Improved Fieldbus Operations with Advanced Diagnostics

Eastman Needed Intrinsically Safe Fieldbus with Maximum Segment Capacity in a Harsh, Hazardous Environment. FISCO and Advanced Diagnostics Was the Way to Go

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By Chris Williams, I&C Engineer, Eastman Chemical Company

Eastman Chemical Company in Columbia, S.C., uses Foundation fieldbus technology and DeltaV hardware throughout one of the plants at our facility. This system has functioned so well that Eastman decided to install fieldbus technology in a second plant. My challenge was to figure out how to install a similar Foundation fieldbus system, but in very different environment.

My Challenge

Our second plant has many Class I, Division 1 and Class1, Division 2 areas with processes that are not only hazardous, but extremely corrosive. Traditional installations required use of rigid metal conduit, explosion-proof fittings and poured seals, or the use of intrinsically safe circuits. Traditional, intrinsically safe Foundation fieldbus segments typically have some amount of signal loss and only permit a maximum of four devices with short trunks/segments. Calculations are required to prove all components will work together to form an intrinsically safe circuit. We wanted to install a system that would function reliably in a harsh and hazardous environment and maximize segment capacity. Also, it was important that our personnel be able to work on the instruments in the field live without having to gas-check the area first.

FISCO (Fieldbus Intrinsically Safe Concept) provides a way around the limitations of normal intrinsic safety. It makes more voltage and current available in hazardous locations—12.8 V and 115 mA. Trunk and spur lengths are 1000 m and 30 m respectively; it verifies safety, system certification is not required, and FISCO avoids having to segregate live-workable and non-live-workable parts of the network in the field junction box.

One of the hesitations I had in bringing fieldbus technology with restricted voltage into the plant was that a system can be clean when it goes in, but with our corrosive environment, connectors get dirty and boxes start to fatigue. Rust and corrosion degrade communication over time and lead to network instabilities. These conditions also create additional resistance that results in voltage drops. Random and intermittent communication problems are hard to pinpoint and eliminate. Usually you find them only after the plant goes down. I wanted to be able to find those problems before they caused a shutdown. Also, I wasn’t sure if 12.8 V would be enough to compensate for corrosion over time, given that the fieldbus devices require at least 9 V to operate.

A Possible Solution

In an effort to find the best solution to our challenge, I attended a seminar at Robert E. Mason Company in Charlotte, N.C., where I first saw a presentation about the Pepperl+Fuchs Advanced Diagnostic Module (ADM). According to Pepperl+Fuchs, the ADM would not only save time during startup and commissioning, but it also would detect degrading conditions or faults occurring on fieldbus segments. Any change in the installation, such as those caused by corrosion or those unnoticed by the operators, would be detected before the change becomes critical to plant performance.

The ADM is part of Pepperl+Fuchs’ FieldConnex product line for fieldbus communications. FieldConnex allows you to design your fieldbus topology specifically for your application. While the fieldbus transmits the data, FieldConnex provides the infrastructure: power supplies, installation equipment, and accessories. The High-Power Trunk concept (HPT), together with FieldBarriers, provides a new approach to creating intrinsically safe fieldbus segments. The entire installation is continuously monitored by the ADM integrated in the FieldConnex power hub.

The High-Power Trunk

Rather than limiting the amount of energy on the fieldbus trunk cable to intrinsically safe or nonincendive levels, the HPT limits the energy on the spur connections to the instruments. Using FISCO with the HPT, the energy on the trunk is increased to 500 mA rather than 115 mA. This increases the amount of energy available for field instruments and facilitates a consistent installation design regardless of the area classification. By limiting the energy in the field with a FieldBarrier rather than in the control room, power is more efficiently distributed to the instruments where it is required. As a result, the segment protection devices are similarly installed for hazardous or general-purpose applications. Consistency, cable runs and cost savings are all increased with HPT.

Another benefit of HPT allows users to standardize on one power conditioning system with optional redundant modules for all areas of the plant. Supplying 30 V/500 mA allows users to achieve maximum cable lengths and maximum loading without using repeaters.

FieldBarriers

FieldBarriers are the core of the HPT. They are mounted near the field devices to provide short-circuit protection as well as energy limitation for explosion protection. They guarantee that the segment remains in operation even during a fault condition on a spur. The HPT expands the FISCO validation methods by considering each spur connection separately. FieldBarriers can be daisy-chained on the trunk and enable the field devices to be serviced without the need for a hot work permit. And they can be used in FISCO, FNICO, hazardous and general-purpose installations.

The Installation

When we brought our original plant system online, we did it using the standard method: pulling the trunk cables apart, performing resistance and capacitance checks. When we had a problem, the technicians went out into the field and determined whether it was coming from the trunk system, spur cables or the instrumentation. It was rather time-consuming to get the bugs out of the system at startup. Yet once we got it online and commissioned, it worked very well. The instrumentation for Foundation fieldbus eliminated the need for technicians to be out in the field doing calibration checks, putting in the scaling factors inside the transmitter, and validating the 4-20 coming back. Startup was extremely fast. In just one day we commissioned and brought over 40 instruments on line.

For our second plant installation, we were told that we didn’t need to go through that same process. Supposedly, with advanced diagnostics, we could shoot our networks and scan them before we hung the instrumentation. After hanging the instruments, we could scan the network again and compare the noise levels, power draws and the health of the network.

I wasn’t convinced this would work. I do know that installation practices have a significant effect on the quality of communication. So we were very careful to install our system perfectly. We installed the first four segments and did all the initial checks: we walked down all the bricks, walked down all the terminations, did the resistance and capacitance checks, conducted a manual scope sweep, and we checked each instrument with a handheld configurator. Then we used the ADM to validate all of that information—and it matched perfectly.

The Advanced Diagnostic Monitor

During checkout, the ADM even detected a problem on one of our bricks that the handheld configurator did not catch. It was over-terminated, but not to the extent that it violated the minimum point where Foundation fieldbus wouldn’t run. We still had a running system. We weren’t getting any communication errors, yet the network was deemed unhealthy by the ADM. We had failed to turn off the built-in terminating resistor on one of the bricks. My confidence in advanced diagnostics was starting to grow.

The next system we brought on-line had eight segments. This time we made sure that the terminators were right and the trunk cables were terminated correctly. After we determined that the segments were good, I used the advanced diagnostic monitor to commission all eight segments. It took only 20 minutes and actually found two problems that my field technicians missed―one where they forgot to turn on the termination at the end of the segment trunk, and another where the polarity was reversed. The errors were corrected in ten minutes, and the segments were back up online and completely healthy.

After that, we hung 40 instruments on the segments and used only the diagnostic system to validate. Not one had a problem. In fact, even our longest run―1500 feet―came on line with no problem as quickly and as easily as the run that was five feet from the cabinet.

Currently we have six different areas in my facility where we’ve installed Foundation fieldbus using the High-Power Trunk concept. We’ve stopped using the handheld, and we no longer haul the oscilloscope out to the field. Once the system is installed, I do the sweep remotely, and we are ready to go. I have total confidence in the ADM.

Even More Features

The diagnostic system also provides a commissioning wizard, alarming and data logging, and a built-in oscilloscope for displaying in-depth detail of fieldbus signals. Once all the instrumentation is on the network, the software allows me to enter the tag names for each node address manually on a particular segment. During troubleshooting, it’s easy to see which instrument has the problem without ever going out into the field. The commissioning wizard is very easy to use and very intuitive.

The diagnostics feature is equally reliable. It measures device signal level, proper termination and cable continuity. It provides a baseline report and comprehensive system documentation. The baseline is continuously compared against actual values and if there’s a problem, it tells me exactly what the problem is, indicates possible causes and then offers possible solutions. Best of all, I’ve never had to go out into the field to diagnose a problem.

The physical layer diagnostic module captures snapshots of all values and can store that data for up to two years for long-term trending. Because it has the ability to create a visual representation of the digital fieldbus communications coming from the control room, it truly provides an “extra set of eyes” on communications, making you aware of a problem before it arises. As a matter of fact, we were having trouble with a valve, and the manufacturer tried to tell me that it was because I had an unhealthy network. I showed him some of the screen shots from the software—he’s never mentioned the health of my network again.

Another neat feature of the system is the remote software. When I commission the segments, I do it remotely, from the comfort of my office. I don’t need to be out in the field fumbling with my laptop. I can monitor the network remotely—even when I’m at home.

We’ve never had any issues or compatibility problems with any of our vendor’s fieldbus products. We have Endress + Hauser, Emerson Rosemont, Yokogawa, SMAR, and all of it is plug-and-play.

The amount of time that we’ve saved with advanced diagnostics has been dramatic. It actually takes longer to walk down the system to set the jumper switches and the boxes than it does to commission the system. The physical walk down and commissioning for the first, four-segment system took a day and a half; on the following eight-segment the physical walk down took about 60 minutes, and the commissioning took a mere 20 minutes. Our confidence level in the network also increased dramatically. When the ADM certifies that the system was good, we know it is good.

From Incident-Based Maintenance to Predictive Maintenance

Advanced diagnostics has enabled me to go from incident-based maintenance to predictive maintenance. When you have a chemical plant running 24/7, and an instrument failure of any kind will bring your operation to a halt, you need a communication system that you can rely on. You need one with greater power capacity, signal quality, consistency, and efficiency. The ADM- FISCO combination delivered on every requirement. It simply outclassed every other product that we used previously.

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