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.

Figure 1. Standards for dimensions enable easily switching single devices without need to modify others. Derived with permission from ISA, www.isa.org.
Figure 2. This sample-handling and preparation system illustrates the size of surface-mount fluid distribution components of NeSSI.
Figure 3. CANopen provides device profiles and function block descriptions. Source: CiA.
Figure 4. Bus operating at 24V dc significantly limits current available to IS device.
Figure 6. Plants trials will demonstrate use of IS CANopen with a NeSSI system.

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.

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