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A vent valve comprises a valve housing defining a vent port and a valve member mounted within the housing so as to define a fluid system chamber and an activation chamber with the housing. The valve member is biased towards a closed position, in which fluid communication between the fluid system cha

A vent valve comprises a valve housing defining a vent port and a valve member mounted within the housing so as to define a fluid system chamber and an activation chamber with the housing. The valve member is biased towards a closed position, in which fluid communication between the fluid system chamber and the vent port is prevented, by fluid pressure within the fluid system chamber acting on the valve member. The valve member is biased towards an open position, in which there is fluid communication between the fluid system chamber and the vent port, by fluid pressure within the activation chamber acting on the valve member. The vent valve further comprises a pressure limiting arrangement associated with the activation chamber to limit pressure therein to a predetermined pressure value.

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1. A lubricator stack vent valve for controlling venting of a fluid from a lubricator stack, the lubricator stack vent valve comprising: a valve housing comprising a lubricator stack chamber and a first bore which opens into the lubricator stack chamber, wherein the valve housing is configured to be

1. A lubricator stack vent valve for controlling venting of a fluid from a lubricator stack, the lubricator stack vent valve comprising: a valve housing comprising a lubricator stack chamber and a first bore which opens into the lubricator stack chamber, wherein the valve housing is configured to be secured to the lubricator stack to present the lubricator stack chamber in communication with the lubricator stack, the valve housing defining a vent port in communication with the first bore;a valve seat defined at an interface between the first bore and the lubricator stack chamber such that a cross-sectional area of the valve seat is equivalent to a cross-sectional area of the first bore; anda valve member mounted within the housing and comprising a seat-engaging surface for engaging the valve seat, and a spool head located within the first bore of the housing such that the vent port is located between the spool head and the valve seat, the spool head being sealed relative to an inner surface of the first bore to define an activation chamber at least partially within the first bore, wherein the spool head defines a terminating axial end face of the valve member which defines a moveable wall of the activation chamber, the lubricator stack chamber and the activation chamber being in pressure communication with each other via the valve member, wherein:the valve member is moveable between a closed position in which the seat-engaging surface is engaged with the valve seat to prevent fluid communication between the lubricator stack chamber and the vent port, and an open position in which the seat-engaging surface is disengaged from the valve seat to provide fluid communication between the lubricator stack chamber and the vent port;the valve member is biased towards the open position by fluid pressure within the activation chamber acting on the terminating end face of the valve member;the valve member is biased towards the closed position by fluid pressure within the lubricator stack chamber acting on the valve member, movement of the valve member towards the closed position being resisted by fluid pressure within the activation chamber; andthe vent valve further comprising a pressure relief arrangement mounted on the valve housing for relieving pressure from the activation chamber when pressure therein reaches a predetermined upper limit pressure value, such that the pressure relief arrangement defines a maximum permitted pressure within the activation chamber which resists movement of the valve member towards its closed position. 2. The lubricator stack vent valve according to claim 1, wherein, when the valve member is in the closed position, a force applied by fluid pressure in the lubricator stack chamber acting on the valve member is related to the cross-sectional area of the valve seat and to the fluid pressure in the lubricator stack chamber. 3. The lubricator stack vent valve according to claim 1, wherein the valve seat forms a knife-edge seal with the valve member; when the valve member is in the closed position. 4. The lubricator stack vent valve according to claim 1, comprising a seal disposed around the spool head of the valve member to provide sealing between the spool head and the inner surface of the first bore. 5. The lubricator stack vent valve according to claim 1, wherein force applied by fluid pressure in the activation chamber acting on the terminating end face of the valve member is related to the cross-sectional area of the first bore and to the fluid pressure in the activation chamber. 6. The lubricator stack vent valve according to claim 1, comprising a biasing arrangement to bias the valve member towards a closed position. 7. The lubricator stack vent valve according to claim 6, wherein the biasing arrangement comprises at least one resilient member disposed between the housing and the valve member. 8. The lubricator stack vent valve according to claim 1, wherein the activation chamber is in fluid communication with an activation port, adapted to allow connection thereto of an activation arrangement supplying pressure to the activation chamber. 9. The lubricator stack vent valve according to claim 1, wherein the pressure relief arrangement comprises at least one of a pressure regulator, a pressure restrictor, a pressure relief valve, and a pressure restrictor for limiting the pressure of fluid in the activation chamber. 10. The lubricator stack vent valve according to claim 1, comprising a lubricator stack port, in fluid communication through the housing with the lubricator stack chamber and adapted to enable the vent valve to be coupled to a fluid system. 11. A lubricator stack comprising a lubricator stack vent valve according to claim 1, wherein the lubricator stack chamber of the lubricator stack vent valve is in fluid communication with fluid in the lubricator stack. 12. The lubricator stack according to claim 11, wherein the lubricator stack vent valve is disposed at or near the top of the lubricator stack, so as to facilitate venting of a first fluid which is displaced by a second fluid of higher density. 13. A method of venting a fluid from a lubricator stack using a lubricator stack vent valve according to claim 1, the method comprising: placing the lubricator stack vent valve in fluid communication with the lubricator stack; andcontrolling venting of the fluid from the fluid system through the lubricator stack vent valve, by: biasing the valve member towards a closed position, in which fluid communication between the lubricator stack chamber and the vent port is prevented, by lubricator stack pressure acting on the valve member;biasing the valve member towards an open position, in which there is fluid communication between the lubricator stack chamber and the vent port, by fluid pressure within the activation chamber acting on the valve member; andlimiting the pressure within the activation chamber to a maximum permitted pressure such that fluid communication between the lubricator stack chamber and the vent port is only achievable when the pressure within the activation chamber is at or below the maximum permitted pressure. 14. The method according to claim 13, comprising applying a biasing force to urge the valve member towards the closed position. 15. The method according to claim 13, comprising opening the valve by filling the activation chamber with a pressurised activation fluid. 16. The method according to claim 13, comprising closing the valve, by releasing the activation fluid from the activation chamber, or by closing the valve under the action of increased lubricator stack pressure acting on the valve member. 17. The method according to claim 13, comprising increasing the lubricator stack pressure so as to automatically close the vent valve. 18. Use of a vent valve according to claim 1, for venting a fluid from a fluid system. 19. The lubricator stack vent valve according to claim 1, wherein the pressure relief arrangement is mounted in a wall of the housing and in communication with the activation chamber. 20. The lubricator stack vent valve according to claim 1, wherein the valve housing comprises a threaded portion at one end thereof for permitting the lubricator stack vent valve to be secured to a lubricator stack. 21. The lubricator stack vent valve according to claim 1, wherein the valve housing comprises first and second housing portions secured together. 22. The lubricator stack vent valve according to claim 21, wherein the first housing portion defines the lubricator stack chamber and the second housing portion defines the first bore.