When is a valve positioner typically recommended as an accessory for a control valve?

A valve positioner is typically recommended as an accessory for a control valve when the valve is experiencing a large degree of hysteresis. Hysteresis refers to the lag between the input signal to a control device and the actual movement or response of that device. In the context of control valves, significant hysteresis can lead to performance issues, such as instability, inaccuracies in maintaining the required flow rate or pressure, and delayed response to changes in the control signal.

A valve positioner helps to enhance the accuracy and responsiveness of the valve by providing a means to accurately position the valve stem in accordance with the control signal received from the controller. It compensates for any discrepancies in valve operation, ensuring that the valve responds more reliably to the control signals. This is particularly critical in applications where precise control is vital for process stability and efficiency.

In contrast, other factors like the size of the valve body, the material of construction (carbon steel), or the configuration of the valve (such as split-body designs) do not inherently necessitate the use of a positioner. These aspects may influence valve selection or suitability for certain applications but do not directly relate to optimizing the control characteristics of the valve as effectively as addressing hysteresis does.