Is Field-Based Control Really All That?

Recent Studies Shown That the Fieldbus-for-I/O-Only Approach Is Likely a Source of Compromised Performance and Unknown Latencies

John RezabekBy John Rezabek, Contributing Editor

"Are you paying me to do the same thing we did on the last job, or is part of my job to seek out and recommend technology that provides substantial additional benefits?" That was the question I posed to my new plant manager when we were trying to choose a control system for his new chemical plant 12 years ago. Only a few years before that, I remember thinking there was no point in learning new systems. Our workhorse Honeywell TDC-3000 was so reliable, so pervasively applied across my employer's enterprises and so loved and trusted by our operations customers that I'd never be buying or implementing anything else. TDC had some bumpy years, but it was chiseled and honed by our peers in the large process industries, and after a few iterations was closing in on "bulletproof."

What I didn't know at the time was that another committed TDC user—Dick Lasher of Exxon Chemical—was busy chiseling and honing another control system: the function blocks and "user layer" of SP-50, which later became the basis for Foundation fieldbus (FF).

As appliers of technology, we turn over a lot of rocks in our careers. Once in awhile we find something shiny. Usually, these shiny objects aren't truly "golden," and as an early adopter of FF, field-based control struck me as neither shiny nor golden. I was convinced that digitally integrated, intelligent field devices were a superior choice to legacy 4-20 mA, but remained wary of using control in devices. It was only after applying it that I became convinced that single-loop integrity and smart, autonomous, certified devices implementing a robust repertoire of standard function blocks would exceed the reliability of our beloved, bulletproof TDC. Dr. Lasher and the other architects of SP-50 had imbued the fieldbus "user layer" with the same aesthetic forged into TDC and its peers—"reliability is king." And by taking pains to exploit the intelligence of microprocessor-based devices, control loops employing FF technology could and would surpass what was possible with the 1990s-era conventional DCS.

Today the use of control in field devices remains an unsettled issue in the minds of many users. What are the benefits and risks? What unforeseen hobgoblins await those who dare to make use of it?

Until recently, perhaps the best anyone could say was, "Well we've been using it, and there have been zero problems." Some giant sites like the Shanghai-area SECCO ethylene cracker complex have had thousands of instances of PID executing in field devices for years—and their projects were executed and completed in record time.

Despite such success stories, many conservative users choose the path of "fieldbus for I/O only," and leave all the control in the host PLC or DCS. Some recent studies have shown, not only is this a "zero gain" with respect to reliability, but also it is likely a source of compromised performance and unknown latencies, especially for loops that require better-than-two-second response times.

When it comes to reliability, few, if any, of our endeavors exceed the demands of achieving a SIL-rated Safety Instrumented Function (SIF) per the guidelines of ISA 84.001 and IEC 61508. A whole science has arisen for evaluating and deriving metrics for these functions, and one might ask, "Why can't the same metrics be applied to basic control loops?" So when we saw Ed Marszal of Kenexis at a recent conference, we posed the same question. Marszal and his company are in the business of providing LOPA, SIL and security analyses for companies govern by OSHA 1910.119 – Process Safety Management of Highly Hazardous Chemicals. At our request, Ed's agreed to use the mathematical tools at his disposal to compare loops implemented using fieldbus to those we do in a legacy DCS, and you may be surprised at the results.

Next month, we'll take a close look at the results, and some of the other recent studies comparing FF to conventional control.