7 Ways Fieldbus Improves Operator Confidence and Performance

Notable contributor to Control magazine and process control specialist, John Rezabek presented on 7 ways the Foundation fieldbus technology improves operator confidence and performance. His abstract:

Using the abundant insights available from digitally integrated field devices, deterministic control-in-the-field, advanced diagnostics, and DeltaV graphics capabilities, operators can quickly assess abnormal situations and take actions that prevent process upsets and shutdowns. With increased confidence in the health of instrumentation and validity of measurements, operators are confident to push constraints and keep the process optimized.

John opened asking if fieldbus is useful to operators and would they even notice if it was taken away. The mission of production is to produce safely, reliability, surpassing quality requirements, with low environmental impact, efficiently, and without unplanned downtime.

Operators want a smooth day without drama and difficult situations. John thought operators would miss 7 things. The first is robustness–not brittle. Does the operator feel comfortable running closer to constraints. Fieldbus devices are supplied with robust power, digital communications means its working or its not, signal status is updated every macrocycle, signal status propagates through loop and mode sheds to manual and holds last value.

John noted that devices and systems report issues in real time, valve position feedback to provide where the valve going and if it is stuck. Also at the physical layer – the wires themselves – can be fitted to detect and report issues in real time.

Stable, optimized tuning from PID loop tuning and running control within the device means the loop operate in a robust manner giving operators confidence. John showed an example using statistical process monitoring in a pressure transmitter to detect when an abnormal condition such as a purge valve not in the right position.

Giving the operators confidence in the robustness of their loops allows them to operate closer to constraints for higher efficiencies.

Operators can review simple diagnostics to gain insights. This helps solve issues process anomalies, such as plugged impulse lines.

Simplified path to extend automation is where the requests an operator makes can be easily accomodated, such as changing a manual valve over to automatc.

John summed up what this confidence operators have in the robustness of their process. Operations has exceptional trust in the “truth” of what their measurement & control system is telling them. While not always happy about pushing constraints, reliable measurement and tight control facilitate their efforts. And, crews have great autonomy and need less support and oversight

2 comments

  1. Jonas Berge says:

    Great material. I have a few personal favorites of my own:

    The real-time status provided by fieldbus enables process problems to be distinguished from device problems, thereby minimizing nuisance shutdowns of the control loop on sensor failure.

    Transmitters are digital, controllers are digital, and positioners are digital. I personally believe it doesn’t make sense to run analog signals between them. Fieldbus does not rely on 4-20 mA which means totally digital closed loop that is and time-synchronized from end to end, from sensor to actuator. Eliminating 4-20 mA I/O cards and their associated scan delays and jitter improves control response time which in turn results in tighter control, particularly compared to remote I/O with additional network and link delays.

    Doing away with 4-20 mA also does away with multiple D/A and A/D conversions. This leads to higher resolution and accuracy. Errors due to current calibration differences between transmitter output and DCS input are also done away with. This is particularly important in flow and level applications such as tank gauging. The “on-scale” errors associated with 4-20 mA signals are eliminated too.

    Fieldbus gets rid of the process variable value skew that sometimes results from mismatched 4-20 mA range settings between transmitters and the control system. Measurement values are also transmitted over the full sensor limit, not limited to a narrow 4-20 mA portion.

    Diagnostics not only detect outright failures, but predictive diagnostics can be used to prevent problems by reporting when ambient conditions are exceeded etc. – in real time.

    Diagnostics is not just for transmitters and control valves, but also the other half of the instrumentation: discrete devices like intelligent two-wire fieldbus on/off valves and electric actuators / motor operated valves (MOV) using fieldbus also provide diagnostics. NAMUR NE107-compliant device alarm rationalization notify the right person without alarm flooding.

    More devices can be added to the existing bus without laying homerun cable or using more I/O cards

    These operation & maintenance benefits as well as project benefits are covered in the FF brochure:
    http://www.fieldbus.org/images/stories/technology/aboutthetechology/overview/fieldbus_brochure.pdf

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