Mining Topic: Lasers

Ignition tests were conducted in a 20 liter chamber that contained the explosions.

What is the health and safety problem?

The Office of Mine Safety and Health Research (OMSHR) strives to reduce adverse safety consequences in the mining industry through effective interventions with new technologies. To this end. laser technologies are being developed for a variety of applications. In the coal mining industry, lasers can be used to detect the presence of dangerous gases, to monitor the level of coal in storage facilities, and to survey the underground mine, just to name a few. However, lasers must not pose an ignition hazard when used in potentially flammable environments found in underground mines and surface facilities. This leads to the question—just how powerful does a laser beam need to be before it can cause an explosion?

What is the extent of the problem?

The rapid development of laser technologies has prompted international efforts to establish safety guidelines for the use of laser-based equipment in potentially explosive environments. Researchers from a number of countries participate with the International Electrotechnical Commission (IEC) 60079-28 Maintenance Team to develop Risk of Ignition by Radiation from Optical Equipment guidelines. Safety guidelines are needed due to the risk of fires and explosions in the presence of methane and coal dust. For instance, lasers could cause smoldering in coal dust layers, leading to the possibility of large-scale burning.

How is OMSHR addressing this problem?

OMSHR researchers have taken an experimental approach to help reduce the risk of ignition by lasers used in potentially explosive environments. Researchers conducted over 1,000 ignition experiments with powerful lasers, studying several variables that can contribute to an explosion. Ignition tests were conducted in a 20-liter chamber that contained the explosions. Explosive concentrations of methane gas or coal dust clouds were put into the chamber. The laser beam was then directed into the chamber using fiber optic cables. If the test resulted in an explosion, the laser power was reduced and the experiment was repeated. The series of tests continued until the laser beam was not powerful enough to ignite the methane or coal dust. Many such series were repeated to study the effect of gas concentration, beam size, and other variables on ignitability.

What are the significant findings?

Laser ignition power thresholds for methane and coal dust were measured. Limiting the laser power to levels below the thresholds will minimize the risk of ignition. Researchers observed that the amount of laser power needed to create explosions was proportional to the laser beam diameter. This suggests that explosions could be prevented even for relatively powerful beams by ensuring that the beam diameter is large enough to reduce the beam intensity. Researchers also measured the time delays for producing laser ignitions, which can be used for designing automatic safety shutoffs. Using these findings, researchers demonstrated the effectiveness of a commercial fiber optic power limiter for preventing ignitions.

What are the next steps?

Results of the work by OMSHR researchers are referenced in the International Electrotechnical Commission standard IEC 60079-28 Explosive atmospheres—Part 28: Protection of equipment and transmission systems using optical radiation. U.S. input to the standard is being coordinated through the ISA 12.21 subcommittee for optically radiating equipment and fiber optics in hazardous locations.

Noteworthy Publications & Products

• Continuous Wave Laser Ignition Thresholds of Coal Dust Clouds (2012-08)
  Laser-based instruments are used in areas where coal dust ignition presents a safety hazard. This NIOSH study determines when an optical beam may be considered a potential ignition source in underground coal mines or coal storage facilities.
• Distributed Measurement of Conductor Temperatures in Mine Trailing Cables Using Fiber-Optic Technology (2012-08)
  Mine trailing cables operated above safe thermal limits can cause premature insulation failure, increasing electrocution and fire hazards. This paper describes an approach to measuring temperatures within reeled cable under dynamic test conditions.
• Ignition of Methane-Air Mixtures by Laser Heated Small Particles (2012-08)
  Optical technologies have progressed rapidly in the past 15 years. NIOSH is conducting a study to help provide a scientific basis for developing appropriate safety guidelines for optical equipment in underground coal mines.
• Ignition Tests With a Fiber-Optic Powered Instrument (2012-08)
  New types of industrial instruments use fiber-coupled laser energy to power remote sensors. The U.S. Bureau of Mines began an investigation of laser-powered fiber-optic instruments in explosive atmospheres in support of the standard-making process.
• Laser Ignition of Flammable Gas (2012-08)
  Emerging laser technologies are quickly gaining acceptance in the industrial workplace. This NIOSH research provides a scientific basis for developing safety guidelines for optical equipment in the presence of flammable methane gas and coal dust.
• Methane-Air Mixtures Ignited by CW Laser-Heated Targets on Optical Fiber Tips: Comparison of Targets, Optical Fibers, and Ignition Delays (2012-08)
  Researchers conducted experiments to estimate the mean and standard deviation of laser powers needed to ignite 6% methane-air atmospheres using single-mode optical fiber tips covered by two types of iron oxide mixed with a ceramic adhesive.
• Optically Powered Remote Gas Monitor (2012-08)
  Many mines rely on toxic gas sensors to help maintain a safe and healthy work environment. This report describes a prototype monitoring system developed by the USBM that uses light to power and communicate with remote toxic gas sensors.
• Remote Fiber-Optic Methane Monitor (2012-08)
  Reports on the U.S. Bureau of Mines development and testing of an instrinsically safe methane monitor based on differential absorption of infrared light.
• Threshold Powers and Delays for Igniting Propane and Butane-Air Mixtures by CW Laser-heated Small Particles (2012-08)
  NIOSH researchers studied laser safety in potentially flammable environments. Researchers measured threshold igniting powers as a function of beam diameter for butane and propane-air mixtures by laser-heating targets placed on optical fiber tips.