Intrinsically Safe NeSSI Nears

An emerging bus standard promises to spur application in hazardous environments.

By Rick Ales, Swagelok Co.

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With the fluid rail standard established, the NeSSI consortium turned its attention to the bus rail, publishing the NeSSI Generation II specification. This calls for upgrading the traditional manual rotometers, gauges and metering valves used in sample systems to smart pressure, temperature and flow sensors that are automated, have self-diagnostic capabilities and are plug-and-play. The new sensors and proposed family of intelligent valves would monitor and operate the sample handling and preparation processes under the supervision of the analyzer controller or the analyzer itself.

Figure 2
Compact Modular Hardware: This sample-handling and preparation system illustrates the size of surface-mount fluid distribution components of NeSSI.

With many of the NeSSI systems expected to be deployed in the potentially explosive atmospheres common in process plants, the consortium identified an IS transducer bus as the key technology for Gen II to become a reality. Unfortunately, the adoption of an IS NeSSI bus standard hasn’t come as easily as the fluid rail standard.

The bus conundrum
On the surface, choosing an open-standard IS transducer bus seems to be a simple choice of one of four possible automation protocols: Foundation Fieldbus, Profibus-PA, HART or ControlNet, all of which already are established in the process and manufacturing industries. However, the NeSSI consortium discovered that none of these buses was particularly well suited for the types of smart devices that would populate a NeSSI Gen II system. The consortium had concerns about architecture complexity, physical size and cost of these options.

Architecturally, existing IS buses don’t match the simple design of NeSSI devices. Typically, IS buses are designed to operate a few sophisticated devices using complex messaging protocols over long distances (e.g., 1.9 km for Foundation Fieldbus), with a determinism suitable for real-time control. Relatively speaking, NeSSI systems aren’t time critical. Gen II components are simple sensors and actuators that support basic configuration and diagnostic functions — and don’t require such a complex messaging protocol. Further, NeSSI devices communicate with a local controller usually located less than 10 m. away.

On the size front, the 1½-in. square ANSI/ISA 76.00.02 footprint places a serious constraint on the amount of electronics that can be fitted onboard a NeSSI Gen II device. The major IS buses, with the exception of HART, require at a minimum: a relatively powerful microcontroller, a protocol controller Application-Specific Integrated Circuit (ASIC), a bus interface and driver circuits. The size and number of required electrical components make it difficult, at best, to squeeze an IS bus — along with the requisite input/output (I/O) circuitry, connectors and other miscellaneous electronics — into the NeSSI footprint.

Lastly, the relative cost of the IS buses is high compared with NeSSI devices. The IS buses were designed to be deployed in larger process control valves and transmitters in which the bus technology represents a fraction of the device’s total cost. NeSSI devices typically cost much less than larger process control valves or transmitters, making the cost of a smart NeSSI device that uses one of the IS bus technologies much higher than its non-networked counterpart.

Other industries, such as semiconductor, consumer electronics and automotive, use simpler, smaller and less expensive buses such as USB, I2C and CAN, which would be ideal for use as the NeSSI bus; however, these options aren’t intrinsically safe.

Given these choices, decision-makers were faced with a conundrum: Do they make one of the IS buses simpler, smaller and less expensive, or do they make one of the simpler, smaller and less expensive buses intrinsically safe?

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