Intrinsically safe fieldbus applications

This article takes a look at ways companies get around limited power and other hazardous environment barriers to connect more fieldbus devices using a technology that goes beyond FISCO.

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Entity Parameters
One of the advantages claimed for FISCO systems is that if the design uses a FISCO Power Supply, FISCO-approved devices, and FISCO-marked cable within the FISCO length limitations, then the segment meets the FISCO requirements and no Entity Parameter calculations need be done-a previously laborious process employed to confirm that the inherent energy storage in any device, plus its connecting cable, were within the allowable parameters in that particular hazardous environment. FISCO removes that issue to save hundreds of man-hours, especially since no-recalculation need be performed should additional devices be added to a segment; a not uncommon event in most facilities.

At first glance, the split-architecture system, being an Entity design, seems to fall back into requiring new calculations per segment whenever additions are made. However, the unique MooreHawke design has intrinsically safe current limiting resistors per spur, making each spur an independent IS loop.

Also, IS fieldbus devices all have the same minimum Entity parameters as a requirement of their Foundation fieldbus approval. This means that while Entity calculations are indeed required, they only have to be done once. This is normally documented for the longest (worst case) spur length, 120m. After that, no device or spur can be any more "worse," so no more calculations are required. Nor does any additional device present any problem.

Considering all of the above, I&C engineers get to have the best of all worlds:

 

  1. The digital efficiencies of fieldbus
  2. The long cable lengths, reduced documentation, and unrestricted device (non proprietary) benefits of the Entity Concept; and 
  3. Vastly increased power (350 mA) that goes beyond FISCO to drive the maximum number of devices.

Increased Flexibility During Installation
Having access to the full 1900meters/120meters per spur (limited only by Ohm's law) allows systems designers to apply the operational advantages of fieldbus to all hazardous areas of a plant, inside or out, almost as if they were looking at a non-hazardous plant. This simplifies installation when facility engineers wire their DCS system, whether retrofitting an older plant or building one up from a "greenfield."

While a collective experience is growing in fieldbus implementation, many installation contractors are still learning the process. One of the issues that frequently causes problems during installation is the fieldbus requirement to have a 'terminator' at each end of the segment. Operationally, these terminators prevent reflection of the high frequency communications signal and subsequent interference. However, correctly determining the right amount of terminators, and where to place them, can prove challenging, leading to a common situation of having too many or too few.

Once again, technology came up with a solution. To reduce any resultant communication errors during segment commissioning, a unique auto-terminator feature was incorporated into the MooreHawke device couplers. This design allows significant time and labor savings during installation because it helps eliminate wiring errors.

Reducing Downtime and Easing Maintenance
The new circuits that help propel fieldbus to increased functionality also work to improve the reliability of hazardous plant operations. As it is, MooreHawke's ROUTE-MASTER system has 99.99993% availability because of the use of passive components, simple design and built-in DC power redundancy.

Further advances have also been made in the field device coupler itself, specifically in the area of spur short-circuit protection¾an important consideration since a single spur fault can bring down the entire segment.

Installing a series fuse per spur is a well-proven technique and many such installations exist today. When a fuse "blows," the fault is isolated quickly, no segment-loading effects can occur, and adjacent devices are completely unaffected. Simple fuses have enormous MTBF and are generally inexpensive. On the other hand, blown fuses must be physically replaced and though this can be done with the segment on line, it can still be a maintenance nuisance.

In keeping with a theme of making fieldbus systems work for the benefit of the plant, technology managed to come up with a simple solution for spur protection when accessing a fuse proves difficult. MooreHawke, for one, offers a feature that provides spur protection via a fold-back technique which locks in a small 2 mA load, just enough to turn on an LED light, and then removes the device from the segment. Once the short is removed, the coupler automatically resets the device on the network.

This approach is distinctly different from earlier current-limiting designs which, as the name implies, simply hold the fault current on the segment at a fixed (and always higher-than-normal) value. A fixed current of 60mA is common, and this can quickly overload power supplies and crash the segment. On the other hand, the fold-back technique completely eliminates any and all fault loads from the segment and allows the rest of the system to remain up and running.

Modern device couplers can also ease the maintenance of control systems through diagnostic capabilities in excess of what Foundation fieldbus or Profibus has to offer by themselves. Some couplers contain LED indicators to provide status checks at a glance. Built-in access points for hand-held communicators such as fieldbus trouble-shooting devices also save time.

A Promise Fulfilled
For plants that process chemicals, pharmaceuticals, petroleum and other products whose production may involve hazardous environments, it appears that recent technological improvements in split architecture systems design have finally delivered what FISCO promised: Intrinsically-safe segment capacity virtually indistinguishable from non-hazardous fieldbus implementations.

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