Intrinsically Safe NeSSI Nears

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

By Rick Ales, Swagelok Co.

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The New Sampling/Sensor Initiative (NeSSI) has provided the basis for modular miniaturized process sampling systems that offer ease of assembly and flexibility while cutting cost of ownership. Not surprisingly, plant acceptance of such NeSSI systems is growing.

A group of analyzer specialists now is working to enable NeSSI to be used in hazardous environments. They envision an analytical system with smart transducers that would be capable of being field mounted at the sample point in a potentially explosive atmosphere and would be easily integrated into the analyzer control system.

To operate safely in a hazardous/combustible environment, the transducers in the sample handling system require an Intrinsically Safe (IS) bus to communicate with the analyzer controller. Such a bus must use very low power to prevent any type of ignition. However, adoption of a standard IS bus has been difficult because none of the existing industrial networks exactly fit all the analyzer specialists’ requirements for a simple, small, inexpensive as well as IS transducer bus.

So, as an alternative, the group looked at Controller Area Network (CAN) communications technology. This is low cost and can be implemented with low-power bipolar and complementary metal-oxide semiconductor (BiCMOS) electronics and proven CANopen device profiles. The group recently completed definition of an IS version of CANopen to meet its transducer bus requirements, resulting in an emerging standard — CiA 103 DSP V1.0.

Historical backdrop
For more than 50 years, plants have relied on process analytics to decrease costs, lower staffing levels, improve quality and increase throughput. Analyzers have evolved into sophisticated chemical sensors and automation instruments that have migrated from the laboratory to the field for integration into real-time process control systems. However, the front-end designs of analytical systems — sample handling and preparation — haven’t appreciably improved for many years.

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.

Figure 1
Component footprint
Figure 1. Standards for dimensions enable easily switching single devices without need to modify others.
Derived with permission from ISA,

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).

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