Hopping on the Fieldbus
Expanded Fieldbus Process Control in Hazardous Environments Comes to the Pharmaceutical Industry
Aunique split-architecture system doubles the number of intrinsically safe devices to provide better control and feedback in hazardous environments. While information technology in the pharmaceutical processing industry is gaining increased importance, there is still no abandonment of the prime dictum: The process is the product.
Without tightly controlled processes, product quantity and quality become jeopardized, and with it go the company's competitiveness. For two decades now, distributed control systems (DCS) employing feedback and control devices have enabled pharmaceutical plants to exercise precise control over their processes, especially given the more efficient communication protocols. However, limitations in installing multiple sensor and other control devices in hazardous environments have traditionally stymied pharmaceutical I and C engineers and DCS integrators in their efforts to maintain tight control.
That has all changed now, thanks to new technology that enlists the use of device couplers in combination with intrinsically-safe, isolated power supplies. These new systems go beyond traditional foundation intrinsically safe concept (FISCO) operating parameters with a full 350mA per segment; enough to drive up to 16 devices. This high current also enables the connection of devices over cable lengths almost double that of FISCO, adding increased flexibility in wiring pharmaceutical plants.
"Folding our processes into a modern control system helped us get around the limitations we previously had in our hazardous areas," says John Hutto, an instrumentation and control engineer for Boehringer Ingelheim Fine Chemicals (Petersburg, Va.), which is part of the Boehringer Ingelheim group of companies, one of the world's leading pharmaceutical corporations. "Better control and data trending helped provide more consistency in our product."
When it came to the hazardous environments often encountered in many pharmaceutical plants, fieldbus falls short when intrinsically safe (IS) techniques are required. While FISCO extends functionality slightly, it typically limits the number of devices to only five, and spurs cable lengths to only 30m.
Of equal concern to plant personnel, FISCO units are complex pieces of electronics, generally based on switch-mode power supplies with duplicated current-limiting networks. This elaborate circuitry creates more heat and reduces unit reliability; an unacceptable tradeoff.
For some time then, many pharmaceutical plant engineers and integrators have been searching for a new solution that would allow them to maximize intrinsically-safe segment capacity and operational ease within hazardous applications to the same level currently enjoyed within non-hazardous implementations.
"Our plant has a fair amount of drums that hold flammables and open man-ways on reactors, and while we try to keep everything closed, there are certain instances where you need to manually have a flammable open to the air for a short time that makes the area a Class 1, Division 1 area," Hutto says.
During the last expansion, the fine chemicals divisions utilized the Moore-Hawke RouteMaster system of power supplies and device couplers within existing DeltaV DCS, Hutto explains.
"This enabled us to get in the neighborhood of up to eight devices per segment," he adds." We could have gotten even more, but we like to limit our devices to those that are part of the same control loop or system, per segment. With ordinary FISCO, we could have only used about five, and that would be the max."
Engineers at MooreHawke, a division of Moore Industries-International, interface developers based in North Hills, Calif., recently developed a split architecture approach using a field-mounted device coupler, and an associated power supply with a safe-area interface.
Subsequently approved by the FM (US) and SIRA (ATEX) certifying organizations, this design enables intrinsically safe fieldbus segments to support up to 350m enough to power 16 devices at 500m while still being intrinsically safe for hydrogen at the individual spur connection. This allows fieldbus designs to be just as cost-effective and efficient in hazardous locations as it has been in non-hazardous applications.
In terms of reliability, the split-architecture power supply steps around the complexity associated with FISCO circuits by the use of a conventional wire-wound resistor which, in I.S. terms, is deemed to be infallible. To further augment the overall systems reliability, the MooreHawke design also incorporates full AC/DC power conversion, simple linear power supply, and full galvanic isolation, with built in redundant supplies.
This high current made possible by split architecture device couplers and power supplies enables the connection of devices over cable lengths almost double that of FISCO� 1,900 meters for trunk lines and 120 meters per spura allowing the operational advantages of fieldbus to extend to all hazardous areas of a plant, inside or out. In conjunction with an auto-terminator feature in the device couplers themselves, this flexibility simplifies the wiring of a DCS system, whether retrofitting an older pharmaceutical plant or building one up from a "greenfield."
"Within the building the longest trunk run was 400 feet, and out in the field the longest cable run we got into was 500 feet," says Hutto when describing the arrangement within his plant. "This helps because we always have one of these couplers available that we can easily tie an instrument into within 30 to 40 feet of any given place in the plant. We can get right up there in the bay and minimize conduit and wiring runs to short distances eliminating clutter verses traditional wiring techniques.
"We went from the fourth-floor to the third-floor in the same vertical column within a bay for one segment, and then had another segment overlap from the third to second," Hutto adds. "Why would you do that? Take temperature control for a reactor."
Part of the reactor (temperature transmitter), he says, may be on the fourth-floor, but the valve to control might be on the third-floor. That way, the temperature-loop on the same segment and avoid inter-segment (between fieldbus card) communications that reduce performance and increase communications loading.
Improved Process Controls
For pharmaceutical plants that process drugs, biologicals, vitamins or food supplements whose production may involve hazardous environments, it appears that recent technological improvements have finally enabled intrinsically-safe segment capacity virtually indistinguishable from non-hazardous fieldbus implementations. For those who believe that "the product is the process," this improvement aims at allowing devotees to stay competitive well into the future.
"While we occasionally experienced some fieldbus issues that were either transmitter or field-wiring related, we've been using the MooreHawke devices for two years now and haven't had any significant problems with the devices themselves," Hutto says. �David Rizzo, D.P.M., is a freelance writer based in Torrance, Calif. He has published one trade book, 150 technical articles and 300 newspaper columns