The One Network

Our Intrepid ‘On the Bus’ Columnist Joins the Search for the Single Network Standard for Digitally Integrated Transmitters

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‘If It’s a Clone, Leave it Alone’

This is a saying I learned from a friend at a major engineering procurement contractor in Kansas City. With pressure from our project managers and the finance community that bankrolls them to keep costs down and accelerate the schedule, the desire to innovate with digitally-integrated field devices is sometimes deferred. I just received an estimate from a specialty valve manufacturer to retrofit a fieldbus-capable positioner to its product for more than three times what the same product costs me. That skid suppliers discourage their clients from deviating from the cookie-cutter product they like to sell is nothing new. Shell Oil overcame this for its Malampaya deep-water gas platform and pipeline, but many of us don’t have the kind of leverage Shell can bring to the table. Faced with these increased costs and delays, we often relent and take the default, usually bottom-of-the-line instruments.

There was a time when speed of execution for oil and gas production projects valued speed over innovation, but there’s evidence this is no longer the case. However when backlogs at the suppliers are high, and you’re told “that will add two weeks to the delivery,” you might be compelled to take the conventional alternative. No one gets fired for specifying 4-20 mA, at least not yet.

And when you’re the subcontractor trying to win the lube-oil skid job on another third-party supplier’s compressor, the diversity of bus technologies such as 4-20 mA and the lack of any global standard make it difficult to propose more than the rudimentary instruments you put on the last one.

We Can’t Maintain What We Have

On the other end of the spectrum from the 1st-quartile refinery are those whose staffs have been cut to the bone or those where attrition continues unabated with few new hires to fill the ranks. Weary baby boomers in their 50s and 60s aren’t anxious to add complexity to the alligator-filled swamp they pull into at 7 a.m. each business day. And, since adding fieldbus or native HART capability to your DCS means ripping out what you installed only 10 or 15 years ago, the cost and disruption to plant operations can be prohibitive.

As one end user reports, “One of the institutional roadblocks is having to install a new DCS, and if your DCS has lots of life left, then it’s very difficult to justify Foundation fieldbus.”

The pilots of our process plants—our operators—often aren’t anxious to learn any new tricks either. Finally, our host suppliers have at times been slow to adapt their products to support fieldbus and native HART. Even today, you can’t easily paste a new DCS that supports fieldbus or native HART to what you have on the ground. Anyone doing an expansion is faced with either installing the soon-to-be obsolete technology of the 1990s or essentially switching to a completely new DCS.

Another end user, now in the throes of appending his DCS vendor’s new system to the old, says that, “If you’re planning to upgrade your DCS, don’t assume your current vendor is the best choice just because you have a good track record with them. Open systems are destructive technology, and it’s not surprising that some vendors have a hard time adapting.” Staying loyal to the supplier of your legacy installed base may be as painful as a new DCS.

Profibus Serves “Breakfast, Lunch and Dinner”

Before the fieldbus wars, German automation leaders asked for a standard digital bus for discrete settings such as automobile manufacturing. Profibus was created as an upgrade from Modbus and a proprietary protocol for integrating discrete devices. Lots of “purple hose” was installed, and North American users of Texas Instruments’ legacy 505 and 545 PLCs found many uses for the gaily colored cable as well.

When real fieldbus solutions for the process industries, like WorldFIP and ISP, began to gel, Siemens and other European PLC makers that supported what we now call “Profibus DP” already had a large installed base. When it became apparent that Foundation fieldbus would likely mean drastic changes for their DP users, suppliers like Siemens went their own way and introduced Profibus PA.

PA integrates with Profibus DP, and PTO spokesman Carl Henning adds it isn’t that different from DP. In fact, the protocols are identical, and only the physical layers are different. It shares the same physical layer and baud rate as Foundation Fieldbus H1, but doesn’t provide control in the field.

As my friend at Siemens likes to say, Profibus offers “breakfast, lunch and dinner,” or solutions for discrete devices (DP), process I/O (PA) and Ethernet (Profinet). This is unlike Foundation fieldbus, which offers two (H1 and HSE). Also, HSE’s implementation by the supplier community has been limited to Smar and ABB. Curiously, these are the same two suppliers that offer native support for Profibus DP and PA.

In terms of applications, the Profibus  end-user community is largely defined by the host systems that support it. Besides suppliers to discrete parts makers such as Siemens and Schneider Electric, only ABB, Invensys and Smar and its clones offer native support for PA. The user base is shaped along the lines of “hybrid” process plant PLC applications, including breweries, pipelines, batch chemical, pharmaceutical and ethanol plants.

So, if you ask someone in Fluor’s or Bechtel’s offices in Houston how much Profibus PA is going into their clients’ projects, the proportion is smaller than Foundation fieldbus. In fact, speakers representing Bechtel at a recent conference in Antwerp estimated that roughly 50% of new projects are installing Foundation fieldbus, while the remainder is mostly conventional and/or HART. Until DCS suppliers such as Honeywell, Emerson and Yokogawa have proven applications using PA, interest in Profibus by the large process industries may be limited to DP, mainly for motor control center integration. “But, globally, the protocols are on par,” adds Henning. While Profinet was positioned to integrate other protocols, including Foundation fieldbus, many large suppliers are content to provide their own, proprietary system-level networks (Figure 1).

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