Explosion protection for optical radiation in hazardous locations
Three protection schemes are under consideration.
Industry uses optical equipment for surveying, communications, sensing, and measurement. Often the installation is inside or close to potentially explosive atmospheres, and radiation from this equipment may pass through these atmospheres, exposing them to a potential ignition source. The explosive mixture may absorb the radiation, leading to a local temperature increase or photochemical process, and ignition or focused laser radiation may cause formation of plasma capable of causing ignition.
In practice, the greatest danger is absorption of the radiation by a solid target (surfaces or particles), causing the heated surface to become an ignition source.
The International Electrotechnical Commission (IEC) has set up Working Group 8 to develop requirements for optical equipment used in hazardous atmospheres.
Continuous wave radiation
Several publications, including a European standard, deal with ignition of explosive fuel/air mixtures by optical radiation. The most likely ignition hazard is a local temperature rise caused by light falling on radiation-absorbing materials.
In view of this mechanism, the European Commission has sponsored a research project to determine limiting values for continuous-wave radiation sources intended for use in potentially explosive atmospheres. Researchers investigated various radiation sources, target materials, and explosive mixtures to obtain minimum values for the radiant power and irradiance capable of causing ignition; 50 milliwatts (mW) has been found to be the lowest igniting radiant power, including explosive mixtures with very low minimum ignition energies and autoignition temperatures. We consider radiation an ignition source if the radiated power is greater than 35 mW or the peak radiation flux is more than 5 mW per square millimeter (mm2).
We have completed additional research, including pulsed optical sources and a combustible target. The findings affirmed the results of the initial research, even when the targets were combustible. Unfortunately, the results show no clear structure similar to the explosion group/temperature class system of IEC/CENELEC (EN50014) or CEN standards (prEN 13 463). On the other hand, ignition of gases in explosion group IIA in combination with temperature classes T1, T2, or T3a requires a minimum of 200 mW for flammable liquids. As this combination covers the majority of flammable gases and vapors, a power limit based on these findings would help considerably in practice.
In the special case of methane, recent research has shown a linear relationship between fiber diameter and minimum igniting power for diameters greater than 100 micrometers (µm). With smaller fibers, the minimum igniting power levels off.
We have investigated pulsed radiation using using two independent methods, relying on a Nd/YAG laser in both cases. We affixed inert and combustible target materials to the end of
62.5- and 400-µm fiber-optic cables or focused the unconfined laser radiation on a small spot on the target material. This resulted in lower ignition energies than with a coated fiber end target.
For the fiber-optic arrangement and laser equipment used, we found that ignitions with 70-microsecond spiked pulses were the most hazardous case. A plasma spark generated on the target material was the primary cause of ignition. Radiative ignition energies may approach the electrical ones, but under normal (atmospheric) conditions they will not fall below the latter. With carbon black as target material, "optical minimum ignition energies" for the combustibles hydrogen, propane, diethyl ether, pentane, and carbon disulfide were a factor of only 2.4 to 9.3 higher than the electrical ones.
Working Group 8 is discussing three protection concepts. The first is inherently safe optical radiation. Based on the research results, the working group is considering experimental ignition threshold values. Continuous wave devices radiating in the visible and near infrared range are unable to ignite a surrounding explosive atmosphere, provided the radiated power is less than 35 mW or the peak radiation flux is less than 5 mW/mm2.
These values are a factor of about 1.5 below the experimental ignition thresholds for the most easily ignited gases or vapors. The above limits, therefore, cover all flammable substances in the explosion hazardous locations. Different values of power and flux may, however, be appropriate for the following:
- Pulsed or intermittent light sources
- Resonance absorption of the radiation in the explosive atmosphere
- Specific mixtures (e.g., oxygen-rich mixtures)
- Devices that emit radiation from an area less than 3 x 10-5 mm2
- Where sensitive substances (explosives, pyrotechnic, and unstable compositions, etc.) are present
One of the questions addressed by the second European research project was classification of gases and vapors. The results have shown there is no direct relationship between the established gas groups and temperature classes and the safe levels of optical power or irradiance. However, all gases and vapors belonging to Group IIA (IEC), Group D (National Electric Code), and temperature classes T1, T2, and T3 (with autoignition temperatures above 200°C) could not be ignited with an optical power below 200 mW, even with small fiber diameters.
When deducing safe limit values from these results by applying a safety margin, engineers must consider the absorptivity and combustibility of the target material, as well as the mixture temperature. A power limit of 150 mW, representing a safety margin of about 2 compared with the lowest igniting power value, is appropriate for fiber-optic instrumentation used in mines, where explosive methane/air mixtures (and coal dust/air mixtures) may occur.
Meeting safety needs
Two cases have to be evaluated for the safety assessment of electrical/optical equipment:
- The optical radiation source is part of a piece of electrical equipment for use in the hazardous (classified) location. Examples are bar-code readers, scanners, data transmission links, and laser pointers to mark measuring points. The electrical equipment must comply with the relevant standards requirements for equipment used in explosive atmospheres.
- The radiation source is located in the safe area. Only the optical radiation, in a free beam or an optical fiber, enters the explosion hazardous location. Examples are spectroscopic analysis of chemical substances and counting systems using optical fibers or level gauging. Here, the source of optical radiation is assessed similarly to an "associated apparatus" in type of protection intrinsic safety "i."
Researchers installed a test setup that allows computer-controlled measurement of the radiant power and pulsed energy of optical sources. Specially designed programmable power sources control semiconductor laser diodes and infrared light-emitting diodes. Further, researchers may vary the enclosure temperature of the diode.
Among the most important results is that semiconductor laser diodes are quite capable of emitting a higher radiant power than the maximum radiant power specified in the data sheets. The data sheet values refer to "kink-free" operation, a nonlinear path for the plot of the radiant power vs. the DC current above the maximum rated power. For the safety assessment, however, engineers must consider a failure in the supply circuit. Because the maximum emitted radiant power depends on the individual sample, the maximum value derived from 10 samples of the same laser diode type is used.
Engineers must perform a practical test if they can't directly observe the threshold values of radiant power or peak radiation flux. Key parameters for the procedure are shape and volume of the test vessel, target materials, power measurement, test mixtures, mixture temperature and pressure, and ignition criteria. Engineers must provide a safety factor, too, because ignition by a small hot surface contains considerable statistical deviations.
The second concept, "inherently safe optical radiation," covers a lot of optical-beam applications and relies on ignition of gases and vapors by particles or surfaces heated by irradiating beams. An explosion occurs only if all the following are present simultaneously:
- Flammable gas or vapor cloud
- Radiating source of the duration and intensity needed to cause ignition
- Appropriate target particle or surface
Complying with the threshold values for the radiated power or the peak radiation flux assumes that all other conditions for an ignition are optimal. When radiation is inside optical fibers or some other medium, however, and there is no possibility of mechanical damage or breakage, there will be no ignition.
If a fiber run is in a Zone 1 hazardous location with the protection required for electrical cables with nonintrinsically safe voltages/currents, then the installation is safe regardless of the irradiance values. The coupling devices and radiation source are located in the safe area in this case or are otherwise protected. Another possibility is to keep the optical radiation inside an enclosure that is protected similarly to electrical equipment.
A third way to prevent critical radiation from becoming an ignition source is the use of cutoff systems that switch off the radiation source at the transmitter if reception at the receiver is lost. The designers must consider the operating delay of interlocks and the reliability and period between inspections that would detect failed elements.
The thermal time constant of the target must be compared with the operating delay of the cutoff system. If the latter is higher than the reaction time of the interlock, ignition risk is low. In case of fiber breakage or other failures, designers should assume at the first instant there is no target at the broken end of the fiber to absorb optical power.
Working Group 8 welcomes comments on this concept from industry, users, and others. IT
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Behind the Byline
Dr. Heino Bothe is head of Department 3.3, Physikalisch-Technische Bundesanstalt, Braunschweig, Germany.
Dr. Ulrich Johannsmeyer is head of Section 3.42, Physikalisch-Technische Bundesanstalt, Braunschweig, Germany.