Alarm Management and DCS versus PLC/SCADA Systems

Aaron Doxtator’s First Question

I am working on a project that I believe many other sites may wish to undertake, and I was looking for best practice information.

  1. Identify the need for a state-based alarm;
  2. Perform a risk review/MOC process with all stakeholders;
  3. Implement the changes; and
  4. Document the state-based alarms.

Nick Sands’ Answer

ISA has a series of technical reports that give guidance on implementation of the standard. TR4 is on Advanced and Enhance alarming, including state-based alarming.

  • Use redundant indications of states to minimize state-based logic failures.
  • Be cautious about suppression of alarms that can indicate the transfer of material or energy to an undesired location (example is high temperature alarms on columns, high levels on tanks…)

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Darwin Logerot’s Answer

In my experience, there are very few if any chemical or refinery units that would not benefit from state-based alarming (SBA). The basic problem is that most alarm systems are configured for only a single process condition (usually running at steady state), but real processes must operate through a variety of states: starting up, shutting down, product transitions, regeneration, partial bypass, etc. In these situations, alarm systems can and will produce multiple alarms that are meaningless to the operator (nuisance alarms). Alarm floods are the natural result. Alarm floods can be problematic in that they tend to distract the operator from the more important task at hand, can be misleading, and can hide important information.

  • SBA at its core is a relatively simple concept — determine the current operating state of the process or system, then apply appropriate alarm attribute modifications. But, as in many situations, “the devil is in the details”. So, best advice is to consult with a knowledgeable practitioner before embarking on a SBA project.
  • Apply SBA only to a well-rationalized alarm system, or in parallel with a thorough and principles-based rationalization.
  • Apply state transition techniques to prevent nuisance alarms when the process transitions into a running state.
  • Utilize commercially available software for SBA, rather than trying to develop custom logic and coding in the control system.
  • Another available resource is the Alarm Management chapter in the Instrument and Controls Handbook recently published.

Greg McMillan’s Answer

I suggest you check out the Control Talk columns with Nick Sands Alarm management is more than just rationalization and Darwin Logerot The dynamic world of alarms.

Aaron Doxtator’s Second Question

While most of my experience has involved using PLC/DCS (or just DCS) for plant control, some clients have expressed interest in shifting away from using a DCS altogether and utilizing exclusively PLC/SCADA. Aside from client preference, are there recommendations for when one solution (or one combination of solutions) may be preferred over the other?

Hunter Vegas’ Answer

I wanted to address your question about DCS versus PLC/SCADA. Historically DCS and PLC systems were very different solutions. A DCS was generally very expensive, had slow processing/scan times, and was specifically designed to control large, continuous processes (IE refineries, petrochemical plants) with minimal downtime. PLCs boasted very high speed processing, were designed for digital IO and sequencing, and typically utilized for machine control and smaller processes. Over the years both DCS and PLC manufacturers have modified their products to expand into that “middle ground” between the two systems. DCSs were made more scalable (to make them competitive in small applications) and added extensive sequencing logic to make them better suited for digital control. At the same time PLCs added much more analog logic, the ability to program in function blocks and other languages, and began incorporating a graphical layer to make them look and feel more like a DCS.

Greg McMillan’s Answer

Most of the PID capability I find valuable in terms of advanced features most notably external-reset feedback and enhancements to deal with large wireless update time and analyzer cycle times are not available in PLCs. The preferred PID Standard (Ideal) Form is less common and multiple PID structures by setpoint weights for integral and derivative modes and the ability to bumplessly write to the gain setting may not exist as well. Some PLCs use the Parallel or Independent Form that negates conventional tuning practices. Even worse, computation of the PID modes in a few PLCs uses signals is in engineering units rather than percent of scale leading to bizarre tuning requirements.

For Additional Reference:

Bill R. Hollifield and Eddie Habibi, Alarm Management: A Comprehensive Guide

Additional Mentor Program Resources

See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).



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International Society of Automation - ISA Official


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