Fluid processes will employ control valves to regulate flow or pressure in between the extremes of fully open and fully closed. Their function and design are specifically different from shutoff valves, which are designed and intended for the isolation of segments of a fluid system. Improperly applying or sizing a control valve can have consequences in operation, productivity, and safety ranging from nuisance level to critical. Here are some items that should always be part of your selection and application consideration.
- A control valve is not intended to be an isolation valve and should not be used for isolating a process segment. Make sure you select the appropriate valve for the function to be performed.
- Select materials for construction that will accommodate the media and the process conditions. Take into consideration the parts of the valve that come in contact with process media, such as the valve body, the seat, and any other wetted parts. Operating pressure and temperature impact the materials selection for the control valve, too. Conditions surrounding the valve, the ambient atmosphere, and specific local conditions that may expose the valve to corrosives should be included in your thinking.
- Install flow sensors upstream of the control valve. Locating the flow sensor downstream of the control valve exposes it to an unstable flow stream which is caused by turbulent flow in the valve cavity.
- Establish the degree of control you need for the process and make sure your valve is mechanically capable to perform at that level. Too much dead-band leads to hunting and poor control. Dead band is roughly defined as the amount of control signal required to affect a change in valve position. It is caused by worn, or loosely fitted mechanical linkages, or as a function of the controller setting. It can also be affected by the tolerances from mechanical sensors, friction inherent in the valve stems and seats, or an undersized actuator.
- Consider stiction. Wikipedia defines it as “the static friction that needs to be overcome to enable relative motion of stationary objects in contact”. This can be particularly evident in valves that see limited or no position change. It typically is caused by the valves packing glands, seats, or the pressure exerted against the disk or other trim parts. To overcome stiction, additional force needs to be applied by the actuator, which can lead to overshoot and poor control.
- Tune your loop controller properly. A poorly tuned controller causes overshoot, undershoot, and hunting. Make sure your proportional, integral, and derivative values are set. This is quite easy today using controllers with advanced, precise auto-tuning features.
- Avoid oversizing control valves. They are frequently sized larger than needed for the flow loop they control. If the control valve is too large, a small percentage of travel or position change could produce an unduly large change in flow, which in turn can make stable control difficult. Unstable control can result in excessive movement and wear on the valve. Try to size a control valve at about 70%-90% of travel.
- Think about the type of control valve you are using and its inherent flow characteristic. Different types of valves, and their disks, have very different flow characteristics. The flow characteristic can be generally thought of as the change in the rate of flow in relationship to a change in valve position.
- Globe control valves have linear characteristics which are preferred, while butterfly and gate valves tend to have non-linear flow characteristics, which can cause control problems. In order to create a linear flow characteristic through a non-linear control valve, manufacturers add specially designed disks or flow orifices that create the desired flow profile.
These are just a few of the more significant criteria to consider when selecting and applying a process control valve. Consider it good practice to discuss your selection and application with a product application expert to confirm your final selection. Combing your process knowledge with their application expertise will provide the best outcome.
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