Possible control valve failure mechanisms include:
Dead band Deadband is defined by a range of control signal inputs that fail to elicit any adjustment in valve position and is an obstacle in maintaining optimal process conditions. The result of a dead band condition may be: 1) reduced sensitivity to control signal variations, 2) delayed response to process control, 3) increased hysteresis, 4) reduced energy efficiency, and 5) increased risk of oscillations.
Stiction If a valve with stiction stops moving, it tends to stick in that position. Then additional force is required to overcome the stiction. The controller continues to change its output while the valve continues to stick in position. Additional pressure mounts in the actuator. If enough pressure builds up to overcome the static friction, the valve breaks free. The valve movement quickly absorbs the excess in pressure, and often the valve overshoots its target position.
Positioner overshoot Most positioners can be tuned. Some are tuned too aggressively for the valve they are controlling. This causes the valve to overshoot its target position after a change in controller output. Sometimes the positioner is simply defective in a way that causes overshoot.
Incorrect valve sizing Changing the valve's position just slightly will create big, non-proportional changes in the fluid's flow so it becomes overly sensitive. The valve's precision will be further reduced by any friction in the system (which will cause stickiness).
Nonlinear flow Nonlinear flow in a control valve can cause a number of issues, including:
1) vibration valve may start to vibrate as problems arise, 2) internal part loosening, 3) reverse flow repetitive reverse flow can cause valve seals to deteriorate rapidly, leading to leaks, 4) water hammer, 5) sticking see stiction above, and 6) leakage.
Converting energy to motion for more than half a century
