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.
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 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.
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.