seats to permit and to block flows between said first and said second product flow paths and said connecting passageway, respectively;
• actuator means including pressure chamber means with at least a first, a second and a third pressure chamber for containing a pressurized fluid, and for containing first, second and third pistons and first, second and third inlet ports for receiving respective pressure signals associated with the first, second and third pressure chambers, respectively, a first pressure chamber located between the second and the third pistons, the second pressure chamber located between the first and the second pistons and the third piston located between the first and the third pressure chambers, the first piston coupled to the second valve disk to open the second valve disk, the second piston responsive to an opening signal in the first pressure chamber to engage the first piston so as to open the second valve disk, and the first piston responsive to a second bleed signal in the second pressure chamber to at least partly open the second valve disk while concurrently applying the second bleed signal in the second pressure chamber to the second piston to apply a closing force to the first valve disk so that the first valve disk remains closed and the third piston responsive to a first bleed signal in the third pressure chamber to at least partly open the first valve disk while permitting the second valve disk to remain closed.