Valve

A valve comprising:

• a first chamber and a second chamber, wherein a movable member is arranged to selectably bring the first and second chambers into and out of fluid communication with one another;
• the movable member arranged such that when in an open position wherein the first and second chambers are in fluid communication, gas at a first pressure in the first chamber provides a closing force to a first surface area of the movable member to overcome an opening force provided by gas at a second pressure in the second chamber on a second surface area of the movable member whereby the movable member moves to a closed position to isolate the first and second chambers;
• and further arranged wherein
• as the movable member moves to the closed position, the second surface area of the movable member exposed within the second chamber is reduced to an exposed annular surface of the second surface area thus reducing the opening force provided by the gas at the second pressure a and providing a bias towards the closed position,
• wherein the movable member further comprises a passageway coupling the first chamber with a third chamber, wherein the passageway comprises a deformable bung and wherein the bung is arranged to be forced from the passageway when a predetermined pressure occurs in the first chamber so as to bring the first chamber and the third chamber into fluid communication.