Certainty of Outcome with Fieldbus

What Are Some of the Key Areas Where Effort and/or Investment Are Needed to Obtain Sufficient Certainty of Outcome for Even the Smallest Project?

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

John RezabekBy John Rezabek, Contributing Editor

David Lancaster, former chief engineer at Bechtel (now retired) and instructor for Trine University in Angola, Ind., has spent some time pondering “certainty of outcome.” As the consultant on fieldbus projects involving tens of thousands of instruments, with engineering happening in three continents and a half-dozen time zones, there’s not a lot he doesn’t know about getting certainty of outcome. Certainty of outcome is important on any project, but delivering it isn’t free. What are some of the key areas where effort and/or investment are needed to obtain sufficient certainty of outcome for even the smallest project?

Training for one. That’s one of Lancaster’s mantras. Even though fieldbus is uncomplicated, it is different, and the liabilities or “gotchas” can have consequences. During the execution of a large project, stakeholders needing training will include your client, engineering and design disciplines, including I&E, controls and possibly process specialists. The installer/constructor, once chosen or available, may well need training as well. What this accomplishes, among other things, is ensuring that everyone speaks the same language, understands how to apply the rules or guidelines of the job and has an opportunity to express his or her concerns about delivering certainty of outcome for their scope of work.

Engineers and designers need guidance from process specialists earlier in a fieldbus job to understand where sensors and valves are likely to end up on the plot plan. Foundation fieldbus needs all the devices involved in executing a loop on the same segment, and one typically can’t wait on the piping designers to identify every process tie-in and control valve before commencing segment design. Process specialists also can give guidance on loop criticality, so end users’ guidelines for segregating highly critical loops can be accommodated. Designers need to know what the field will need to deliver a quality installation. For example, segment topology can be simple (“star” or “chicken foot”), more complex (multi-drop trunk), or a hybrid. Installers relying on local skills in regions where craft availability is low may lobby for a simpler design, even if it means more labor or materials. Clients can dream of saving money by eliminating a junction box drawing for fieldbus, but it may end up costing them when the installer needs daily guidance to interpret segment drawings.

Even though fieldbus is uncomplicated, it is different, and the liabilities or “gotchas” can have consequences.

With Foundation fieldbus, end users can choose whether to employ control in field devices, control in the host, or both. This can influence host procurement, since some hosts don’t support control in the field, and those that do sometimes don’t support every function block that’s offered in a device. Despite certified interoperability, host systems are known to play more nicely with their same-branded brethren than with devices from competitors, which can also affect the availability of specific function blocks or block functionality. The sooner this sort of information is known the better because  there’s nothing like the procurement cycle to get promised functionality in writing.

Training can also bring to light diagnostics capability. Users can set alarms or alerts on scores of diagnostic parameters, so the end user can save a lot of effort by having them delivered with the preferred settings preset by the factory, conceivably saving weeks or months of implementing the proper settings on every device.

Working on live segments in a running plant has great potential for creating an undesirable outcome. Have you re-run segment calculations to ensure the spur lengths and segment power are sufficient? Will the addition of another few function blocks make the required macrocycle longer? You need answers to these questions.

We can deliver certainty of outcome. Typically, it is not an investment our management is averse to making. With the proper training, communication and execution, practitioners of fieldbus can be certain that their outcome will be a positive one.

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