Over a four-year period, a mine manager and mining crew at two underground limestone mines had a significant impact on assisting National Institute for Occupational Safety and Health (NIOSH) researchers in reducing contaminants and improving the air quality in enclosed cabs of mobile mining equipment. Because of this positive cooperative working relationship, NIOSH researchers were able to evaluate the following:
The impact of a powered intake air unit versus a static design,
Use of an in-cab pressure monitor to determine the integrity and functioning of filtration and pressurization systems,
Changes in air quality using different filter configurations and filter dust loading characteristics in the filtration and pressurization systems, and
HEPA versus MERV 16 quality filters on the air quality inside the enclosed cabs.
Gaining partners
Over the past 15 years, the Pittsburgh Mining Research Division (PMRD) for NIOSH has partnered with many different companies and organizations to improve the health of miners working inside enclosed cabs of mobile mining equipment. In addition, NIOSH has also partnered with an array of different surface and underground metal/non-metal mining companies to perform research studies on reducing dust/contaminants and improving the air quality inside of enclosed cabs.
J.H. Fletcher Co. face drill and roofbolter machine were evaluated at two different underground limestone mines over four years.
After the completion of one of these studies, the results were published and presented at the SME Annual Meeting. When the presentation ended, Doug Hardman, president of J.H. Fletcher Co., approached the NIOSH researcher and informed him of his company’s desire to build upon NIOSH’s research with the design of its filtration and pressurization system with the goal of improving the air quality to miners working in its equipment. From this conversation, a cooperative working relationship was established with J.H. Fletcher Co. and this longstanding relationship continues to this day.
Initially, shop testing was performed on a few different pieces of Fletcher’s mining machinery at its headquarters and manufacturing facility in Huntington, W.Va. Next, engineers with Fletcher’s metal/non-metal underground sector laid the groundwork for testing at an actual mine site to evaluate two new pieces of its equipment equipped with the newly designed filtration and pressurization system. This cooperating site was the Sidwell Mining Co. underground limestone mine, south of Zanesville, Ohio.
NIOSH’s point of contact was Ted DiNardo, the mine manager at this relatively new underground limestone mine. From November 2010, through July 2011, NIOSH performed research during 13 different visits to this mine. Unfortunately, at the end of December 2011, DiNardo informed the researchers that the Sidwell underground mine was closing and all its underground assets, including the equipment, were being sold to the nearby Shelly Materials Co. (part of Oldcastle Materials/CRH). However, as part of the sales agreement, all of the original Sidwell underground employees, including DiNardo, were offered jobs with Shelly, which was opening a brand new mine only 4 miles away from the previous site. Once the new underground mine progressed to an extent where testing could be performed, NIOSH continued its research with the mine manager and underground mining crew. Fortunately, since the same two pieces of equipment tested at the previous mine — a single-boom face drill and roofbolter machine — were purchased, NIOSH was able to continue its research on the exact same equipment. From April 2013, through the completion of research in November 2014, NIOSH traveled to this underground limestone mine on 19 different occasions as a part of this cooperative research effort.
Making research advances
As a result of this cooperative research effort, advances were made in four areas.
1. The impact of a powered intake air unit versus a static design.
It was determined that a powered intake air system was the optimal design over a static type system. On a powered unit, the intake air has its own fan, so the air is delivered at positive pressure through ductwork to the main HVAC unit. One benefit to this is that a known quantity of intake air is always provided to the enclosed cab. Obviously, as the intake filter loads with dust, the intake air quantity decreases, but there is a known and reliable air quantity range from a clean to a fully loaded filter. The intake air is a critical component in the enclosed cab’s positive pressurization level along with the cab’s integrity (ability to be reasonably sealed). In a static design, the actual intake air quantity is more variable based upon the loading rate of all the filters used in the system, and it is difficult to determine or control the intake-to-recirculation air ratio. Because of this, the actual amount of intake air delivered to the enclosed cab is variable and is dependent on numerous factors.
Having a known quantity of intake air delivered to the enclosed cab is not only of great benefit in creating positive cab pressurization, but is critical in regards to CO2 levels and ensuring the equipment operator does not become asphyxiated while working in the enclosed volume. NIOSH researchers believe a minimum quantity of at least 25 cfm of intake/outside air per person should be maintained to dilute CO2 quantities exhaled by each worker. Since almost all enclosed cabs in the mining industry are designed for a single operator, a recommended lower limit for pressurized intake air would be somewhere around the 40 cfm range in order to achieve a minimal cab pressurization, while also ensuring a level of safety in regards to the CO2 issue. A good rule of thumb for an acceptable pressurized intake air range would be between 40 and 140 cfm.
Another advantage with a powered intake air unit is it can be designed to minimize dust loading on the intake filtering unit by using one of two proven techniques: (A) a self-cleaning filter technique, or (B) a centrifugal design which spins out the over-sized dust particles (>5.0 microns) before reaching the intake filter. A common self-cleaning method is to use a reverse-pulse or back-flushing technique in which a compressed air system is activated to blow the dust cake off the intake filter. This reverse-pulse can be used either on a regular time interval or based upon a differential pressure across the filter. With the centrifugal design, the system spins the oversized particles out of the unit and back into the atmosphere to minimize the number of particles being deposited on the intake filter. This system has an approximately 90 percent efficiency with particles greater than 5 microns.
Both the self-cleaning and centrifugal techniques have been tested and shown to be very effective at providing a known quantity of intake air to the enclosed cab while minimizing dust loading on the intake filter.
2. Use of an in-cab pressure monitor to determine the integrity and functioning of filtration and pressurization systems.
With any new filtration and pressurization system, the starting positive pressure should be established and recorded. Then, over time, as the system filters load with dust and contaminants, the intake airflow decreases, which also decreases the cab’s positive pressure. Since filter loading rates are different in all cases based on contaminant levels where the equipment is located in the mine, using a filter cleaning or changing schedule based on time is not the preferred method, because a mechanical filter becomes more efficient as it loads with dust and contaminants and develops what is commonly known as a “dust cake.”
Knowing the positive pressure of the cab provides the equipment operator or maintenance personnel with the ideal filter changing time based upon the required intake airflow. The enclosed cab’s positive pressure will be highest with new filters and then as the filters load with dust, the intake airflow will decrease, which also directly decreases the cab’s positive pressure. Once the cab pressure decreases to the point where the intake air is at the minimal level, it signifies that a new intake filter needs to be installed. Conversely, a rapid increase in positive cab pressure also indicates a system failure. This could include such things as a hole or tear in the filter media, a clog in the recirculation system such as a plastic bag or a rag covering the recirculation inlet, or even a maintenance worker removing a used filter and then forgetting to replace it with a new one. It is also possible, over time, for the cab integrity to be compromised by a damaged door, window gasket, or seal being damaged or removed.
An in-cab pressure monitor provides the most real-time indication of the cab’s performance over time. There are a number of commercially available pressure gauges that have an alarm option to signify that the intake filter needs to be changed. Using one of these pressure gauges is a very effective way to monitor the cab filtration and pressurization system functionality.
3. Changes in air quality using different filter configurations and filter dust loading characteristics in the filtration and pressurization systems.
Different filter combinations were evaluated in both pieces of equipment during the testing performed at the Shelly Materials Co. mine. Table 1 shows the various combinations and the resulting protection factors on the roof bolter machine. These tests were performed under static conditions which indicates that no one was entering or exiting the enclosed cab during the particle count testing, and thus no dust was allowed to enter by opening the cab door, nor was anyone inside the enclosed cab to stir up any in-cab dust sources. Because of this, these protection factor (PF) values are at their highest.
Table 1: Roof bolter protection factors with various filter combinations.
These results provide a significant amount of insight into the effectiveness of the various filters in the system. Obviously, the lowest PF measured was when testing a used intake and recirculation filter combination with no final filter. In the next evaluation, a final filter was added to the system, resulting in a significant increase in the protection factor from a value of 3 to 76. Somewhat surprising was the huge increase in the next two combinations when the recirculation filter was removed from the system. A PF of 300 was achieved with a new intake and final filter, which was a substantial increase over the previous values. The researchers do not believe this would be typical with most filtration and pressurization systems, but for this system, adding the recirculation filter into the system was detrimental because the recirculation filter was under-sized, which significantly reduced the recirculation airflow and thus, the air cleaning potential of the system. A negative aspect of not having the recirculation filter in the system is that dirt and dust from inside the cab is drawn into and is deposited in the HVAC system, thereby increasing maintenance issues. An alternative solution to improving this cab filtration system would be to increase the size of the recirculation filter to increase its airflow capabilities. Finally, note the significant increase in the protection factor from 300 to 465 when evaluating a used intake and final filter. As stated previously, a mechanical filter becomes more efficient as it loads with dusts and contaminants and creates a dust cake, which results in a significant increase in the air quality and the resulting PF.
4. HEPA versus MERV 16 quality filters on the air quality inside enclosed cabs.
When most health and safety professionals today think about filtration efficiencies and their correlation with protecting workers’ health, the normal assumption is the higher the efficiency of a filter, the greater the protection afforded to workers. The term HEPA quality was established by the U.S. Department of Energy and signifies a filter that has a 99.97 percent filtering efficiency for ≥ 0.3-micron particles. The Minimum Efficiency Reporting Value, commonly known as a MERV rating, is a comparative value designated by American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) to compare the effectiveness of different filtering efficiencies. A MERV 16 rated filter must be capable of at least a 95-percent filtering efficiency on 0.3- to 1.0-micron particles, thus being significantly less efficient than HEPA rated filters, which must meet a 99.97-percent filtering efficiency on 0.3- to 1.0-micron particles. Going against normal assumptions, for this testing, NIOSH hypothesized that HEPA quality filters were too restrictive for the mining industry and speculated that the MERV 16 rated filters were the optimal choice.
To test this hypothesis, both MERV 16 and HEPA quality filters were evaluated in the filtration and pressurization system of the roofbolter and face drill machines at the Shelly Materials Co. mine. Testing was started in May 2013 using a MERV 16 intake and final filter and repeated on roughly a monthly basis until November 2013. In May 2014, this test was repeated with all aspects being identical except now using the HEPA quality filters. Statistically, at the 95 percent confidence level, there was no difference between the PF values for either the face drill or the roof bolter machine using both types of filters. However, since the MERV 16 rated filters were less restrictive, they provided greater airflow and cab pressurization. In addition, since these filters are less costly and do not have to be replaced as often as the HEPA filters, it was concluded the MERV 16 filters were the optimal choice for both pieces of mining equipment in this case study comparative analysis.
Sharing results
Figure 1: Respirable dust filter cassettes from inside and outside the enclosed cab of a face drill machine during testing on July 30 and 31, 2014, at underground limestone mine.
Throughout the four years of testing highlighted in this article, there were constant discussions with J.H. Fletcher Co., Sidwell Mining Co., and Shelly Materials Co. to provide updates on the results of the research being performed. These discussions were also held with the miners on a monthly basis when the NIOSH researchers were performing their testing. Periodically throughout the study, a lunch-time meeting was scheduled during which Ted DiNardo would have the mining crew come out to the mine office for lunch and allow the NIOSH researchers to provide the crew with a PowerPoint presentation showing updates of the research results obtained during this testing. These meetings were always well received and provided a two-way flow of information.
At one of the last meetings, one slide that had a tremendous impact on the miners was comparing the outside and inside gravimetric dust filters for the face drill machine for July 30 and 31, 2014 (Figure 1). Since these gravimetric samples were taken with a 10-mm Dorr-Oliver cyclone device, they represent the particles that are approximately 10 microns and less in size that would deposit into the inner regions of a miners’ lungs. The white areas of the inside filters represent what the miner would be breathing inside the enclosed cab through his working shift. The dark areas of the outside filters represent what the miner would have been breathing if there was no filtration and pressurization system to clean the air before entering the enclosed cab.
Based on these findings, it is not difficult to comprehend the tremendous impact of the improved air quality from the filtration and pressurization system and the long-term effects on improving the health of miners and minimizing the potential for developing silicosis, black lung, chronic obstructive pulmonary disease, or any other types of lung diseases.
A note of appreciation
NIOSH would like to state its gratitude and appreciation to the following individuals for their assistance and friendship during this research:
Ted DiNardo, mine manager, Sidwell Mining Co. and Shelly Materials Co.,
Jerry Siddle, face drill operator, Sidwell Mining Co.; Mine Foreman, Shelly Materials Co.,
Jay White, roofbolter operator, Sidwell Mining Co.and Shelly Materials Co.,
Harry Montell, face drill operator, Shelly Materials Co.,
Adam Drake, equipment operator and blast crew, Sidwell Mining Company and Shelly Materials Co., and
Tom Appel, mine engineer, Shelly Materials Co.
In addition, NIOSH would like to acknowledge Douglas Hardman, president; Ward Morrison, retired product manager; and Sean Farrell, former product engineer, Rock & Industrials Minerals Products Division, with J.H. Fletcher & Co.; for their assistance and long-standing support of this research effort over a number of years. The cooperation and dedication of all these individuals was instrumental in the success of advancing the science to improve the air quality inside of enclosed cabs of mobile mining equipment.
Andrew Cecala is the principal supervisory mining engineer for the dust, ventilation, and toxic substances branch of NIOSH’s Office of Mine Safety and Health Research. John Organiscak is a senior research engineer. They are based at NIOSH’s Pittsburgh, Pa., office.
Disclaimer
Mention of any company or product does not constitute endorsement by the National Institute for Occupational Safety and Health. The findings and conclusions in this report are those of the authors and do not necessarily represent the views of NIOSH.