Discussion on the overall rotation of the pneumatic control valve stem, pneumatic rod, and coupling
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During operation of the on-site pneumatic control valves, there have been numerous instances where the valve stem, the pneumatic rod, and the coupling (the component that connects the pneumatic rod to the valve stem) all rotated as a unit. The positioner feedback rod is connected to the coupling; if the coupling rotates, the hook between the positioner feedback rod and the feedback arm can easily come loose, resulting in the disappearance of the displacement feedback signal. Large rotations can also damage the diaphragm of the actuator. Recently, during the operation of the unit, a situation occurred in which the pneumatic rod, valve stem, and coupling rotated as a whole, resulting in the detachment of the hook on the feedback arm and the loss of the feedback signal, which affected the unit’s status. This valve is a pneumatic control valve produced by EMESON; the groove on its coupling works in conjunction with the guide rails on the yoke to prevent the coupling from rotating at large angles. I’d like to discuss with everyone why the coupling rotates There is a certain gap between the coupling groove and the guide rail; does the valve design allow the coupling to rotate slightly? Are there any standards for the degree of rotation allowed by a coupling?
What you’re experiencing is slight rotation, which isn’t enough to cause the feedback rod of the positioner to come loose. Many valves these days have components designed to prevent excessive rotation, though this will result in friction. I’ve identified several reasons for the rotation of valve actuators from several different brands that we use on site: 1. Improper alignment during installation, which leads to slight rotation; 2. Loose fixing screws on the diaphragm head (if any), causing excessive rotation; 3. A loose coupling (this depends on the design of the coupling in each brand)
It can be confirmed that there is no manual operation of the coupling; the coupling is disassembled to put the pneumatic rod and the valve stem in a completely free state. With good alignment, the coupling is tightened again. Even when the valve is open, the pneumatic rod, the coupling, and the valve stem as a whole can still deflect. At present, we believe that when the valve is in the open position, the large pressure difference upstream and downstream of the valve can exert a deflecting force on the valve disc, causing the coupling and pneumatic rod to deflect as a whole.