Control Valve Impact on Process Variability

The March/April edition of Intech magazine features an article by ModelingAndControl.com‘s Greg McMillan–Key design components of final control elements. The article is derived from Greg’s new book, Essentials of Modern Measurements and Final Elements in the Process Industry.

Final control elements, such as control valves, are used to control the process by changing the flow. Greg notes:

By far, the most common final control element is the control valve, with its attendant positioner, actuator, and other components.

Other final elements include variable speed pumps, compressors, blowers, fans, and dampers. In some cases, using a variable frequency drive (VFD) instead of a control valve with its associated pressure drop can save energy:

…particularly in large utility flow applications…

Greg defines the term, backlash, as:

The amount of play (looseness or gap) in the connections between the shaft, stem, and closure component…that creates deadband and determines, in part, how well the valve will respond to small changes in signal.

He describes shaft windup as the result of rotating a stem or shaft in a ball or disk valve that has excessive seal friction.

Sticktion is the case where the control valve sticks for a period before moving to the requested position. In the article, Greg summarizes the effect of sticktion and backlash:

For small changes in valve signals, the resolution limit from sticktion and deadband from backlash that prevent a good response and create a sustained oscillation (limit cycle) are observable. Current test methods established by the ISA-75.25.01-2000 (R2006) standard address the effect of step size on response.

Greg showed these oscillatory effects in a recent deminar (demo/seminar), PID Control of Valve Sticktion and Backlash.

Control valve backlash, shaft windup, and/or sticktion can impact the process by increasing variability. A loop in automatic trying to compensate for these conditions oscillates:

…from the continuous hunting of integral action to find a position it cannot attain exactly.

Greg notes that sliding stem (globe) valves are least affected by these conditions. Digital valve controllers, also referred to as smart digital positioners, help to reduce these issues with fast-feedback control and built-in diagnostics.

Greg suggests some best specification practices for your control valves. If you’re looking at replacing or adding control valves to your plant, be sure to read the article to see these best practices as well as things to think about with variable speed drives.

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