Voices: Rezabek

A Logical Path to Device Criticality

If You're Aiming to Improve the Usefulness of Your Digitally Integrated Intelligent Field Devices, There's Help Available to Help You Get Moving Down This Road

This article was printed in CONTROL's December 2009 edition.

John RezabekBy John Rezabek, Contributing Editor

Ten years ago, early adopters concerned about the real reliability of fieldbus technology devised guidelines for device criticality to optimize segment loading. In detailed design, the process control team assessed the potential of each device to cause an untimely process interruption. Those that would immediately shut down the process were assigned the highest criticality, and those whose failure would be relatively inconsequential, the lowest. The critical devices were segregated on lightly loaded segments and H1 interface cards, and those with low criticality were loaded on segments to the maximum practical number, with considerations for physical device location and function. The total number of control valves on critical segments was set as low as one. So we felt better, at least, that we would have less risk manipulating or maintaining a non-critical device and having an adverse impact on a critical control loop.

Experience has borne out, however, that H1 is so reliable that some users now load all segments more or less equally (e.g., aiming for 12 devices per segment). Transmitters and valves from non-critical services are combined on segments with the most critical ones, assuming the practical matters of proximity make it worthwhile. The latest draft of the Fieldbus Foundation AG-181 systems engineering guide has incorporated this as a recommended practice.

Criticality ranking may have some use, however, for getting the best value from device diagnostics. As instruments and systems are released that support NAMUR NE107 diagnostic alarm prioritization and routing, we're seeking a method for determining which device alarms get enabled and given a high priority. For example, setting the alerts for low instrument air supply on every valve positioner sounds like a great idea, until the night the whole header slumps, and your operator has to deal with potentially hundreds of redundant alerts. Some experienced practitioners are using the old criticality rankings to devise alerts, and pare the potentially vast number of device diagnostics down to the few that may be of real value.

For most reliability-focused users, the task of actually doing this ranking is a little daunting. It can prove challenging to assemble the right resources and people to devote their time to it. So, some consultants have appeared to help us.

For example, I have found the PlantWeb Services division of Emerson Process Management useful. I think users will find its methodology for arriving at a "maintenance priority index" (MPI) compellingly logical.

To rank a device, begin by dividing the plant into functional systems, for example, "steam system" or "boiler 1." Apply such metrics as the system's impact on safety, environmental compliance, product quality and throughput to get a "system criticality ranking." So, for example, one determines that boiler 1, which has no spare, has a relatively high system criticality compared to the instrument air system fed by redundant compressors.

Next, operations specialists assess the importance of the assets that enable them to keep the system on-line, in effect asking, "If I lose this, what will be the effect on the process?" So, in the case of the boiler, operations may assign a high "operational criticality ranking" (OCR) to boiler feedwater pumps or steam drum level instruments.

Following the derivation of OCR, the asset's "failure probability factor" is applied, which I'd read as "mean time to fail." So, an unreliable level instrument on the critical boiler will end up with the highest MPI. Such community-derived prioritization has some side benefits, among them the mutual acknowledgement of maintenance that can be deferred to planned maintenance.

If you're aiming to improve the usefulness of your digitally integrated intelligent field devices, there's help available to help you get moving down this road. Getting it right can make a measureable difference in the effectiveness of your maintenance efforts. 

More from this voice

Title

Save Money. Calibrate Less?

Have Our Calibration Skills and Practices Quietly Migrated to Being Largely "Plug-N-Play.”"Or Are They "Plug-N-Pray?"

04/03/2009

Selling Diagnostics to Management

Managers Are Paid the Big Bucks to Make Decisions That Many Times Boil Down to "How Much Insurance Do I Want to Buy?

04/05/2012

Selling Diagnostics to Management, Part 2

Dear Plant Manager, We See a Chance to Get Distinctive and Strategic Value From Intelligent Devices; Give Us a Chance to Prove We Can Do It

05/07/2012

Siemens–Not Just for Profibus Anymore

In PCS 7 We May Have the First SIL-Capable Logic Solver With Native Support for Foundation Fieldbus

06/06/2011

Simplifying Fieldbus Device Calibration

Creative End Users Have Been Exploring the Use of 802.11 Wireless to Display their DCS Interface on a Wireless Laptop or Notebook PC

08/12/2009

Smart Pipe--One Bus to Rule Them All

What Revolutionary Technology Is Coming Along That Will Kill Fieldbus?

06/05/2012

Surprise! Field-Based Control Beats DCS

It Is Evident That Device-Based Control Exceeds DCS-Based Control in Reliability and Performance

03/04/2010

The Island of Misfit Instruments

The Island of Misfit Instruments Could Become a Great Place for Learning and Help Shape the Future When the Aging Systems Will Be Replaced

10/05/2010

They'll Make a Better Software Fool

Because We're Working With Hazardous Processes, We Have to Think Through the Consequences of Every Errant Mouse Click

10/11/2013

Tinker with Defaults to Get Value From Smart Devices

Empower Your Staff So They Can Show Thoughtful, Proactive Use of Intelligent Devices, says Writer John Rezabek

07/24/2014

Tolerate less redundancy

Today, with Foundation fieldbus, the old redundancy paradigm no longer applies. Chances are, though, it isn’t free. So where should you apply it to achieve the fault tolerance you need?

12/05/2006

Training Wheels for Fieldbus

Even in Lean Times, There Are Ways to Get a Fieldbus Testbed If You Think Creatively

02/06/2009

Trunk Testing Tribulation

It's Challenging to Power Down Segments While the Plant is Down, Let Alone While a Process Is Up. Powering Down Is Not an Attractive Option

12/04/2012

Using Fieldbus in your HMI

Digitally Integrated Field Device Information Is Useful to Your Operator

10/06/2008

Using Your Best People

Getting to Know Those Working Around You. How Did They Become an "Instrument" Person?

03/02/2012

Want Open Standards? Work at It

End Users Have to Insist Their Favorite Suppliers Support Open, Interoperable Solutions, or We'll Remain Saddled With the Closed, Proprietary Ones

12/30/2011

We Get It - Wireless Works

Can Anyone Remember an Instrument Technology That Was Marketed With Such Persistence and Zeal?

11/01/2010

When to Use Control in the Field

Exploiting Control in the Field Is Never an All-or-Nothing Proposition

11/02/2012

Which Bus--If Any--for On-Off Valves?

Profibus PA and Foundation Fieldbus Give the End User Flexibility to Integrate On-Off Valves Wherever the Process Places Them

08/31/2011

Why Industrial Couplers Aren't Commodities?

Maybe We Should Ask If Couplers Can Be Procured on the Basis of Cost Only

02/26/2013