Using an incorrectly applied or sized control valve may have significant ramifications for operation, productivity, and, most importantly, safety. Here is a brief list of fundamentals to consider:
Control valves are not isolation valves:
Control valves do not isolate a process and do not offer a bubble-tight seal, and utilization in a shutoff capacity is unwise.
Choose the suitable materials for the job:
The valve body, seat, and wetted materials must all be compatible with the process under control. Before selecting a valve, evaluate the pressure ratings, operating temperatures, and material compatibility.
Sensor placement:
Place the flow sensor upstream of the control valve when configuring the control loop. When the flow sensor placement is downstream of the control valve, exposure to an unstable fluid (bubbles) created by the flashing and turbulence of the flow in the valve cavity is possible.
Control precision and mechanical constraints:
Consider the degree of controllability you need and the inherent Deadband produced by your valve and associated components. Deadband is the built-in movement that occurs in a control valve between the signal change and the direction of the valve, which exacerbates by worn or poorly designed couplings between valve and actuator, mechanical sensor tolerances, friction in the valve stems and seats, or an undersized actuator. Due to opening/closing oscillations, too much deadband leads to poor controllability (hunting).
Stiction:
Stiction is the "stickiness" in valve action induced by packing gland, seat, or force against the disk friction. It may happen if the valve sticks in one position for a prolonged time or is constantly traveling in a minimal range for an extended period. The actuator must apply more force to break the disk free, resulting in overshoot and poor control.
Tuning the loop controller and/or positioner:
A poorly configured loop controller or positioner is often the source of poor control and loop instability. Advanced auto-tuning capabilities in PI (proportional with integral), PD (proportional with derivative), and PID (proportional with integral and derivative) controllers have replaced human (often trial and error) loop tuning.
Valve sizing should be correct:
Control valves are often oversized, permitting maximum flow at just a tiny percentage of total travel. Minor adjustments in valve position have a significant impact on flow. A high valve-position-to-flow ratio promotes continual "hunting," which leads to excessive valve wear. A decent rule of thumb is to size a control valve at around 70% to 90% of its travel.
What sort of flow characteristics does your valve produce:
The flow characteristic of a control valve is the connection between the position of the valve disk, gate, or globe and the change in flow rate through the valve under normal circumstances. A linear flow characteristic is desirable. However, different valve designs have varying flow characteristics, some of which are linear and others that are not. Globe control valves have linear flow properties, while butterfly and gate valves have non-linear flow characteristics. Manufacturers will often create specifically shaped disks or orifices to "characterize" the valve's flow to improve linearity.
The above is a brief list of the most common things to consider when applying control valves. There are many other criteria to consider. It is suggested in the strongest terms to consult with an experienced application expert before selecting or using a control valve.
