Lessons Learned from Refuge Alternative Research by NIOSH: Occupancy derating of refuge alternatives in underground coal mines.
Collectively, these three tragedies drew national attention to the plight of miners trying to escape during an emergency, and led to major changes in how one thinks about coal mine emergency preparedness and response. Today, although using a refuge alternative should still be a last resort in an emergency, miners and mine operators need to be well prepared for that possibility, and part of that preparedness is understanding the exact conditions that miners would encounter in a refuge alternative once it was occupied.
To that end, in cooperation with a company specializing in thermal simulation and infrared analysis software, and in collaboration with five cooperating mines, the National Institute for Occupational Safety and Health (NIOSH) has conducted testing and developed benchmarked thermal simulation models to examine thermal conditions inside occupied refuge alternatives.
Government Legislation and Refuge Alternatives
The Mine Improvement and New Emergency Response Act (MINER Act) was signed into law on June 15, 2006. It called upon NIOSH to conduct research on refuge alternatives, which includes investigating their utility, practicality and survivability.
On December 31, 2008, MSHA published its rule requiring refuge alternatives for underground coal mines. The rule states that they must be capable of sustaining trapped miners for 96 hours.
Because heat buildup in a confined space such as a refuge alternative is a health and safety concern, the rule mandates a maximum apparent temperature of 95[degrees]F within an occupied system. The apparent temperature is a heat index calculated from two factors: air temperature and relative humidity. As an example, an air temperature of 83[degrees]F and a relative humidity of 90% would result in an apparent temperature of 95[degrees]F. Obviously, higher air temperature and /or higher relative humidity would result in higher apparent temperature.
Refuge Alternative Thermal Environments
Heat transfer in an occupied refuge alternative involves complex processes and multiple heat sources. The heat sources include the miners and the unit's carbon dioxide scrubbing system, but the main source is the metabolic heat from the miners themselves, involving the burning of calories to sustain the body. In an occupied refuge alternative, miners would transfer heat via conduction (from the miner to the refuge alternative and the mine), convection (from the miner to the air inside the unit), and radiation (from the miner to the surrounding environment in the refuge alternative). Also, in hot conditions, the body naturally loses heat by sweating. Therefore, as the sweat from miners would evaporate in an occupied system, the humidity inside would rise. Considering all these factors, the environment within an occupied refuge alternative would certainly be hot and humid.
NIOSH conducted its first heat and humidity tests on four refuge alternatives in 2007. Because the MINER Act required NIOSH to submit a report on refuge alternatives by the end of 2007, and these tests were done to gather important information for the report, these early tests had to be completed very quickly, so there was too little time to develop sophisticated devices to represent the heat and moisture input of miners. In these tests, NIOSH used incandescent light bulbs and ultrasonic humidifiers to represent the heat and humidity of miners occupying a refuge alternative. The results showed that the apparent temperature limit was exceeded for two of the four units. However, the use of incandescent light bulbs and ultrasonic humidifiers to represent the heat and moisture input of miners may not have been accurate.
In 2012, NIOSH launched a new effort to more accurately examine the thermal environment of occupied refuge alternatives, developing simulated miners to represent the heat and humidity of actual miners. These simulated miners were built using 30-gallon barrels and a water-filled core with attached electrical resistance heaters. The simulated miners were designed based on the principle that their surface area should be similar to the exposed skin surface area of actual miners in a refuge scenario, and the simulated miners should output heat at the same rate as actual miners in a refuge situation. In addition, the simulated miners were designed to output the moisture (sweat) of actual miners.
During testing, the surface temperatures of the simulated miners were shown to be similar to that of actual miners. In addition, the moisture output of the simulated miners was similar to the expected moisture loss of actual miners. Thus, NIOSH determined that the simulated miners provided a good representation of the heat and moisture output of actual miners.
Simulated miners are necessary for performing refuge alternative heat and humidity tests because using human subjects would not be practical and using simulated miners eliminates exposing human subjects to health risks. Using simulated miners also provides consistent sources of heat and humidity so that test conditions are repeatable.
NIOSH used the simulated miners to conduct more than 30 96-hour heat and humidity tests on refuge alternatives in the NIOSH Safety Research Coal Mine and the NIOSH Experimental Mine near Pittsburgh, Pennsylvania. Some tests involved portable refuge alternatives while others involved built-in-place refuge alternatives, with the bulk of the tests conducted with the refuge alternatives at full capacity. In other words, a 23-person tent-type refuge alternative was tested with 23 simulated miners, a six-person rigid-type unit was tested with six simulated miners, and so on. All the refuge alternatives were tested with the simulated miners providing heat and moisture. Some additional tests were conducted "dry" without moisture, and a few others were conducted with the refuge alternatives at more or less than full capacity.
The specific refuge alternatives tested were as follows:
* One 10-person tent-type training unit;
* One 23-person tent-type unit;
* One six-person rigid-type unit; and
* One 30-person built-in-place unit.
Note that the above capacities are based on the requirement for refuge alternatives to provide 15 square feet (ft2) of floor space per occupant.
The testing conditions mimicked the situation of refuge alternatives being supplied with oxygen from cylinders versus air from a borehole air supply. During these tests, NIOSH measured the temperature and relative humidity inside the refuge alternatives and the temperatures of the mine air and mine strata. The NIOSH mines are about 60[degrees]F year-round, so the refuge alternative air temperatures were roughly 60[degrees]F at the start of the tests. By the end of the tests, the air temperature inside the refuge alternatives increased by about 15[degrees]F to 20[degrees]F, so the final air temperatures in the refuge alternatives ranged from 75[degrees]F to 80[degrees]F. Also, the relative humidity reached roughly 85% to 95% within about eight hours after starting the tests, and remained there for the duration of the tests.
After the tests, water droplets covered the roofs and walls of all the refuge alternatives, and the floors of all the refuge alternatives had puddles of water in low spots. For the tent-type units tested as part of this research, the support tubes around the bottom of the refuge alternative formed a trough between the wall and the floor, which was filled with water that had condensed on the walls before running down to the trough. These accumulations of moisture show that the conditions inside an occupied refuge alternative would be very humid.
Ideally, refuge alternative heat and humidity testing would be conducted across a range of initial mine temperatures, mine strata thermal properties, and mine entry sizes. However, this is not practical. Therefore, using thermal simulation models would be necessary to explore the effects of these parameters on refuge alternative air temperature and relative humidity. In addition to testing thermal conditions with simulated miners, NIOSH worked with ThermoAnalytics Inc. (TAI) to develop thermal simulation models of two tested refuge alternatives: a 23-person tent-type unit and a six-person rigid-type unit. TAI used NIOSH data to benchmark the thermal simulation models, and thermal simulations were conducted with computer models of simulated miners and computer models of actual miners.
These thermal simulation models were used to examine how various factors, such as initial mine air temperature, initial mine strata temperature, mine strata thermal conductivity, and mine entry size affect the thermal environment of a refuge alternative. The initial mine strata surface temperature was found to be the primary factor, and it was found to be more important than the initial mine air temperature. Lower mine strata thermal conductivity increased the final temperature by a few degrees, and smaller mine entries increased the final temperatures in a refuge alternative by a degree or two.
NIOSH and TAI also used the thermal simulation models to determine the final temperature, relative humidity, and apparent temperature for the 23-person tent-type RA and six-person rigid-type RA across a range of initial mine temperatures, from 55[degrees]F to 75[degrees]F, in 5[degrees]F increments. Initially, the simulations were carried out with the refuge alternatives at their full capacity, and if the maximum apparent temperature limit mandated by the MSHA rule was reached, then the refuge alternative occupancy was reduced, or derated.
In the thermal simulation models of the 23-person tent-type RA and the six-person rigid-type RA, for an initial mine temperature of 65[degrees]F, the conditions inside the refuge alternatives were nearly at the maximum apparent temperature limit of 95[degrees]F. For initial mine temperatures of 70[degrees]F and above, the 95[degrees]F apparent temperature limit was exceeded. At initial mine temperatures of 70[degrees]F or above, the occupancy of the refuge alternatives had to be reduced, or derated, so the 95[degrees]F apparent temperature limit was not reached.
Work With Cooperating Mines
Because the NIOSH test mines are at about 60[degrees]F year-round, the refuge alternative heat and humidity testing described here could not be performed at other mine temperatures. In addition, there was no way to vary the thermal conductivity of the NIOSH test mines. Furthermore, it is impractical to conduct in-mine refuge alternative heat and humidity testing at mines across the U.S. at other mine temperatures and with other mine strata thermal conductivities, as it would interfere with operations.
To ensure that NIOSH heat and humidity research on refuge alternatives was as complete as possible, NIOSH wanted to examine the resulting internal thermal environment of refuge alternatives across a wide variety of actual production U.S. coal mines with different mine temperatures, mine strata compositions (thermal properties), and mine entry sizes. In support of this effort, five mines across the U.S. worked with NIOSH to measure the fluctuating mine air and mine strata temperatures near existing refuge alternatives for a time period of at least one year to determine the worst-case thermal conditions at each mine. The mines were located in the Eastern, Midwestern, Western, Southern, and Southwestern regions of the U.S.
Based on the thermal simulation models described above, NIOSH and TAI used the mine temperature data, mine strata thermal properties, and mine sizes of each of the five mines to carry out simulations to determine the final temperature, relative humidity, and apparent temperature of the 23-person tent-type and six-person rigid-type refuge alternatives tested by NIOSH. Simulations used the worst-case temperatures for the mines to determine if the apparent temperature limit would be reached. If the apparent temperature limit was reached, the occupancy of the unit(s) was reduced and the simulation was rerun to determine the maximum number of miners that could occupy the refuge alternative for the mine in question without exceeding the apparent temperature limit.
In these simulations, based on data gathered from the five participating mines, the 95[degrees]F apparent temperature limit was exceeded in some cases and derating was therefore necessary. The following observations were made:
* For the Eastern, Western, and Southwestern mines, derating was not necessary for either refuge alternative.
* For the Midwestern mine, which had a worst-case mine temperature of 69[degrees]F, the 23-person tent-type RA would have to be derated by two, to 21 miners. The six-person rigid-type RA would not have to be derated.
* For the Southern mine, which had a worst-case mine temperature of 81[degrees]F, the 23-person tent-type RA would have to be derated by 22, to only one miner. The six-person rigid-type RA would exceed the apparent temperature limit with even just 1 miner. These more extreme cases for the Southern mine underscore how critical the initial mine temperature is with respect to occupancy derating.
As these results show, derating a refuge alternative's occupancy is not a "one-size-fits-all" proposition. By definition, each unit to be derated must be considered as an individual case based on the refuge alternative type and mine.
Applying Occupancy Derating
Results from NIOSH testing and benchmarked thermal simulation models show that the MSHA-mandated apparent temperature limit of 95[degrees]F may be reached in refuge alternatives depending on initial mine strata temperature, mine thermal conductivity, and, to a lesser degree, mine entry size. If the apparent temperature limit is reached, it would be necessary to derate the occupancy of a refuge alternative or to use some type of cooling system.
Obviously, the necessary derating discussed here depends on the mine-specific refuge alternative installation. The most critical parameter to consider is the initial mine strata temperature, which, in many cases, is approximately equal to the mine air temperature. Table 1 shows an approximate occupancy derating based on mine temperature. Importantly, this table was derived from the specific conditions and locations described here--i.e., NIOSH's Safety Research Coal Mine and Experimental Mine, and the refuge alternatives used for NIOSH tests and simulations. Derating percentages for refuge alternatives in actual mines should be based on a combination of mine temperatures, thermal conductivity values, entry sizes, and the specific refuge alternative used in the installation.
With the concept of occupancy derating to reduce a refuge alternative's apparent temperature now thoroughly explored, NIOSH is conducting research on the use of cooling systems to reduce the temperature and relative humidity. Use of cooling systems might be more desirable, especially considering the high initial temperatures in some mines, where solutions other than derating could be necessary. Cooling strategies such as battery-powered air conditioners, cryogenic air supplies, and carbon-dioxide-based cooling systems may be possible solutions in these cases. To that end, through external research contracts, NIOSH is currently working on developing a battery-powered air conditioner and a cryogenic air supply for refuge alternatives.
Dave Yantek is a lead research engineer for the Electrical and Mechanical Systems Safety Branch of NIOSH's Pittsburgh Mining Research Division. He can be reached at: email@example.com. Joseph Schall is a health communications specialist for the Health Communications, Surveillance, and Research Support Branch of NIOSH's Pittsburgh Mining Research Division. For further information on NIOSH's refuge alternative research, go to: www.cdc.gov/niosh/mining/researchprogram/projects/project_RefugeAlternatives. htm I.
The findings and conclusions in this paper are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health (NIOSH). Mention of any company or product does not constitute endorsement by NIOSH.
Caption: NIOSH-developed simulated miner is used for refuge alternative heat and humidity testing.
Caption: Simulated miners inside a 23-person tent-type refuge alternative being tested in the NIOSH Experimental Mine.
Caption: Thermal simulation results of a fully occupied 23-person tent-type refuge alternative using the TAI human thermal model. Temperatures are indicated by color, with blue indicating the lowest temperatures and red indicating the highest temperatures. To simulate real conditions, some occupants are lying down and some sitting up.
Caption: Thermal simulation results of a 10-person training unit tent-type refuge alternative using models of simulated miners. Temperatures are indicated by color, with blue indicating the lowest temperatures and red indicating the highest temperatures.
Table 1-Example of derating percentages for refuge alternatives in the NIOSH Safety Research Coal Mine and Experimental Mine based on heat and humidity testing and thermal simulations. Mine Temperature % Derating Needed to Prevent Range Apparent Temperature From Exceeding 95[degrees]F 60[degrees]F 0% to 30% to 65[degrees]F 65[degrees]F 20% to 50% to 70[degrees]F 70[degrees]F 25% to 70% to 75[degrees]F 75[degrees]F 60% to 90% to 80[degrees]F
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|Title Annotation:||REFUGE ALTERNATIVES|
|Comment:||Lessons Learned from Refuge Alternative Research by NIOSH: Occupancy derating of refuge alternatives in underground coal mines.(REFUGE ALTERNATIVES)|
|Author:||Yantek, David; Schall, Joseph|
|Publication:||Coal Age (1996)|
|Date:||Dec 1, 2017|
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