Intrinsically Safe NeSSI Nears

An emerging bus standard promises to spur application in hazardous environments

By Rick Ales, Swagelok Co.

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A1999 ISA symposium spurred an effort to radically rethink such system designs that led to the creation of NeSSI. A consortium of end-users, suppliers and schools, working under the umbrella of the Center for Process Analytical Chemistry at the University of Washington, Seattle, developed the vision and specifications for NeSSI sample systems, including: 1) modular miniaturized sampling components; 2) smart transducers; 3) field mounting capabilities at the sample point in potentially explosive atmospheres; and 4) easy integration into plant control systems.

The NeSSI specification defines two interface “rails” that allow for easy system integration. A fluid rail enables modularity by providing a standard interface to connect flow paths between devices. A bus rail enables the smart transducer and field mounting capabilities by providing IS power and digital communications between the smart sampling system components, the analyzer and the process control system.

In August 2002, a 1½-in. square NeSSI component footprint for the fluid rail was adopted in the ANSI/ISA 76.00.02 standard, which defines properties and physical dimensions for surface-mount fluid distribution components (Figure 1). This fixed footprint enables changing a single element without having to modify the entire system, streamlining design and maintenance.

 Adoption of ANSI/ISA 76.00.02 led to the emergence of what are referred to as NeSSI Generation I miniature modular sample systems. Their modular platform components consist of a surface-mount layer made up of devices such as valves, filters and adapters; a substrate layer that provides the flow path between the surface-mount components; and a manifold layer that provides the flow path between two or more parallel substrates (Figure 2).

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.

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.

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