SEEING the LIGHT: Fiber Optics Illuminate Process Control and Instrumentation

Fiber-optic technology—it's not just for decorative lighting anymore. In the 1970s, decorative lighting using fiber-optic cable made a statement (dancing pinheads of ever changing colored light) in college dorm rooms, bachelor pads, and the homes of the avant-garde. What might have been creating really neat novelty lighting at that time was not yet setting the industrial world on fire.

12/01/2000


KEYWORDS

 

  • Process control and instrumentation

  • Fiber optics

  • Process sensing

  • Networks and communication

Sidebars:
Looking out for number one—fiber-optic style
'Going long'—Linking control in a very large refinery/petrochemical complex
Blowing that fiber around

Fiber-optic technology—it's not just for decorative lighting anymore. In the 1970s, decorative lighting using fiber-optic cable made a statement (dancing pinheads of ever changing colored light) in college dorm rooms, bachelor pads, and the homes of the avant-garde. What might have been creating really neat novelty lighting at that time was not yet setting the industrial world on fire.

Fiber optics has been adapted to industrial use for a number of reasons. In communications, whether in plant networks or process instruments, fiber-optics technology offers advantages over traditional copper wiring or cable. Data can be transmitted digitally and at greatly increased speed. Fiber cable is superior to copper wires because it is immune to corrosion, vibration, and electromagnetic (EMI) and radio frequency interference (RFI). In network applications, it has superior transmission quality and is generally easier to install and maintain.

Mike Shulim, director of engineering for DST Controls, a system integrator located in Benicia, Calif., placed noise immunity at the top of his list of reasons to consider fiber optics in a control network situation. "Additionally, fiber-optic networks have 'no' life," Mr. Shulim commented. "Copper-based systems degrade with age. After 10 to 15 years speed of transmission drops and internally generated noise increases. Fiber-optic systems have an effective life three times that of copper. Definitely a long-term solution."

Additionally, fiber-optic conductors are much smaller, lighter, and more flexible than copper wire. Development of low-cost optical transducers and advancements in optical-fiber manufacturing now makes it cost competitive with conventional wiring. Bandwidth is very high compared to conventional wire cable (two hair-thin strands of optical fiber can transmit the equivalent of 24,000 telephone calls at the same time. It takes two copper wires contained in a much thicker cable to carry just one call. Termination and spicing of fiberoptic cable (especially in the field), once its Achilles' heel, has improved greatly since its appearances in the 1970s, ushering it into prominence in modern digital communication.

Mr. Shulim points out that use of optical fibers in control installations, especially retrofits and system expansions, is cost comparable to running copper conductors. "Users can run fiber-optic cables in the same conduit and trays as existing power cables with no 'noise' concerns and save on installation time and cost."

According to Mr. Shulim, the smaller, flexible cable is generally easier to install, however, technicians must pay closer attention to detail. Overstretching of cable and poor termination can prove disastrous. Quality variations in poorly specified cable can also be a problem. Says Mr. Shulim, "As little as two inches of bad cable can disable a system. In general, fiber optics is less forgiving of installation errors."

Change of scale

Fiber-optic technology has been adapted to control instrumentation applications often for the same reasons that make it a good choice for networking. FiberFlex, a nuclear continuous level detector developed by Ohmart/Vega Corp. (Cincinnati, O.), incorporates a fiber-optic bundle for the device's scintillator.

Nuclear continuous level measurement works by directing a narrow fan of radiation through a vessel to a detector or scintillator. Within a fiber-optic scintillator, radiation creates photons, then measured with electronics and correlated with liquid level. As the process level rises, it shields the detector from the radiation. The more radiation the detector "sees," the lower the process level (discernable to 1% of span); less radiation detected means a higher level.

To avoid "blind spots" and ensure accurate readings, scintillators must conform to the vessel wall opposite the gamma ray source. As the name implies, flexible, lightweight fiber optics allows FiberFlex to be easily installed to measurement lengths of 23 ft.

According to Kevin Carmichael, chief engineer at Ohmart/Vega, "nuclear devices are often the technology of last resort" when it comes to measuring continuous level. Nuclear measurement is used for level values in processes where materials are extremely hot, corrosive, hazardous, or viscous. In short, because of high cost and complexity, it is used when no other technology will work.

Clear vision

Not all fiber-optic materials are created equal. In fact, communications with its "on and off " digital mode can use a different grade of fiber optics than do instruments that must convey a clear image of what the sensor "sees." Used in medical examination equipment such as endoscopes, these materials have been adapted to industrial temperature sensors. In the case of infrared sensors, fiber-optic cable can be used to connect a sensing head that must be mounted separate from the processing electronics. Fiber optics' intrinsic immunity to temperature and electromagnetic interference enhances this type of application.

Marathon FR1 noncontact ratio thermometer developed by Raytek Corp. (Santa Cruz, Calif.) is among practical examples of this technology. The sensor is said provide the accuracy and stability of a two-color ratio-thermometer even in high temperature (in this case, 200 °C) and electrically "noisy" environments. The fiber-optic cable is available in lengths up to 33 ft and can be replaced in the field without need for recalibration. The FR1 is currently available in three models, covering a range of 500 to 2,500 °C. It is intended for harsh operating environments, including metals and glass production.

A common use for this type of fiber-optic instrument is in carbon anode "baking." In the reduction of aluminum, carbon anodes are consumed during the electrolysis process. Efficient manufacture of these anodes, which are often larger than a cubic meter and consumed by the hundreds, is key to aluminum production. Anodes are molded from a slurry of calcified green coke and distilled coal tar. They are then baked in gas/oil-fired pits at temperatures greater than 1,000 °C for several days to develop the best physical and electrical properties. Controlling the baking process requires accurate monitoring of temperature at the top of the baking pit.

Originally handled by Type S thermocouples/protection tubes, pit air temperatures can now be measured more accurately using fiber-optic sensors. Leveraging fiber-optic sensor accuracy, immunity to noise, and ability to mount electronics remotely, many aluminum smelters have embraced fiber-optic sensor technology for this demanding application.

Fiber-optic cable can also be used for level sensing and provide both intrinsic safety and eliminate RFI/EMI concerns. Kinematics & Controls Corp. (Brooksville, Fla.) uses a plastic fiber to send red visible light to a prismatic tip located in the sensor head of a level switch. If the switch is dry, light reflects back to the remotely mounted controller. However when the liquid level reaches and submerges the sensor, light refracts into the media and none is returned. The controller then produces the appropriate output for signaling PLCs or driving relay coils.

Commonly used to monitor levels of flammable liquids, the sensor cables are available in 20-ft lengths for applications up to 100 ft away. Tiny fiber-optic heads, as small as 0.25-in. dia., can fit in tight locations. Use of fiber optics eliminates heat sensitive components at the sensor, allowing the device to sense liquids at temperatures above 300 °F, if required.

Splitting things up

Separating electronics from a sensor head via a fiber-optic cable has also been adapted to photoelectric sensor design. Without ability to transmit light to its electronics, photocells would have to integrate sensor and electronics in the package. Size of such packages (even with the degree of electronics' miniaturization now available) can inhibit mounting flexibility and convenience.

According to Charley Strobel, senior tech support engineer, for Keyence Corp. of America (Woodcliff Lake, N.J.), separating the two components offers real advantages. In the case of its FS-V10 Series photoelectric sensors, Keyence provided a small sensor head that could be used in "tight" mechanical locations and allow users to mount electronics in any convenient remote location up to 33 ft away.

Because remote electronics are not "in the way" of the sensor and can be packaged as desired, both electronic and mechanical features can be optimized. "Bigger" electronics can lower response times, provide space for convenient readouts and manual controls, and allow features such as automatic and/or manual calibration. These types of sensors often detect small targets, such as registration and guide marks in web-processes that produce rubber and construction materials (foam insulation, wallboard, etc.).

Bad environments

Damp and corrosive atmospheres can effect traditional copper wiring. Even in standard conduit installations, connections in unsealed or poorly assembled electrical boxes can corrode severely. Additionally, poorly shielded wiring is at the mercy of EFI and RFI. Control engineers faced with altering, expanding, or retrofitting a control system often need to use existing cable trays or pull extra wiring through existing conduit. Addition of new sources of electrical interference, such as motors, generators, standby-power equipment, or factory lighting and their cumulative effect on the control system can be overlooked.

To circumvent these types of problems, Poultry Management Systems Inc. (PMSI, Saranec, Mich.) is using Lucent Technologies' (Avon, Conn.) FiberWire fiber-optic industrial communications system as the backbone of its "chicken house" control package. These control systems are designed to control every aspect of the chicken house environment, including controlling feed, water, light, air circulation, and egg production. The specialized egg production control package has been built to control the flow of eggs coming out of the houses by interconnecting with the environmental control system.

FiberWire links each house's computer to a central computer system that calculates which chicken house is "laying" best on a daily, weekly, or up-to-the-minute basis. This information then helps determine the proper environmental control needed for each house to produce the optimal quantity of eggs. According to Bill Kaufman, PMSI fiber specialist, "FiberWire's construction and resistance to damage, corrosion, and EMI/RFI immunity make it a perfect choice for the system.

"Chicken house environment is dark, damp, and filled with ammonia and methane. Many of these complexes run on 230/460 Volt, three-phase power extending all over the facility creating noise even with shielded communication cables. We had recently retrofitted an older site in Michigan where over $20,000 was spent trying to chase down the noise before turning to a fiber solution," Mr. Kaufman says.

Electrical isolation of fiber-optic cable offered another advantage in this application. PMSI's farms may contain anywhere from 12 to 32 chicken houses, 500 ft long and 60 ft wide. Each house is made of steel and full of wire, a perfect attraction for lightning! "You can imagine what would happen if we would have used copper wiring," says Mr. Kaufman. "Lightning would be conducted all the way through the system taking out everything in its path."

Making inroads

Fiber-optic technology has illuminated many applications since its humble "curiosity" status. According to Mike Shulim of DST Controls, "We have seen its use jump 200% per year for a while now in the controls projects we are doing."

Even with its success in high-tech control and communication systems, those fiber-optic lamps and sculptures are still available for those looking to make an interior decorating statement. If you are looking, I have a few web sites for you!




Looking out for number one—fiber-optic style

Trouble in a fiber-optic network is not the same as on a conventional one. Because information is so compressed in a fiber-optic system, a single fiber break can do much more damage to the flow of data. Turan Erdogan, director at the University of Rochester's Institute of Optics (Rochester, N.Y.), says an "inchworm-sized" device situated inside the cable, clinging to a single fiber, can precisely measure how efficiently it's performing.

Previous devices that performed this function were complicated and quite large, hampering adaptability within the system. The new device has no moving parts and will fit anywhere a fiber can go. This line-tester for the fiber-optic era self-monitors each strand and indicates when there is trouble. The detector works by comparing component wavelengths up- and downstream, using a tiny photodetector on a chip. It can accurately report what wavelengths are moving through the monitored fiber.

Web address for University of Rochester's Institute of Optics is

'Going long'—Linking control in a very large refinery/petrochemical complex

Reliance Industries Limited (RIL), India's largest private sector company, recently constructed and commissioned what is said to be the world's largest and most highly integrated refinery and petrochemicals complex at Jamnagar on India's West Coast.

It includes the world's largest "greenfield" refinery, the world's largest paraxylene plant, Asia's largest polypropylene plant, a 360 MW (India's largest) captive power plant, a marine tank farm, a complete distribution terminal, plus India's largest private sector port. The complex uses the latest and most advanced processes, automation, information technology, and networking available. RIL joined with main engineering contractor, Bechtel (San Francisco, Calif.) to perform all design work. It was constructed at an investment of $6 billion US. The complex uses Foxboro (Foxboro, Mass.) I/A Series control systems, Triconex ESD (Irvine, Calif.) process protective systems, and a SAP (Norwood, Mass.) R/3 ERP system.

A total of 310 miles of fiber-optic cable was installed to connect the various automation systems within this massive complex. Systems and associated networking for each were supplied by Foxboro India (I/A Series DCS and terminal automation systems, plus Triconex ESD systems), Wormald U.K. (fire and gas control system), Bently Nevada U.K. (machine condition monitoring system), and Rockwell Automation U.K. (PLC systems).

Approximately 155 miles of fiber-optic control network links the five major I/A Series systems within the complex. Systems controlled include the refinery and marine terminal (the largest I/A Series system installed anywhere), the aromatics plant, the polypropylene plant, the complex's captive power plant, and port operations Fiber-optic "Nodebus Extenders" connect physically distributed control node segments within common I/A Series systems. This includes the one that controls the refinery and marine terminal, which are located at total local area network (LAN) length of more than 12 miles apart within the complex. However, RIL placed them on a common control system to ease data transfer between the Foxboro-provided advanced offsite applications (tank information, oil movements, and blend optimization/supervision) and the refinery unit controls.

A fiber-optic carrierband LAN also connects all five control systems, plus other plant systems, onto a common Plant Information Network (PIN). The PIN provides authorized users throughout the complex with access to any real-time process display or historical information across the five control systems. This network is almost 13 miles in length and is composed almost entirely of commercial off-the-shelf fiber-optic components.

In addition to providing the most cost-effective solution for accommodating the very large distances involved, fiber optics provide other benefits over conventional copper media. These include superior electrical isolation to protect equipment from ground loops as well as electrical discharges from severe thunderstorms common at the site, and freedom from RMI and EMI interference. Additionally, the fiber-optic system's high bandwidth handles the large amount of communications traffic—data, voice, and video—that flows over the Reliance Jamnagar PIN today, as well as the even higher communications rates for future expansions.

Blowing that fiber around

Industrial control applications that can use fiber-optic LANs can also benefit from technology developed by Sumitomo Electric Lightwave Corp. (Research Triangle Park, N.C.). In large, complex, and growing process industry sites, the demand for frequent moves, changes, and additions to an existing network can wreak havoc on installation/maintenance scheduling and budgets and disrupt plant critical operations.

The FutureFlex Air-Blown Fiber-Optic (ABF) cabling system is an infrastructure of compact tube cable used in place of traditional inner duct to form the LAN topology. The tube cables hold up to 19 smaller individual tubes within the outer jacket. Once the tube bundle is in place, installation of the individual fiber bundles can begin. Essentially, the required cable bundles are blown through the cable tube on a stream of air or nitrogen gas at speeds up to 150 fpm using a proprietary cable-blowing head. Two technicians are required for the operation, one at each end of the intended run. If configurations change, cables can also be blown out of a tube (and reused) and new ones blown in.

According to Jimi Hendrix, application engineer for the system, Sumitomo manufactures the blowable fiber bundles with 2, 6, 12, or 18 fibers in the bundle. The fibers are covered with a special polyethylene foam outer jacket that is dimpled like a golf ball, allowing the fiber to float on the air or nitrogen stream. A typical installation goes 3,500 ft at 100-150 ft/min, depending on run geometry.



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