Flow control valve assembly

A valve assembly comprising a housing defining therein an inlet through which material can flow to the housing, a closure member having a planar closing surface and being mounted within the housing to be movable between an inoperative position remote from the inlet and an operative position aligned with the inlet, said closure member being mounted in the housing such that the plane of the closing surface is axially spaced from the inlet and such that the closure member is displaceable axially of the inlet towards said inlet, said spacing between the closing surface of the closure member and the inlet being adjustable, the valve assembly further comprising a deformable sealing ring mounted on the housing to surround said inlet and having a sealing surface protruding into the housing, and means for deforming the sealing ring whereby, on deformation, said sealing surface protrudes further into the housing, the arrangement being such that, during displacement of the closure member from its inoperative position to its operative position, the closing surface of the closure member first engages with the sealing surface of the sealing ring to be wiped thereby and to make initial sealing contact therewith, secondly, deformation of the sealing ring by the deforming means, with the closure member in its operative position, increasing the sealing contact between the sealing ring and the closure member, thirdly, subsequent pressurisation of the interior of the vessel serving to adjustably displace the closure member towards the inlet to increase further the sealing contact between the deformed sealing ring and the closure member, and sufficent pressurization serving to sealingly engage the valve closure member with the housing after the sealing ring has been fully deformed.