A valve assembly comprising: a valve housing defining therein a fluid flow channel between a first end arranged to be connected to a source of pressurised fluid, and an outlet, a frangible member disposed within and across the fluid flow channel so as to block flow from the first end to the outlet, rupture means for rupturing the frangible member to allow flow from the first end to the outlet; biasing means for biasing the means for rupturing into contact with the frangible member to cause rupture thereof; retaining means for retaining the rupture means out of contact with the frangible member to prevent rupture thereof; release means configured to release the retaining means to allow the biasing means to bias the rupture means into contact with the frangible member; wherein the retaining means comprises a bearing extending to engage the biasing means and hold it in a compressed state, against its bias; and securing means for securing the bearing in the engaged state, wherein the release means comprises a trigger attached to the securing means which, when actuated, moves the securing means to release the bearing away from engagement with the biasing means whereby the biasing means expands to bias the rupture means into contact with the frangible member to cause rupture thereof; characterised in that the bearing comprises two or more ball bearings mounted in a channel around the housing so that they can rotate relative to the housing, the ball bearings extending into the housing to retain the biasing means in a non-biased state and the release means comprises a basket formed over a part of the housing and extending across at least part of the channel so as to hold the ball bearings in the state where they extend into the housing. a valve housing defining therein a fluid flow channel between a first end arranged to be connected to a source of pressurised fluid, and an outlet, a frangible member disposed within and across the fluid flow channel so as to block flow from the first end to the outlet, rupture means for rupturing the frangible member to allow flow from the first end to the outlet; biasing means for biasing the means for rupturing into contact with the frangible member to cause rupture thereof; retaining means for retaining the rupture means out of contact with the frangible member to prevent rupture thereof; release means configured to release the retaining means to allow the biasing means to bias the rupture means into contact with the frangible member; wherein the retaining means comprises a bearing extending to engage the biasing means and hold it in a compressed state, against its bias; and securing means for securing the bearing in the engaged state, wherein the release means comprises a trigger attached to the securing means which, when actuated, moves the securing means to release the bearing away from engagement with the biasing means whereby the biasing means expands to bias the rupture means into contact with the frangible member to cause rupture thereof; characterised in that the bearing comprises two or more ball bearings mounted in a channel around the housing so that they can rotate relative to the housing, the ball bearings extending into the housing to retain the biasing means in a non-biased state and the release means comprises a basket formed over a part of the housing and extending across at least part of the channel so as to hold the ball bearings in the state where they extend into the housing.