Product category:
Fieldbus systems, Fibre-optic systems
News Release from: Fieldbus Foundation | Subject: Fieldbus Review questions to JR
Edited by the Processingtalk Editorial
Team on 18 March 2008
Fieldbus: device diagnostics make the
difference
In this interview, John Rezabek, chairman of the FF End User Advisory Council, describes developments in intelligent device diagnostics, and how fieldbus features help end users manage plant assets
Rezabek is employed as a process control specialist for ISP Chemicals in Lima, Ohio, and has extensive experience as an end user of Foundation technology FF: How have intelligent device diagnostics evolved over the years?
This article was originally published on Processingtalk on 18 Nov 2003 at 8.00am (UK)
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JR: In the early days, we pretty much just had old HART diagnostics ported over to fieldbus.
I remember looking at these diagnostics and wondering what value I'd get from them.
Even today there are diagnostics out there like "A/D Board Update Error" or "EPROM failure" and some devices still provide little else.
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We were promised diagnostics that would, for example, detect cavitation in an upstream pump.
Today, I know of at least three of the major transmitter manufacturers offering plugged impulse line detection.
In my mind, these capabilities are much more interesting and potentially useful to end users, as our problems are more in the application and process connection than in the device itself.
Some very interesting work has been done using statistical process monitoring and the "noise" component normally filtered out of the signal.
Applications.
At ExxonMobil and Shell have used this information to detect evolving abnormal conditions in fired heaters and FCC feed risers.
This sort of "Abnormal Situation Avoidance" is worth an order of magnitude or two more than the still-significant benefits one gets from the "total productive maintenance" sort, realized through predictive device wear and the like.
FF: How do Foundation fieldbus diagnostics differ from those of other protocols?.
JR: I'm not a user of Profibus, but my friend, Tanmoy Basu of Siemens, tells me there's little difference in the scope and range compared to Foundation fieldbus.
Profibus PA is a master-slave system, but it has no formal "user layer" like Foundation fieldbus, so host system control schemes may not be equal in how the diagnostic information is used.
HART has basic diagnostics and I know of at least one vendor that's porting plugged impulse detection and possibly Statistical Process Monitoring (SPM) to the HART side.
Users at neighbouring facilities, like me, have had challenges utilising the status information in control strategies, even though our systems have I/O cards that natively support HART communications.
You can see the diagnostics using asset management software or a vendor-specific snap-on, but in most cases, you'd better be looking at it when an issue arises.
If there's -20F wind chill at 3am and an impulse line freezes in 15minutes, it will be "old news" when you arrive in the morning.
Foundation fieldbus has a higher power budget and all devices on the network send their alerts and/or modify standard (and granular) signal statuses in the same scan (macrocycle) that the condition is detected.
Since Foundation technology has a user layer and standard function blocks, they all support changing signal statuses in the same way.
Fieldbus PID, for example, sheds mode to "manual" and holds last output by default, without any user-created logic.
Along with timely and configurable alarming, this affords an unparalleled degree of abnormal situation avoidance.
FF: In what ways are the technologies similar in terms of diagnostics?.
JR: Profibus PA has the same physical layer and the same power budget as Foundation fieldbus.
Thus, it only makes sense that vendors would offer the same diagnostics.
One vendor tells me that the same sophisticated diagnostics - in particular, an in-service "stiction" diagnostic - are available in their HART offering.
The main difference appears to be that the diagnostic must be requested or "scheduled" in the HART device.
I saw one Asset Management system that allowed the diagnostic to be scheduled, for example, once a day.
I'm not sure how long the diagnostic runs, but I'm sure one that runs continuously - like those implemented in Foundation fieldbus devices - is much more likely to detect an impending failure or poor performance.
FF: How do users typically employ current device diagnostic capabilities?.
JR: The most interesting thing I've seen is the work done at ExxonMobil and Shell, where SPM capabilities are being developed to detect flame instability in fired heaters, or poor fluidisation in an FCC riser.
At our site, we use simpler schemes.
We have a few applications where we've implemented plugged impulse line detection and SPM.
All of our valves in "critical" services have the in-service stiction diagnostic.
FF: What is the value of investing in Foundation technology over other choices, particularly as it relates to asset management, maintenance, etc?.
JR: I would say the technologies and concepts that form the basis for the benefits are what ARC refers to as the "object-based" nature of Foundation fieldbus.
The devices in fieldbus are autonomous, supporting "publish and subscribe" and messaging.
In the plant, that means signal statuses, diagnostics, and alerts are originated at the device and "pushed" to the host - no polling or master-slave transactions are required.
The Foundation fieldbus user layer was crafted by its originators to be a control system - not simply a device network for instruments.
What this means for asset management software packages and maintenance is that devices themselves perform diagnostics, alert the user, and store diagnostic and historical information independent of the host or a snap-on server.
Unlike buses that cater exclusively to AMS, fieldbus also functions as a control system, providing a degree of fault tolerance that legacy platforms would struggle to approximate.
Because all the device intelligence is interwoven with the signals interconnecting function blocks, our plants are less likely to end up "in the weeds" when abnormal situations, or more commonly, self-inflicted mishaps, come about.
FF: How can the cost of implementing fieldbus controls be justified in terms of increased uptime, better maintenance, greater availability, etc?.
JR: At Brownfield sites, I'd argue the "added" cost isn't that great.
If you really want your DCS to benefit from digital integration of field devices, there's a high likelihood it will need to be replaced.
Most of the DCSs of the 1990s do not, and will not, support anything beyond proprietary protocols.
If you're replacing your DCS and the I/O cards, why not install fieldbus? The installed base of legacy "smart" instruments may speak HART, but you'll need at least a board upgrade to get the most useful diagnostics currently being offered.
If you're building a Greenfield site, your degree of "future-proof" is higher with Foundation fieldbus.
I'm told even my valve positioners purchased in 1999 can be "flashed" to a version with the latest performance diagnostics.
Many of the devices being sold today are promising to support "flash upgrades" so your exposure to ripping out obsolete hardware is much smaller.
Foundation fieldbus was built from the ground up to function as a control system.
No other bus being applied in the large process industries can make that claim.
Consequently, the Foundation solution delivers robustness and fault tolerance with distinction.
Other schemes can approximate it, but only with extensive effort and custom programming on the part of the end user.
With Foundation fieldbus, it's built-in, which frees both basic and sophisticated end users to focus on greater value-added activities.
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