![]() If CO is moving more than 0.5% each cycle, there is a problem. When trends of control valves with hysteresis and backlash problems are developed with the controller in manual mode, the quantity and magnitude of process variable changes isn’t consistent with controller output changes. (See Stiction diagram.) Hysteresis and backlash in manual mode diagram Visit Control Engineering Tutorials at Stiction is frequently observed in control loops operating in automatic mode as a continuous cycling ‘sawtooth’ pattern of the controller output (CO) and a square wave pattern of the process variable (PV) that produce process upsets and unnecessary valve wear. For more articles about control valves, loop tuning, and process analysis, ![]() Contributing factors include packing gland torque, viscosity of process fluids, and plug and seat characteristics. Stiction is the result of the sum of all the static friction in the moving parts of the valve. Stiction -formed by combining the words stick and friction-refers to the extra effort required to get a valve moving from a dead stopped position. Most common are stiction, hysteresis and backlash, and improper valve sizing. There are many ways for control valves to degrade process performance. Testing the valves in-place, during normal operation, can reduce rebuild costs, improve process operation, and focus maintenance efforts. Off-line and bench-testing procedures can be infrequent, inadequate and costly. Typically, many valves are inspected or rebuilt during off-line outages. For a plant to perform optimally, control valve performance must be tracked and maintained. Since control valves are mechanical devices, their performance is less than ideal, and degrades over time. These workhorses are required to move quickly and precisely to desired positions, 24 hours a day, while requiring as little maintenance as possible. I f instruments are the senses and controllers are the brains, then control valves are the muscles of the process industries. ![]() Control valves with stiction problems often display a ‘sawtooth’ pattern when the controller output is trended. ![]()
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