Meaning
Resistance forces generated by the sealing material around a valve shaft oppose the movement of the actuator. While some stem packing friction is necessary to prevent the escape of process fluids to the atmosphere, excessive force can hinder the accuracy of control. This mechanical load must be overcome every time the valve moves to a new position.
Control Performance
High levels of resistance cause the stem to move in jumps rather than a smooth motion. When stem packing friction is too high, the positioner may overcorrect, leading to oscillations in the process variable. Adjusting the packing nut to the correct torque is essential for balancing seal integrity with the responsiveness of the valve.
Wear Rate
Constant movement against the packing material causes it to compress and eventually wear down. If stem packing friction decreases too much, a leak will develop at the top of the valve.
Calculated Load
Actuator sizing must include the worst case friction values expected during the life of the valve. Changes in the chemical composition of the fluid or the operating temperature can alter the stem packing friction significantly. Engineers use these values to ensure that the automation has enough power to move the valve even when the seals are tight.
