Gate valves are located intermittently along the length of gas and oil pipelines. Such valves are adapted to generally remain open but under dislocations in the fluid flow within the pipeline caused by, for example, a leak in the pipeline, the valves are each adapted to close thus shutting off the flow of oil or gas until the dislocation is located and repaired.
Gate valves and associated actuator systems to perform such functions are known. Such an actuator system and gate valve is disclosed in our U.S. Pat. No. 4,423,758 to Ellett entitled EMERGENCY SHUT DOWN DEVICE. Certain new and inventive improvements have been made, however, in the gate valve actuator system there disclosed.
In the apparatus disclosed in the aforementioned reference, the preload in the compression spring within the housing of the actuator is constant. In order to change this preload in the spring, which may be required, for example, when the actuator is used with a different valve, it was necessary to disassemble the Ellett device to vary the spring loading. The disassembly was difficult and time consuming.
A further disadvantage with the Ellett apparatus related to the position of the piston after the preload in the compression spring was changed. When a spacer was added between the pull tube and the end plate, the piston would move upwardly with the pull tube with the result that the stroke necessary to completely close the valve would be shortened by a like amount and this reduced length stroke might not fully close the valve. To compensate for this stroke reduction, a spacer was added between the piston and the pull tube. Because the stop tube is integral with the pull tube, however, and does not move relative to the pull tube, it was necessary to either provide a new pull tube and stop tube or to reduce the length of the stop tube by cutting or machining. This was also costly and unnecessarily time consuming.
Yet a further disadvantage in the aforementioned Ellett apparatus related to the necessity in actuators to have different stroke lengths for different valves. In the Ellett apparatus, there was no adjustment for the stroke possible and, therefore, there was little universality of the actuator for valves of various sizes.
Yet a further undesirable feature of existing actuators relates to the use of the indicator rod which indicates whether or not the valve is open or closed. It is desirable that the indicator rod attached to the piston be of a configuration which allows the valve to be mechanically maintained in an open position in the absence of pressure in the cylinder.
Yet a further disadvantage with existing actuators relates to removing the actuator from the valve while maintaining the valve in the open position there being a large force exerted on the piston by the compression spring. Disassembly of the actuator and, in particular, removing the hub from the end plate under such conditions can be dangerous.