Incorrectly applied or sized control valve may have significant ramifications for operation, production, and, most importantly, safety.
Here is a brief checklist of things to think about:
- Control valves are not isolation valves. Don't apply them as such.
- Always carefully choose the suitable construction material. Consider the valve components that come into contact with the process medium, such as the valve body, seat, and other "wetted" portions. Consider the control valve operational pressure and temperature exposure. Finally, examine the surrounding environment and any corrosives that may arise and affect the outside of the valve.
- Place the flow sensor upstream of the control valve. When the flow sensor is downstream of the control valve, it senses a less stable flow stream created by turbulent flow in the valve cavity.
- Consider the degree of control you need and ensure that your valve's design will provide the controllability you require.
- Excessive dead-band leads to hunting and poor control. The change of input signal necessary to induce a change in valve position is dead-band. Dead-band is affected by worn or loosely fitting mechanical linkages or a controller setting. It may also be affected by mechanical sensor tolerances, friction in the valve stems and seats, or an undersized actuator.
- Think about stiction. Stiction is the propensity of valves that have had very little motion or have not moved at all to "stick." It is usually produced by the valves, packing glands, seats, or the pressure applied to the disk.
- Make sure your loop controller is appropriately tuned. Overshoot, undershoot, and hunting are symptoms of a poorly tuned controller. Check your proportional, integral, and derivative controller settings. Today, employing controllers with powerful, accurate auto-tuning capabilities that have replaced the old-fashioned trial and error loop tuning approach, is relatively simple.
- Avoid oversizing your control valve. Control valves are usually oversized for the flow loop they control. A tiny change in valve position has a considerable influence on flow. If the control valve is too large, only a fraction of its valve travel occurs, causing the valve to hunt, leading to severe wear. Always size a control valve to allow 70-90 percent of travel.
- Consider the kind of control valve you're employing and its inherent flow characteristics. Flow properties vary greatly across valve types and their disks (or profiles). The flow characteristic is the change in flow rate in response to a change in valve position. The linear features of globe control valves are preferable, while the non-linear flow characteristics of butterfly and gate valves might pose control issues. Manufacturers add specifically designed disks or flow orifices that provide a custom flow profile to establish a linear flow characteristic via a non-linear control valve.
