Valves for controlling or preventing fluid flow

A valve for controlling fluid flow, which valve comprises:

• a shut-off element displaceable in an axial direction between a first position corresponding to an open condition of the valve and a second position corresponding to a closed condition of the valve.
• a valve seat with which the shut-off element is capable of fluid-tight sealing engagement when in the said second position,
• drive means operatively connected to the shut-off element and capable of transmitting a drive to the shut-off element to effect the said displacement thereof between the said first and second positions thereof,
• a connecting member capable of transmitting the drive to the shutoff element, the shut-off element being arranged to move in said axial direction (i) relatively to said connecting member when driven by said drive means, and, (ii) with said connecting member when said connecting member is moved axially,
• a housing defining a chamber,
• a diaphragm within said chamber and having a central region attached to said connecting member and a peripheral region attached to said housing, the diaphragm supporting said connecting member, and being controllable independently of the drive means, so as to move said connecting member, and thereby provide said displacement of the shut-off element with the connecting member in response to pressure difference across the diaphragm, and
• diaphragm control means capable of controlling the said pressure differences across the diaphragm and operable to control the said movement of the diaphragm,
• the action of the diaphragm on the connecting element being superimposable on the transmission by the connecting element of the drive from the drive means to the shut-off element in such a manner that when the drive means acts to displace the shutoff element, the diaphragm is not constrained to suffer a corresponding displacement equal to that of the shutoff element, and
• the said pressure difference being so controllable that
• (1) at least when the said drive means is operative to drive the shutoff element between the first and second positions, the diaphragm is urged by the said pressure difference away from the valve seat, and
• (2) at least when the said drive means is inoperative and the shut-off element is at the second position, the diaphragm is urged by the said pressure difference towards the valve seat to move the shut-off element into sealing engagement with the valve seat.