A field network for process gas chromatographs is indispensable if you have two or more. Why? A typical process gas chromatograph is microprocessor-based with multiple analog outputs—so why consume multiple 4-20 mA analog inputs on your DCS if you can bring all the data in over, say, Modbus?
The same is true of current process spectrometers. If there are issues, one can view chromatograms or spectra from a safe climate-controlled control house or shop location, change or run calibrations, and do extensive troubleshooting, if issues persist. Even the relatively simple pH analyzer can be examined—if digitally connected—to better assess its health or issues that must be addressed, before striking off, getting permits, advising operations, and putting loops in manual prior to maintenance. Field networks integrating microprocessor-based devices are a great aid to maintainers, or at least those who are knowledgeable and motivated to use them.
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If you’re familiar with this boost to productivity and reliability, why not do the same for the remaining field devices? Practically all of them have microprocessors, and many—even temperature transmitters—have multiple variables. That’s why several end users deployed Foundation Fieldbus (FF) and Profibus PA (PA). While savings copper (less wire), labor (fewer terminations), and rack room real estate, such economies were obscured by other project costs, such as the cost of other instrument and electrical infrastructures. But after startup, many fieldbus champions were discouraged by the light use of field diagnostics, and many asset management consoles collected dust. What was missing? Unlike analyzer techs, who were brought up on digitally integrated analyzers, instrument techs’ work habits were already well established, and leadership to provide knowledge and motivation was absent or ineffective.
"Field networks integrating microprocessor-based devices are a great aid to maintainers, or at least those who are knowledgeable and motivated to use them."
Why do I bring this up (again)? End users must have a solid plan—and a record of successes—for utilization of field-device intelligence. Once the needed FF proxy switch technology is available, migrating FF to Ethernet-Advanced Physical Layer (APL) will take conviction and investment. How much cash remains to be seen. Having recently acquired some “managed” commercial and industrial-grade switches, you’re looking at costs on the order of a big-screen MacBook Pro (each)—just for a switch. If one’s field junction box houses a couple APL switches, the installed cost could easily be well into five figures per box. Last year’s FieldComm Group Technical Recommendation, FCG TR10365, warns that existing junction boxes could get too hot, especially if local power supplies are added to ensure the switches have clean, redundant DC power. Unlike FF and PA, this would involve bringing additional AC circuits—ideally from diverse AC power distribution infrastructures—into every APL field junction box. I imagine a gradual migration of FF/PA devices to APL devices, as the legacy devices fail due to old age or become unreliable. APL devices could consume up to four times more power. A model favoring redundant field DC power may preempt providing power over existing 18 AWG fieldbus trunks.
End users may hope for a migration less burdened by cost and complexity. Meanwhile, we put our supplier community in a predicament. The market for FF-to-APL switches will not likely take off like a rocket until FF sites are under the gun—when a fieldbus device fails and no replacement can be obtained, even on eBay. Perhaps our infrastructure suppliers should start a side business, buying and refurbishing FF and PA field devices as they’re exchanged for APL instruments. But it’s a good bet that an FF or PA device purchased today will still function, barring extreme abuse, past 2045 or even 2050.
If we’re not already in an enterprise actively exploiting device intelligence in a disciplined way, and documenting our victories, such as minimizing the time techs are out in extreme climate conditions or hazardous areas, we must start now by providing needed leadership, knowledge and motivation.
