Control Valve Position Instability

Total control valve stem position mechanical stability is the total absence of valve stem movement when the signal to the actuator is constant. Position instability that is valve stem movement is not an absolute phenomenon. It occurs in many control valves to some degree. In most applications where it exists it is not noticeable or does not exceed the acceptable limit for the application.

Measure the maximum amplitude of the unstable motion when the signal to the actuator is constant. The amplitude measurement technique may be that which is deemed appropriate for the application such as a linear scale dial-indicator or motion transducer.

To determine the instability percentage use the following equation to calculate the percentage of the rated valve travel that is unstable:

Instability Percentage = (Maximum unstable motion amplitude*100)/ (Rated travel)

Example:

Rated Travel = 50 mm

Unstable Motion Amplitude = 2.5 mm

Instability Percentage= (2.5 *100)/ 50  = 5