1 September 2002
Miniature modular is NeSSI
By David Simko
As the capabilities of process analysis have continued to develop and improve, there has been an increase in the variety of measurements that the more sophisticated analyzers can take.
Industry seeks to reduce the cost to design, build, and install the systems as well as operating and maintenance costs. One important way to meet these needs is to make sampling systems miniature, modular, and intelligent.
The Center for Process Analytical Chemistry (CPAC), a joint industry/academic research consortium located at the University of Washington, has moved forward with a New Sampling/Sensor Initiative (NeSSI) through the efforts of CPAC leadership and the industrial affiliates.
The purpose of NeSSI is to "facilitate the state-of-the-art evaluation and ongoing development of the next-generation modular sampling system designs." A key element in the modularization strategy is open architecture.
ISA's SP76 Composition Analyzers Committee added a subcommittee charged to develop interface seal standards to apply to functional fluid control components and the fluid path substrates of a miniature, modular, smart sampling system.
Along with NeSSI, CPAC issued a request for proposals for design concepts for the new systems to all interested parties. The request included drawings of six real systems in actual use in process plants. They ranged from a relatively simple system to a complex eight-stream system.
Six concepts using modular technology originally developed for specialty gas handling systems used in the semiconductor industry lay out in modular form, using a configurator software package.
The advantages of the modular approach are many:
Development and layout time and costs are lower, due to tools such as system configurator software. The reduced size, weight, and footprint make it easier to couple the sample system to dedicated field-mounted analyzers and place them at the sample point, reducing the need for long, heated sample transport lines.
- The small modular sample systems have less internal surface area than a traditional system, reducing the amount of possible adsorption of material from the fluid.
- The internal volume of the system is less, making it easier to purge or flush the system and conserve expensive analyzer fluids.
- The systems are easier to assemble.
- The footprints are small, and the work can likely be done on a bench rather than a rack.
- Installation is simpler, requiring only a few connections. For example, in the case of the demonstration unit described earlier, only six connections are required.
- Maintenance is easier, as functional components are surface mounted. By removing four bolts, one can extract any component.
The NeSSI Generation I systems described resolve the physical issues of modularity by applying commercially available surface-mount functional components and control solutions.
Generation I systems involve simple automation, where functional components such as pressure transducers, thermocouples, and mass flow controllers that are monitoring the physical properties of the process fluid are connected through I/O modules to a controller. The signals are conventional 4–20 mA. Communications are point to point and one way and accommodate only one device per wire.
CPAC has developed a specification for NeSSI Generation II systems that addresses the connectivity and communications issues that will lead to intelligent control of these sample systems. Some key features of these next-generation systems defined in the specification are as follows:
- Compact, smart pressure, temperature, and flow sensors
- Smart valves with built-in electric-to-pneumatic actuators
- Multidrop sensor bus communications, which may be wireless
- Sensor Actuator Manager (SAM) to provide control and connectivity to the sample system
The smart functional components should have true plug-and-play capability. Because these sampling systems may be located in potentially explosive environments, the functional devices, the sensor bus, and the SAM should certify safe for use in these kinds of hazardous areas. The preferred protection scheme is intrinsic safety, which would allow removal and insertion under power in such an area.
NeSSI Generation I systems can use commercially available I/O-based control solutions. NeSSI Generation II systems are intended to use an intelligent fieldbus-based control architecture.
Communication is to be two way, so when integrated into a system, these devices will be able to talk to one another through the SAM. The architecture permits many devices and variables per wire.
Many of the devices required to build NeSSI Generation II systems are not yet commercially available, but suppliers are working on their development.
Generation I and Generation II systems differentiate between modular and smart issues. CPAC has developed a road map detailing the development timelines for these systems.
CPAC also looks to the longer-term future and describes the development of NeSSI Generation III systems, which are expected to incorporate microanalytical devices such as lab on a chip. Smart miniature modular sample systems will provide the platform for such development. IT
David Simko is an ISA member. He works at Swagelok Co. as the marketing resources manager.
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