Field Automation and Compressed Air System Mistakes Tips

Michel Ruel, a frequent source on process control improvement in both my Control Talk Blogs and Columns, offers his concise list of the more memorable mistakes made with actuators, compressed air systems, transmitters, and variable-speed drives. Many of these are mistakes could have been avoided by just being aware of the potential problem. The solutions are straightforward but are much more expensive to correct "after the fact."

Here is Michel's list:

1. Transmitters

  • Transmitter installed too far increases dead time; location based on easy access for maintenance rather than process control common sense
    - Worst real case, a density transmitter installed 2000 ft away for maintenance by plant personnel A in lieu of about 10 ft at plant B
  • Flowmeter installed too close to valves or elbows
    - E.g. a Vortex meter between 2 elbows
  • Magnetic flowmeter installed on a horizontal pipe
    - Should be on a vertical or at least where pipe is always full
  • Level transmitter installed at the bottom of a reservoir
    - Should be slightly higher to avoid sediments into the transmitter
  • Level transmitter with wrong zero adjustment
  • Level transmitter sensitive to froth e.g. capacitive

2. Compressed air

  • Air lines for instruments (control valves) without filters and water collectors
  • Air lines with slope in the wrong direction, bringing water to the instruments
  • Compressed air with high dew point; air dryers not maintained properly
    - E.g. With piping outside in North Canada (temp below -40 °F but dew point about -30 °F)
    - E.g. no control of dew point, no maintenance of air dryers
  • Mixing instrument air and process air
  • Air supply pressure to the valve too low
    - E.g. design is 90 PSI, air supply is 80 PSI
    - Warning, some Digital positioners are very sensitive to air supply
  • Valve-positioners without pressure regulator

3. Valve-Positioner-Actuator

  • Positioner configuration
    - E.g. linear actuator configured as a rotary
    - Error in stoke
  • Actuator not strong enough
    - A buyer mistake (to reduce costs)
    - A designer mistake, not enquiring for line pressure
    - Air supply pressure too low
    - Bad design

4. Variable speed drive (VSD)

  • Valve and VSD on the same line using the same signal
    - A real bizarre case! Air flow to a burner; a fan with a variable speed drive in series with a valve, both receiving the signal from the flow controller
  • VSD on a pump with low limit too low; not enough static pressure to fight column of liquid

Michel Ruel has been a great source of expertise as seen in Control Talk Columns "Show Me the Money" - Part 1 and Part 2.

Michel is joined by Greg Shinskey and Jacques Smuts in the Control Blog Series “How can we provide Mechanical and Process Engineering Regulatory Control Guidance” - Part 1, Part 2, Part 3, and Part 4.

Most recently Michel shared his Fuzzy Logic Control success story in "Ruel Rules for the Application of PID, MPC, and FLC"

Michel Ruel is Department head, Optimization and Advanced Control (Top Control) and Director Expertise Centre - Quebec City Partner BBA

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  • <p>why is compressor air flow measured in Nm3/min and not in m3/min</p> <p>How to convert m3/min to Nm3/min to select my flow meter</p> <p>A compressor discharge flow at its outlet can be nm3/min,but if the same flow is measured say,at the header of 3 compressors working together,would this flow be in nm3/min again.? I doubt that the flow is not at STP in header,how to make the calculation of flow at this point</p>


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