Recently I had an operator tell me that a ball valve will be forced open/closed by the force of the liquid flowing through it. (i.e. when the valve is nearly closed the force of the flow will "snap" it closed, and similar for opening.)I have seen throttled ball valves rattle open (not closed). It is not a snapping action, but more of an incremental thing. It has usually taken a couple of hours in strongly vibrating service for a ball valve to go from 5% open to 15% open (where the throttling stops) and then it will tend to stay there forever. This is one of the many reasons that I will not spec a ball valve for throttled service.I have done the free-body diagram on the forces on a ball and while it is a bit indeterminate (positive feedback is hard to get into equilibrium), the forces to slam a ball in either direction don't seem to be available in a flow stream, even at high flow potential and very high dP.

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