Control Valves–Running out of Control

Companies spend millions of dollars automating their plants for a variety of reasons including increased productivity, improved quality, reduced human error, reduced manufacturing time, and better safety.

Why, then, are as many as 30 percent of control valves running in manual mode? There are some relatively simple things that can be done so that manufacturers can get more out of their automation investment, such as:

  1. Understand what you currently have—Engineering and maintenance need a solid understanding of what is installed in the plant and how it affects their process. Daily activities should be prioritized accordingly. Organize your control loops into the following categories:
    • Process Critical—loops that directly control production rate
    • Performance Critical—loops that influence critical loops
    • Safety Critical—emergency shutdowns or safety management
    • Environmental Critical—loops that have an impact on regulatory agencies
  1. Proper sizing of equipment—If your valve is too small or too large, control cannot be achieved. Typically, a control valve should operate somewhere between 60–80 percent open at the maximum flow rate.
  2. Run loops in automatic—Loops are placed in manual for a variety of reasons, but it should only be done on a temporary basis.
  3. Calibrate instrumentation—You are only as good as the input you receive. If the instrumentation is giving you inaccurate data, then control will never be attained.
  4. Proper sampling times—Here are a few guidelines:
    • Flow: 1–2 seconds
    • Level: 1–5 seconds
    • Pressure: 1–5 seconds
    • Temperature (fast acting): 5–15 seconds
    • Temperature (slow acting): 15–30 seconds
    • pH, Concentrations: 5–30 seconds
  1. Loop enabling—Disable loop control when appropriate, i.e., cleaning.
  2. Preventive maintenance—Prevent leaks and stiction. A loop cannot be controlled efficiently without working seals and limit switches that function properly.
  3. Know when to retune a loop—Here are reasons you may retune a loop:
    • Procedural changes
    • Equipment changes
    • Material changes
    • Seasonal changes

There is much to be gained by optimizing control valves and your automation investment.

I’d love to hear what you think about this blog post. Let me know in the comments.