Abstract: The valve's flow control is provided by sliding concentric tubes which make or break metal to metal contact in reaction to pressure changes in the inlet and outlet piping.Power for bi-directional operation of the valve is provided entirely by the forces of fluid pressure and gravity, with no mechanical or electrical means of assistance (such as springs, rods, pistons, screws, or electromagnets) being required.The valve's status is recognized by means of a proximity sensor.When the concave surface of the stopper drops to the slightly rounded (convex) edge of the tube (with the radius of the stopper's concave surface being greater than that of the tube's convex surface), a tight and complete seal is made. That seal remains until the surfaces are separated. Separation occurs at the instant the stopper starts to rise in response to the incoming fluid pressure exceeding the opposing forces of gravity and any reverse fluid pressure acting against the stopper.