Control Valve Problems
There are 5 basic control valve problems in poorly performing control loops: - Dead band - Stiction - Positioner overshoot - Incorrect valve sizing - Nonlinear flow characteristic Dead band (generally caused by friction) A valve with dead band acts like there is backlash between the controller output and the actual valve position. Every time the controller output changes direction, the dead band has to be traversed before the valve physically starts moving. Although dead band is often a mechanical issue such as loose mechanical linkages, it may also be due to excessive friction in the valve, or to an undersized actuator, or to a defective valve stem positioner.
Hysteresis is also an issue in that the characteristics characteristics of the value may be different depending on the direction of travel. A control valve with dead band will cause oscillations in a level loop under PI or PID control if the controller directly drives the control valve (non-cascade), (non-cascade), i.e., no control positioner. A control valve with dead band can also cause oscillations after a set point change in control loops on self-regulating processes – especially if Integral action is even slightly excessive.
Stiction Another common problem problem is stiction, short for Static Friction. It means that the valve internals are sticky. If a valve with stiction stops moving, it tends to stick in that position. Additional force is required to overcome 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 and finally moves. The valve
movement quickly absorbs the excess in pressure, and then it overshoots its target position. Then valve movement stops and the valve sticks in the new position.
Frequently, this overshoot causes process set point overshoot. When the valve sticks at the new position, the controller output reverses direction and the process repeats in the opposite direction causing oscillation. This is known as a stick-slip cycle. If loop oscillations are caused by stiction, the controller output cycle may resemble a saw-tooth wave, while the process variable may look like a square wave or an irregular sine wave. Stiction might be caused by an over-tight valve stem seal, by sticky valve internals, by an undersized actuator, or a sticky positioner.
Positioner Overshoot Positioners are fast feedback controllers that measure valve stem position and manipulate the actuator until the desired position is achieved. Most can be tuned, but may be too aggressive for the valve they control. This causes the valve to overshoot its target position after a change in controller output. Sometimes the positioner is defective. If the process controller is also tuned aggressively, this combination leads to severe oscillations in the control loop.
Valve Sizing Valves should be sized so that full desired flow is obtained at about 70%-90% of travel, depending on the valve characteristic curve and service conditions. In most cases, control valves are sized too large for the flow rates they need to control. This leads to the valve operating at a small opening even at full-flow conditions. A small change in valve position has a large effect on flow leading to poor performance as valve positioning errors such as stiction and dead band, are amplified by the oversized valve. Nonlinearity Nonlinear flow characteristics can also lead to tuning problems. A control valve flow characteristic is the relationship between the valve position and the flow rate through the valve under normal service conditions. Ideally the flow characteristic should be linear. With a nonlinear characteristic, optimal response will exist at only one operating point. The loop may become unstable or sluggish as the valve position moves away from this operating point.
The intrinsic characteristic is measured on the bench. It is important to understand how this may change when the valve is placed into to service. Pressure variations on pumps and pipelines can lead to changes in the valve characteristics in service.
