Metal legos streamline sampling and control

Keywords: process analyzers, ControlGlobal.com, Control magazine, sampling, control, sensors, NeSSI, process control, Mike Spear and chemical processing

The new sampling and sensor initiative (NeSSI) delivers a smaller, smarter approach that is resulting in savings now, and improved process control soon. So where does NeSSI go from here?

By Mike Spear, Editor-at-large for sister publication Chemical Processing
 
INCREASING PRICE competition is forcing chemical companies to reduce capital and operating expenditures, and, of course, do it without compromising their plant’s reliability and performance. Easier said than done, perhaps, but the New Sampling/Sensor Initiative (NeSSI) for delivering process samples to analyzers can help make it happen.

Since it began about five years ago, NeSSI has driven the move to modularize and miniaturize process sampling systems. Presently operating under the Center for Process Analytical Chemistry (CPAC) at the University of Washington in Seattle, NeSSI started as a group of equipment manufacturers and operating companies.

The group initially sought to build on a standard for modular, surface-mounted systems in the semiconductor industry. It was modified for use in the oil, chemical and petrochemical industries by the SP76 working group of ISA—the Instrumentation, Systems and Automation Society. In 2002, the resulting design became the basis for the international ANSI/ISA-76.00.02 standard for sample flow control, conditioning and analysis. This modular standard basically requires compliant sampling systems to consist of a series of close-coupled, 1.5-in.-square manifolds, each carrying one of the components fastened to the substrate with four hex (Allen wrench) screws.

Saving Sugars Cubes
NeSSI’s potential savings were first quantified in 2000 by an ExxonMobil team, which reported that plants switching to NeSSI could save 40% on their sample transport and conditioning system, and eliminate the need for climate-controlled housings. It also projected a 35% savings by reducing sample volumes, as well as carrying and purge fluids, and by requiring less maintenance and support.

“After the field experiences with ‘Generation I’ NeSSI systems, we now have a solid platform to build on,” says Jeff Gunnell, lead specialist for process analytics with Scotland-based ExxonMobil Chemical. “There has been one round of iteration [in design] since 2001, and these systems work fine in the field within a realistic operating window.” ExxonMobil has 30 modular systems installed, including two sites in Baytown, Tex., and one in Sarnia, Ontario, Canada. These systems sample hydrocarbon process streams, and are approaching “Best Practice” approval status at ExxonMobil. “We’re considering them for some major projects, and that could mean huge numbers of systems,” adds Gunnell.

ExxonMobil adds its modular sampling systems come from two of the three main vendors supplying substrates and other flow components for ANSI/ISA-76.00.02-compliant systems. These are the Instrumentation Division of Parker Hannifin, Swagelok, and Circor Instrumentation Technologies.

	FIGURE 1: UP ON BLOCKS
	ADVERTISEMENT In Swagelok’s MPC system, devices, such as valves, mount on substrate modules containing specialized channel and flow components.

From Handling to Control
While many of its Generation I modular-sampling are serving well worldwide, NeSSI’s Generation II is moving beyond physical fluid handling to integrating the modular systems into plant control hierarchies.

Rob Dubois, another NeSSI member and senior analytical specialist at Dow Chemical, says many modular systems are used in ethylene-hydrocarbon and ethylene oxide production applications and on a continuous-emissions-monitoring system for NOX at his plant in Fort Saskatchewan, Alberta.

"Technically, the NeSSI systems work well and many of the modular components that previously had been missing are now available or are coming to market,” he says. “However, implementation has been slower than hoped for due to higher costs when compared to conventional systems. Right now, we’re missing a ‘single-block,’ combined flow transmitter/indicator, which would really simplify and ease construction. Analyzer systems may have become ‘smart,’ but the sampling system remains archaic. Today, the industry uses a hodgepodge of analyzer I/O, PLCs, DCSs, databases and proprietary systems.”

Generation II systems reportedly will solve this problem with a sensor/actuator manager (SAM) operating system, which will act as the communication channel between DCSs and analytical systems. Higher up, NeSSI systems communicate to the DCS using OPC over TCP/IP Ethernet, while Foundation Fieldbus (FF) is preferred at the field level. The CPAC/NeSSI steering committee incorporated FF into NeSSI’s Generation II draft specification in February 2005 because its modular systems need FF’s intrinsically safe operating capabilities.