|DeZurik Control Valve|
Control valves are not intended for isolating a process and should not be used as such. They are not intended to provide a bubble tight shutoff.
Choose the proper body, seat and wetted materials for the process being controlled. Consider material compatibility, pressure ratings, and operating temperatures prior to valve selection.
When designing your control loop, put the flow sensor upstream of the control valve. Putting the flow sensor downstream of control valve exposes it to possible bubbles caused by the flashing and turbulence of the flow in the valve cavity.
Consider to what degree of deadband your valve and related components may produce. Deadband is the “slop” in a control valve between the signal change and the movement of the valve. Sources of excessive deadband can be worn or poorly designed linkages between valve and actuator, the inherent tolerances from mechanical sensors, friction in the valve or an undersized actuator. Too much deadband can cause poor control due to opening/closing oscillations.
Think about any causes or sources of stiction. Stiction is exactly what it sounds like - “stickiness” in the valve movement caused by friction, and occurs after the valve has been immobile, or travels in a very narrow range, for extended periods of time. Only after additional force is applied by the actuator does the valve disk (gate or globe) break free, which can then cause overshoot and poor control.
Proper tuning of the loop controller and/or positioner. Many times poor control and instability is caused by overly aggressive controller and positioner settings. Historically, PI, PD and PID controllers had to be manually tuned for optimum control. Nowadays, controller auto-tuning features are available that mitigate the tuning challenge.
Proper valve sizing. Many times control valves are oversized for the flow rates they are supposed to control. This allows for full flow at only a small percentage of travel. This leads to the valve operating at small openings even at full flow conditions, thus small changes in position equal a large effect on flow. Such a high ratio of valve position change to flow makes control very difficult and causes excessive wear on the seat and trim of the valve and exacerbates problems with stiction and deadband. A rule of thumb is to size control valves at about 70%-90% of travel.
A linear or non-linear flow characteristic. A control valve’s flow characteristic is the relationship between the valve disk, gate or globe position, and the flow rate through the valve under normal conditions. Ideally the flow characteristic should be linear, but many types of valves have non-linear flow characteristics because of the shape of the disk, gate or globe. Globe control valves have fairly linear characteristics, while butterfly and gate valves have very non-linear flow characteristics. Many valve manufacturers “characterize” their valves for better linearity by modifying the orifice or disk with a special shape.
Of course, there are many other many criteria to be considered before selecting and applying a control valve, and should only be done in consultation with an experienced application engineer.