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 is a range of input values that result in zero output. For a control valve, a controller signal fails to trigger a direction change within this range, and additional output results in the valve overshooting its target. Deadband leads to increased dead time, creates load disturbance errors, increases oscillations in the control loop, and can damage pressure relief discs or vessels. Hysteresis is a similar condition that can create process cycling around the setpoint, slow controller response, and increased control variance.
Stiction is static friction that can cause a valve to stick in position. When the valve breaks free, excess pressure causes it to overshoot its target position, rendering the controller incapable of reaching its desired setpoint. Common causes are over-tightened valve stem seals, sticky valve internals, undersized actuator, media viscosity, and sticky positioner, among others.
Oversized 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, small changes can significantly affect flow. (Undersized control valves can cause various issues, as well, but this condition is less common.) If other valve positioning problems exist, oversized valves will further amplify the negative effects.
Finally, nonlinearity 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.
About Omni
Omni Instrumentation Services, Inc. is a premier instrumentation and electrical contractor, providing superior total care solutions since 1986. Services include instrument installation, tubing installation, instrument calibration, control loop testing, startup and commissioning, power and lighting, process control wiring, BMS wiring, telecommunications and data wiring, fire alarm wiring, security wiring, process network wiring, and control panel fabrication. Omni Instrumentation Services, Inc., is certified in Foundation Fieldbus, Profibus, DeviceNet and ASI-Bus installation.