Intrisic Safety in the Digital Age

In Which We Sort Through the Complexities of Building Intrinsically Safe Fieldbus and Ethernet Networks in Hazardous Plant Areas

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Who Are You Calling a Hybrid?

Pepperl+Fuchs has developed a hybrid approach that combines nonincendive techniques for the fieldbus trunk with intrinsically safe techniques for the fieldbus spurs. Pepperl+Fuchs calls this hybrid approach the “high-power trunk” concept. The hybrid approach uses nonincendive energy limits for the fieldbus trunk (where allowed) to enable more power to be available for fieldbus devices. Only the spur connections to fieldbus instruments located in IS locations are limited to IS levels. This is accomplished using short circuit-protected junction boxes with built-in barriers. Pepperl+Fuchs calls these “segment protectors” for nonincendive applications and “field barriers” for intrinsically safe applications.

According to Schuessler, the high-power trunk concept, “…allows end users to get the maximum number of devices on a segment, while also being able to achieve maximum cable length. Depending on the applications, the protection (energy limitation) is done in the field, inside the junction box. By not limiting the energy on the trunk, the high power trunk concept offers the same advantages seen in general-purpose applications in hazardous location applications.”  Of course, this also means that the hybrid approach cannot be used in applications requiring an intrinsically safe trunk.

While this approach allows the fieldbus spurs to be worked on while energized without a hot work permit, this is not the case for the trunk. However, according to Schuessler, “Fieldbus users do not normally perform live maintenance on a fieldbus trunk cable because of the high risk of losing an entire segment due to a single short on the trunk cable.”

With this hybrid approach, users are free to select between instruments with either Entity or FISCO hazardous area certifications. Options are also available for users to implement power redundancy and online physical layer diagnostics not available with the FISCO approach.

Moore-Hawke’s O’Neill sees a problem with the hybrid approach, “The high power trunk looks like a great idea; unlimited segment current delivered by mechanically protected cable to field barriers which generate IS power for devices. However, the design is constrained between the maximum practical fieldbus conditioner voltage (28V) and the need to deliver 16V or so to the field barrier. It doesn't matter if the HPT can supply 1A since that 12V differential will disappear in just 12 Ohms of fieldbus cable (240 m). Nothing escapes Ohm's law!”

Let’s split the difference

Moore-Hawke has developed its own alternative approach, which the company calls, “split-architecture.”

With this design approach, the current-limiting resistor is split into two parts; one in the barrier, which is located in the non-hazardous, “safe” area, and the other in the field device coupler. This configuration enables a large current to flow in the trunk, which is then further reduced by in the resistor in the device coupler (one per spur) to allow connection to conventionally approved devices. According to Moore-Hawke, commercially available systems can demonstrate segment capabilities of at least 350mA and full compatibility with common IS device approvals. This would appear to eliminate the major issue associated with implementing fieldbus networks in hazardous locations: the number of instruments you can install per segment. 

Furthermore, according to O’Neil, “Since these systems are conventionally designed using diodes for voltage limiting and resistors for current limiting, there are no limitations on segment length or spur cables.

At the recent Fieldbus Foundation General Assembly in Antwerp, Miodrag Pramenko from Serbia Gas explained why his company selected a split-architecture system for the re-instrumentation of the Banatski Dvor underground gas storage facility, “We wanted to use intrinsic safety to maximize our security, but we needed a cost-effective and efficient way of achieving this. Our systems integrator (WIG, Belgrade) were experienced in systems design with a split-architecture solution which met all these needs.”

Boehringer Ingelheim Chemicals, a bulk active pharmaceutical ingredient manufacturing facility located in Petersburg, Va., also implemented the Moore-Hawke split-architecture  IS approach for a recently installed automated solvent distribution system.  Here, in addition to maximizing the number of devices allowed per segment, it was important to be able support long segment lengths. This would allow primary and final control elements physically located on different floors of the facility to be connected on the same fieldbus segment. Boehringer Ingelheim also wanted an intrinsically safe system that would allow technicians to troubleshoot and work on the system without requiring a hot work permit or having to shut down the process.

As with FISCO, the split-architecture approach uses worst case Entity parameters to significantly reduce the time and effort required to determine the intrinsic safety of a fieldbus system. According to O’Neill, “Having current-limiting resistors per individual spur in the split-architecture system means that each spur is a separate circuit for the purposes of intrinsic safety, and since the worst-case length of that spur is also known, it is very easy to demonstrate the worst-case spur cable plus fieldbus device is safe, and that no segment can be worse. A single page of calculation will suffice for the whole plant.”

And Now for Something Really Dynamic…

In April 2008 at the Interkama industry trade show in Hannover, Germany, Pepperl+Fuchs announced a new technology now in development in Europe that shows great promise for eliminating many of the previous power limitations relative to installing fieldbus devices in hazardous plant locations. In fact, Pepperl+Fuchs is claiming that the new technology, called “dynamic arc recognition and termination,” or “DART,” will even allow higher-powered devices to be used in the hazardous areas. Examples include industrial PCs, LED lighting systems, high-power sensors, analyzers and solenoid valves.

According to Pepperl+Fuchs, DART detects the characteristic voltage change caused by a spark and quickly turns off the circuit before the spark’s temperature is sufficient for ignition. For fieldbus instrumentation, DART should be able to provide up to eight watts of power per segment, even inside explosion hazardous plant areas. According to a Pepperl+Fuchs news release issued at Interkama, “DART concepts are proven and patented. And the market will start to see products in 2010. Leading experts consider this technology as the beginning of a totally new era in process automation.”  To this end, the company is looking for suitable partners to develop new products and applications. “We are interested in a constructive dialog with product managers all over the world,” stated Michael Kessler, director of the Pepperl+Fuchs Components and Technologies business unit.

User Interest is Growing

According to Lee College’s Chuck Carter, end-user interest in FISCO and other approaches for implementing fieldbus in hazardous environments is growing. “In regard to fieldbus, I have to admit I was one who was somewhat dubious about whether or not FISCO and the other IS approaches would survive, much less flourish. This was largely because of the cool reception I would get whenever I brought up the topic in our industrial fieldbus courses. This has changed over the last year, and I find more and more of the attendees are paying great attention to that portion of the course and anxious to get more information that might apply to their own unique situations.”

According to Bruce Bradley, PE, a project engineer for Boehringer Ingelheim Chemicals, “Implementing Foundation fieldbus coupled with hazardous area classifications may approach information overload. But believe it or not, with today’s technology and product offerings, fieldbus is simpler than any time before to implement.”

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