Regarding the constant throttle orifice of the control valve positioner
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Why can’t the control valve be adjusted to its full closed position? Adjusting the constant orifice will solve this issue. What is the function of the constant orifice?
The constant-throttling orifice serves to regulate flow; it is located quite low, making it easy to get clogged, and clearing it restores normal operation.
For pressure feedback systems, the openings are small and prone to getting clogged by tiny particles in the clean air; simply pushing in a strand of hair will do.
Find a manual for the locator and study its working principle (structure diagram) carefully, and you’ll understand it roughly.