US Mine Safety and Health Administration mining engineer William Francart presented his work at last year’s 11th US/North American Mine Ventilation Symposium at the Pennsylvania State University, highlighting statistics of hazards, frequency, ignition sources and other factors as part of his work and noting several elements that could easily reduce fires.

“The use of belt air to ventilate working sections has been a major reason for mine operators installing carbon monoxide monitoring systems in belt conveyor entries of underground coal mines in the United States,” Francart said, adding that the growth of belt air ventilation has really seen an increase in popularity following the 2003 rulings that negated the need for a pre-project petition that also required the use of CO monitoring systems.

CO monitoring systems are also seeing an increase in use recently, especially over the use of point-type heat sensors (PTHS), due to their efficiency and accuracy. In general, Francart said, mines have found the return on investment (ROI) as a safety and fire prevention feature to be incalculable.

Looking at years of reports recording past fire incidents, the MSHA researcher found that 15–20% of all those reported since 1980 were belt fires (identifying “reportable” as those events not extinguished within 30 minutes of discovery).

Because most fires result in the establishment of a report – an MSHA 7000-50a – that includes root-cause analysis, the ability to review 62 belt entry fires that had occurred during a 26-year period from 1980–2005 was easily compiled for analysis and review to determine if conveyor maintenance may have been a contributor (one was not included because it was still pending).

Francart elaborated on the significant effect these occurrences can have on production at operations for which these belts are the arteries delivering output to the surface.

“Depending on the location of a fire within the belt, coal production can be disrupted for days, weeks, and historically has even been the death knell to a coal mine,” he said, adding that infrastructure and even roof control conditions can be compromised.

He noted, however, that technology is again coming to the industry’s aid. “New fire detection system technology is evolving, allowing mine operators to detect fires before they cause extensive damage,” he said.

No matter how much technology a mine adds, Francart said there’s no substitute for manual maintenance by a human being. “Proper maintenance of the belt and belt components may have prevented many of the 61 reported fires included in this analysis,” he said.

Also noted as a part of the researcher’s work was a downward trend in belt entry fire frequency during those same years. The exceptions to that statement both were in this new century, with four in 2002 and three in 2003, and otherwise averaged about one to three fires per year per 1000 mines.

Francart stated that while many mine fires were attributed to ignition sources including compromised safety controls, insufficient maintenance and using equipment in a manner other than intended, conveyor belt-oriented blazes often include the factors of accumulation and spillage, deficient design, insufficient lubrication and poor roller replacement practices. Slippage, misalignment and derisory examination schedules are also cited as potentials that can lead to the frictional heating (a primary cause in this study) that becomes the ignition source.

Conveyor transfer points and belt drives are often considered to be principal ignition points, he said, because of the amount of electrical components in those areas and the propensity for spillage and build-up at others. In fact, the areas were specifically noted as part of MSHA’s 2003 regulations; all belt air being used on active sections was required with the rulings to be installed downwind of belt drives.

“It is interesting to note that the number of frictional ignitions along the belt was equal to the number identified as originating at belt drives,” Francart said.

“While it is probably that many of the indeterminable sources were also at or near drives, the numbers indicate ignition along the belt is a significant concern.”

With the two primary forms of technological prevention for the fires being CO monitors and PTHS systems, he said the anecdotal evidence the agency has done over the years has shown the former of the two to be the most effective. Advantages including early warning for evacuation and combating the flames has allowed the systems’ popularity to increase exponentially; in fact, according to statistics, more than 20% of active mines currently employ CO detectors – not due to regulations, but because of their faith in the units’ overheat sensor systems.

Francart said that while belt air seemed to be the preferred method of larger mines historically, smaller mines have recently taken to the trend as of late. He said that “may strengthen”, as the technology employed in CO monitor research and development makes them even more discriminating in what it senses.

He added that he supports intake belt air as a fire prevention method, as statistics have shown it to be positive.

“While extensive data on non-reportable fires is not readily available, there is little doubt that the use of belt air as a section intake has reduced the incidence and, in some cases, the severity of belt fires,” he said.

As many as 17 of the fires in his study were determined to have been related to frictional heating, according to Francart’s report. Therefore, “it is probably that one in four fires in belt entries is related to frictional heat as a belt drive.”

Using the identifiers of ignition sources, prevention techniques are more easily determined, such as increased maintenance of rollers, reducing slippage, and working to reduce drags and misalignments. Regulated use of slippage and sequence switches also aids in this.

Francart identified three types of switches: magnetism, centrifugal force and photo-electricity, adding that a common choice for underground operations is a system which includes slip switch proximity censor. Using these methods in combination with regular maintenance, roller change-outs and belt checks are key.

“The use of belt air to ventilate the working section has directly improved the fire detection capability in … coal mines due to the required use of CO monitoring systems,” he said, noting that technology has played a large part.

“However, mine operators cannot rely on the use of CO monitoring systems alone for early fire detection.

“Only prevention through proper maintenance and examination can effectively prevent reportable belt entry fires.”