As one of the more successful communication protocols for process control, fieldbus has proven its merit in bringing projects online more efficiently, allowing better diagnostics, and control, and reducing maintenance costs and inventories. Fieldbus, which comes in various flavors such as Foundation Fieldbus and Profibus, is a local area network (LAN) designed to replace analog. Whereas analog (4-20 mA) provides a single output in only one direction, Foundation fieldbus, and similar protocols, provide multiple, two-way, communication between instruments and controllers.
Fieldbus, and other protocols, allow far more exacting process control, greater autonomy of control loops, more accurate trending, greater centralized monitoring, and lower installation costs through easier wiring and faster commissioning compared to conventional analog. Such protocols are a first step in realizing the benefits of top-to-bottom plant integration. However, Foundation Fieldbus fell short when it came to the hazardous environments often encountered in many chemical plants and refineries.
Users of intrinsically safe (IS) devices in conventional control schemes have become accustomed since the late 1980s to the flexibility and ease of use afforded by the Entity Concept. Factory Mutual (FM) in the U.S. led the way in simplifying the process of confirming the safety of IS loops. As a result, the Entity Concept now governs how every non-fieldbus IS loop is designed and documented.
Initially, fieldbus implementations went the same way: use a conventional IS interface, apply the industry-standard Entity Concept and the loop (now a segment, in protocol terminology) would be safe. The problem was conventional IS interfaces under the Entity Concept allowed only 80mA or so, barely enough to drive four devices at an average draw of 20 mA per device. Fieldbus segments with only four devices somewhat defeated the point of early fieldbus justifications; plants still had lots of cable, and hardware costs went up.
Technology stepped up to the challenge in the form of Fieldbus Intrinsically Safe Concept (FISCO), which was developed in the late 1990s. Work in Germany had established that if cables and device parameters were defined by boundary values, modern electronic current-limiting designs could allow more current than allowed by old limits and still remain intrinsically safe. Before FISCO, a barrier between an instruments power supply and a hazardous environment had to be tailor-made for each individual instrument. With FISCO, power is safely distributed to instruments via the fieldbus. By taking advantage of this new technology, FISCO succeed in making more current available in hazardous locations a full 115 mA in worst-case (hydrogen) areas, enough to comfortably drive five devices, rather than the 80 mA (four devices) allowed by the Entity Concept (Figure 1).
Figure 1. FISCO allows fieldbus to enter intrinsically-safe environments.
Improvement, indeed, yet FISCO makes this incremental gain at the expense of operational limitations. These power supplies are complex pieces of electronics, with switches, line conditioners and filters. This complex circuitry creates more heat and reduces unit reliability (complexity = more components = lower mean time between failure (MTBF)). Furthermore, a primary requirement of FISCO design is that the maximum allowable trunk and spur lengths fall to 1,000 m from 1,900 m, and to 30 m from 120 m, respectively. In addition, all devices and cable must be FISCO-compatible, further limiting choices in installing fieldbus networks. For some time then, many instrumentation and control (I&C) engineers have been searching for a new solution that would allow them to increase the capacity and operational ease of intrinsically-safe segments to the same level currently enjoyed by non-hazardous implementations.
The split architecture solution
At last, technology has come through once again with a solution. The capacity barrier of FISCO has now been significantly surpassed by a novel split architecture design that has already proven itself in the field. Engineers at MooreHawke division of Moore Industries-International, North Hills, Calif. developed this new technology by re-examining traditional approaches to pushing the capacity limits of intrinsically safe segments.
It quickly became evident that the primary cause of low segment power in fieldbus applications was the placement of the main current-limiting resistor at the point of highest current: the IS interface between the safe area and the hazardous area. In response, MooreHawke developed a split architecture approach using a field-mounted device coupler and an associated power supply with a safe-area interface. Here, the total resistance requirement is obtained via a split resistance; a small resistor is used in the IS interface and a larger resistor is placed in the field device coupler. The small (trunk) resistorsees a large current (sum of all devices), but only generates a small voltage drop. The larger (spur) resistor sees a small current (single device) and so only generates a small voltage drop.
Subsequently, MooreHawkes split architectural design has been certified by organization including Factory Mutual (U.S.), Scientific Instrument Research Association (SIRA-U.K.), Atmospheres Explosibles (ATEX - France). This fieldbus design allows segments to support up to 350mA enough to power 16 devices at 500 m while still being intrinsically safe for hydrogen at the individual spur connection. Now, fieldbus is as cost-effective in hazardous locations as it has been in non-hazardous applications.
In terms of reliability, the split-architecture power supply steps around the complexity associated with FISCO circuits by the use of a conventional wire-wound resistor, which in IS terms, is deemed to be infallible. To further augment the overall systems reliability, the MooreHawke design also incorporates full AC/DC power conversion, simple linear power supply, and full galvanic isolation, with built in redundant supplies. Here again, fewer components translate into greater reliability the MTBF should rise.