Monday, November 11, 2013

Five Common Control Valve Problems

Some control valve problems can be relatively easy to detect, but others are more difficult to identify without performing specific tests.  Before attempting to tune a control loop to improve loop performance, here are five of the most common control valve problems to look for:  

Deadband, sometimes known as hysteresis, can be caused by backlash between the control output and actual valve position.  It can also be due to mechanical friction or looseness.  This can cause oscillations under PI or PID control.  

Stiction is static friction that can cause a valve to stick in position.  When enough pressure builds to force the valve to break free, the excess pressure will often cause the valve to overshoot its target position.  When valve movement stops, it sticks in the new position.  Common causes of stiction are over-tightened valve stem seal, sticky valve internals, undersized actuator, or sticky positioner.  

Positioner overshoot often results when a valve positioner is defective or tuned too aggressively, and changes in controller output can cause the valve to overshoot its target position.  

Oversize control valves can lead to poor control performance. Full flow should be obtained at 70-90% depending on conditions, and if valves are sized too large for the flow rate, small changes can significantly affect flow.  If other valve positioning problems exist, oversize valves will further amplify the negative effects.  

Finally, nonlinearity is another issue that can lead to tuning problems.  If a control valve’s flow characteristic is nonlinear, control loops tend to become sluggish or unstable when the valve position moves away from its operating point.