Abstract: An internal valve for pressurized fluid containment vessels having vessel ports. The valve includes a valve seat associated with the vessel port and a valve member movable between a closed position against the valve seat and an open position wherein fluid can flow through the vessel port. A control chamber positioned in the vessel is connected to the valve member. A motive mechanism acting on or in the control chamber, by a control system outside of the vessel, controllably moves the valve member to at least one of the open or closed positions. The control chamber is sealed from fluid in the vessel by a sealing system, which includes a flexing surface against which fluid pressure in the vessel acts to thereby bias the valve member towards one of the open or closed positions.