| 摘要 |
A valve actuator (1) is provided which comprises an actuator stem (4) extending from a housing (2) which contains a sleeve (17). The actuator stem (4) is driven so as to cause the sleeve to compress a stack of springs (22) within the housing until bolts (25) engage the sleeve, locking the springs in a compressed state. At the same time an armature (28) on the actuator stem is attracted to an electromagnet (29) within the housing. The actuator stem can then be driven to open or close a valve in a controlled manner without first compressing or releasing the springs. In an emergency, the electromagnet is switched off, releasing the armature thus unlocking the stack of springs. The released springs push the piston which partially retracts the actuator stem into the housing causing the valve to be closed.
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