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Solutions for Uncalibratable Valve Positioners
Release time:
2026-04-07 15:22
In practical applications, valve positioners sometimes cannot be calibrated using conventional adjustment methods. This is because, under normal conditions, the zero-position spring operates in its linear range, where its length variation is limited; consequently, the mechanical travel of the span-adjustment mechanism is also constrained, which in turn restricts both the adjustment range of the zero-position spring and the amplification factor of the span-adjustment mechanism. Under such circumstances, if a control valve with a very large or very small gain is being adjusted, it becomes exceedingly difficult to properly calibrate the associated positioner—effectively rendering the standard calibration method ineffective. However, as shown in Equation (4), other adjustable parameters include the spring constant of the zero-position spring and the effective length of the feedback lever. Since the zero-position spring typically operates within its linear region, this discussion will focus solely on calibrating the valve positioner by adjusting the effective length of the feedback lever. To do so, the pin connected to the valve stem can be moved closer to the positioner, thereby shortening the effective length of the feedback lever; this increases the lever’s mechanical advantage, resulting in a larger stroke. Conversely, moving the pin farther from the positioner increases the effective length of the feedback lever, reducing the stroke. Therefore, when this approach is combined with the conventional calibration method, the range of stroke variation can be expanded, making it easier to calibrate the valve positioner.
The steps for calibrating a valve positioner using the feedback lever method are as follows:
a. Apply a 4 mA DC signal and adjust the zero point using the standard zeroing method;
b. Apply a 20 mA DC signal and adjust the span using the standard span adjustment method;
c. Repeatedly perform steps a and b;
d. If the zero point and span cannot be calibrated, adjust the pin on the valve stem to change the effective length of the feedback lever, thereby increasing or decreasing the stroke: shortening the effective length of the lever increases the stroke, while lengthening it decreases the stroke.
e. Repeat the above steps until both the zero point and the span meet the specified requirements.
2020-01-19
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