Valve Hysteresis

The difference between up scale and down scale results in instrument response when subjected to the same input approached from the opposite direction.

Example: A control valve has a stroke of 1.0 inch and we give the valve a 9 psig signal. The valve travels 0.500 of an inch. We then give the valve a 12 psig signal, and the valve travels to 0.750 an inch. When the valve is then given a 9 psig signal, the stroke is measured at 0.501. That represents hysteresis.

Hysteresis can be caused by a multitude of variables, packing friction, loose linkage, pressure drop, etc. If someone asks you what the hysteresis of your control valve is, it is a bum question because hysteresis is more aptly applied to an instrument than to a control valve.

There are simply too many variables in the valve and the system to answer the question properly. The control valve only responds to the controller signal and will move to a position to satisfy the controller - thus negating the effects of hysteresis.

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