Device for controlling fluid flow

A device for controlling the flow of a fluid through a conduit from an upstream side of the device to a downstream side of the device, the device comprising:

• a valve aperture;
• a cylindrical mounting member arranged on a downstream side of the valve aperture;
• a valve member movably mounted on an outside of the cylindrical mounting member and arranged to move reciprocally to selectively open and close the valve aperture, thereby controlling flow of the fluid through the valve aperture;
• a control volume defined between the cylindrical mounting member and the valve member;
• an arrangement for introducing a control pressure into the control volume;
• and
• a seal arranged between an outer surface of the cylindrical mounting member and an inner surface of the valve member for substantially sealing the control volume;
• wherein the valve member is acted on by a pressure of the upstream side and the control pressure so as to be moved by a difference between these pressures.