Adjustable springs for pressure vessel closure

A closure device for a lower opening of a pressure vessel, the pressure vessel having an annular flange encircling the opening, comprising in combination:

• an annular movable support encircling the pressure vessel above the flange and being axially movable relative to a longitudinal axis of the pressure vessel;
• an annular stationary support secured to the flange below the movable support and extending radially outward therefrom;
• a plurality of coil springs evenly spaced around the pressure vessel, each of the coil springs having an upper end in engagement with the movable support for axial movement therewith and a lower end in engagement with the stationary support;
• a plurality of hydraulic cylinders connected between the movable support and the stationary support for moving the movable support axially relative to a longitudinal axis of the pressure vessel between an upper position and a lower position;
• a head having an upper side with a circular mating surface for abutment with the flange for closing the opening;
• hook means spaced around and carried by the movable support for axial movement therewith for lifting the head into abutment with the flange for closing the opening when the movable support is in the upper position, with the springs biasing the head into tight engagement with the flange;
• a plurality of adjusting rings, each mounted between one of the supports and one end of each of the springs;
• and
• means for selectively and independently moving each of the adjusting rings relative to said one of the supports along an axis of each of the springs to increase and decrease the bias of each of the springs independently of the other of the springs.