01 June 2003
Shedding Light on Industrial Fiber Optics
By Ellen Fussell
A strand of hope for fiber's future
"With sufficient power you can generate an explosive environment with fiber optics," said Don Baker, operations manager at P&D Electric Inc. in Shelbina, Mo. "If you get a broken fiber in the right environment of gas or dust, you can create an explosion. That's been proven."
As an engineer working in construction, Baker knows it's important to understand when cabling is suitable for a certain environment. "The cable has to be suitable so the gas doesn't permeate the cable or travel down the cable," he said. To prevent this from happening, "you could put the fiber in a conduit just like you do with high-voltage wiring," he said. "You could put in a conduit system that would guarantee it would be just as sound as any C-500 installation. You could develop an intrinsically safe switch or device, so if the fiber breaks, it automatically shuts off the energy source and removes the energy required to ignite the gas or dust." The problem, Baker said, is identifying what gases in what environments are really hazards. What power levels are hazards?
ATTENUATION ANOTHER ISSUE
Infrared meets fiber optics
Infrared technology and fiber optics have become well acquainted in the past years as chemical, petrochemical, pharmaceutical, and other polymer manufacturing plants use Fourier transform infrared (FT-IR) and Fourier transform near IR (FT-NIR) to measure polymers to control its polymerization process online. (Polymers are small molecules strung together and are the basis of all plastics.)
Online analyzers can monitor the chemical composition in a process, and using fiber optics (through near IR) during this process allows analyzers to put the probe directly into the process pipe—gaining real-time measurement of the pipe's composition.
Manufacturers can make process control decisions based on real-time analytical or compositional data. With FT-NIR and fiber-optic interface, manufacturers can measure immediate composition in the pipe. So there's hardly any time delay except a few seconds. (See April 2002 InTech's tutorial on infrared.)
Testing at companies such as Dow and Lucent ensures communications fiber can tolerate low temperatures. "Optical fibers used in aircraft might get to minus 60ºC when the aircraft is up at 35,000 feet. So your communications fiber has to withstand low temperatures in that case too," Harner said. Experts learned how to create cables to survive those temperatures in the 1990s, "but we weren't doing communications. We were doing sensors," he said. "And we all learned something new."
Harner explained in a paper presented at the 2002 International Federation of Process Analytical Chemistry meeting how his team found a certain cable manufactured for Dow Chemical experienced attenuation when the temperatures went below freezing. Loss of light shouldn't have anything to do with low temperature, but low temperature causes fiber to lose light. If you lose light and it's flat across the absorption spectrum, it won't hurt anything. But if in addition to losing light, absorption occurs in specific wavelengths, there is a problem, he said. (In the visible realm, colors—violet and true red—represent the absorption spectrum, but fiber optics work in the infrared.)
"Even though we had specified for the operation at low temperature—Fourier transform near infrared measurement—we knew there was a possibility of a problem," he said. "But we didn't realize there was a problem. The issue was that we had contracted with the vendor to create a cable for us with optical fiber inside," Harner said. "We had talked about low-temperature operations, but the cable was manufactured improperly. Nobody tested it."
The issue was more than loss of light, Harner said. "There was a spectrum of the fiber growing in as the temperature went down, which caused a problem. There was another absorption going on due to fiber," he said. "Typically when you build a system, you assume the fiber is transmitting light reliably; the transmission light doesn't change with outside influences. So we're trying to measure that with the tip of a probe."
Other issues to address are "what kinds of installation practices we're going to adopt in the real world to put in fiber so that we don't lose the economic advantage of fiber," Baker said. Every piece of fiber has a transceiver that takes the light and converts it to energy. "Those transceivers have to be suitable for the locations you're in. You can't go into classified areas and put in a transceiver that's not suitable for that environment," Baker said.
Although he admits he hasn't personally seen any instances where something exploded, Baker said that doesn't mean the potential isn't there. "We need to be careful where we put fiber and have some knowledge of the potential hazards," he said. "Just because it hasn't happened, we know it can because it's been proven in the lab." (SP12.21 chairman Tom Dubaniewicz made films with coal dust and methane gas exploding from broken fiber.) The publication of the new 12.21 standard will reveal "what levels and environments you need to worry about," he said.
WHEN TO USE FIBER OPTICS
Fiber optics might come into play when transmitting light during chemical composition measurements in the process area—in measuring caustic composition and polymer composition. "In any hazardous location, where you have a concentration of flammable material, oxygen, or fire, optical fiber has the possibility of providing the ignition source," Harner said. "In the case of fiber optics, it's heating up a target particle to a high enough temperature to ignite the flammable gas. The distinction is if you have an electrical spark controlling electrical ignitions in hazardous areas, which is where all these intrinsically safe issues come from," he said.
One of the differences in optical fibers is "you have to break the fiber, but you also have to have a target, like a dust particle or something that absorbs the light and heats up to above the auto-ignition temperature," he said.
"If you've got powers to cut steel, you could cause an ignition. But we're talking milliwatts of power," he said. There is a lower limit that research has shown—below that limit you cannot cause an ignition with a certain optical power. Harner said the ISA SP12.21 committee is working on setting that lower limit and the requirements to get ignition.
GO OR NO GO
Fiber-optics gurus never used to worry about installing fiber in the past, Harner said. "When we started using fiber optics in process areas twenty years ago, we considered them intrinsically safe. We put them in areas classified as hazardous. Then the information in the past ten years shows under the right conditions, you could cause an ignition by optical power transmitted through an optical fiber. It's not well known that that's the case."
Sometimes optical fibers are put into vaults underground with other electrical cable. So if that fiber is carrying electrical power, what happens if you cut that power? Methane is a flammable gas. If you break an optical fiber, it could cause a fire in this underground vault. That's the issue.
"Normally you'd be terminating fibers with the power levels off," Harner said. "But if you're polishing the fiber (making a connection in an optical fiber—transmitting light from one end of the fiber to the other), and the ignition is on, would you be polishing under hazardous conditions? Probably not. But you need to consider it." IT
New standard offers answers
The ISA SP12.21, Fiber Optics, subcommittee is working to determine how much power it takes to create explosive energy levels with fiber optics, and what might be done to prevent it—setting limits for using optical fibers in hazardous areas (April InTech Standards department).
One of the reasons the committee is writing the report is to educate installers of optical fiber that this is a potential hazard. "In flammable or classified areas, granted there's a very restrictive set of conditions that has to occur to get it to ignite," said Rich Harner of Dow Chemical. "But if there's any possibility, we don't want to cause an ignition in a production area or actually in a telephone or communication situation that uses fiber."
The report explains when an optical beam may be a potential ignition source. It suggests engineering and installation practices for reducing optical ignition hazards and introduces three protection concepts for preventing optical ignitions: inherently safe, protected fiber-optic cable, and optical radiation interlock.
The first concept—inherently safe—is a protection concept analogous to intrinsically safe electrical circuits. The report recommends safe powers, irradiances, and energies for different Class I materials or material groupings and summarizes results of laboratory studies conducted over the past fifteen years. The technique of protected fiber-optic cable allows a rugged cable design to prevent the optical fiber from breaking and allowing a potentially ignition-capable laser beam to interact with a flammable atmosphere. The document summarizes mean time between fiber breakage estimates for determining when the protection technique may be suitable for a given zone or division, or for determining when the protection technique can be combined with other techniques to prevent ignitions in some of the more hazardous locations.