Report: APP CMHS Project 1




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3.6.3. Respirable Dust Control

Respirable dust exposure has long been known to be a serious health threat to workers in coal mining, overexposure to respirable coal mine dust can lead to coal workers’ pneumoconiosis (CWP). CWP is a lung disease that can be disabling and fatal in its most severe form. In addition, miners can be exposed to high levels of respirable silica dust, which can cause silicosis, another disabling and/or fatal lung disease. Once contracted, there is no cure for CWP or silicosis. During 1970–2004, CWP was a direct or contributing cause of 69,377 deaths of U.S. underground coal mine workers. Recent x-ray surveillance data for 2000–2006 show an increase in CWP cases. The goal, therefore, is to limit worker exposure to respirable dust to prevent development of these diseases. This section will identify the best practices that are available to control respirable dust levels in underground coal mining operations in the U.S. This section will also discuss sampling instrumentation and sampling methods, dust control technologies for longwall mining and continuous mining

Current Technologies

Respirable Dust Samplers

The most common type of sampler used in the coal mining industry is the gravimetric sampler (Figure 65). This device is designated for use in compliance dust sampling by the Federal Coal Mine Health and Safety Act of 1969. It consists of a constant-flow sampling pump, a size-selective cyclone, and a filter cartridge.

In addition to gravimetric samplers, a real-time dust sampler has been approved by MSHA for use in underground mines, but not for compliance sampling purposes. The personal DataRAM (pDR) has dust-laden air pass through a sensing chamber and passes a light beam through the dust. A sensor measures the amount of light scatter caused by the dust and relates this scatter to a relative dust concentration. This concentration is correlated to the time when the sample was measured and stores this information in the internal data logger. The sample data can then be downloaded to a computer for analysis. Figure 66 illustrates a typical graph obtained with the pDR, as well as a photo of the pDR. The time-related dust data can be analyzed for specific time intervals (e.g. head-to-tail passes on longwalls), with average dust concentrations calculated for these intervals.

Figure 65 Gravimetric sampling pump, cyclone, and filter cassette

Figure 66 Example of dust measurements obtained with the pDR

The personal dust monitor (PDM) is another real-time sampler that has been developed and tested by NIOSH, approved for use in underground coal mines by MSHA, and reached commercial production [Volkwein et al. 2006]. The PDM uses the tapered-element oscillating microbalance (TEOM) to obtain a real-time, gravimetric-based measure of respirable dust concentrations. The TEOM is a hollow tube that vibrates at a known frequency with a filter mounted on the end. As respirable dust is deposited onto the filter, the TEOM frequency changes, which can be related to a dust concentration. The PDM provides the wearer with a readout that displays the cumulative dust concentration to that point in the shift and the percent of the permissible exposure limit that has been reached. This information can be used by the wearer to monitor dust exposure during the shift to prevent overexposure. The sampler is incorporated into standard cap lamp housing and has the sampling inlet located at the cap lamp (Figure 67).

Figure 67 PDM with TEOM removed

Dust Sampling Strategies

To effectively control the respirable coal and silica dust exposure of mine workers, it is necessary to identify the sources of dust generation and quantify the amount of dust liberated by these sources. Once the dust sources are identified and quantified, dust control technologies that offer the greatest protection to the mine workers can then be applied. To quantify the amount of dust liberated by a source, dust sampling must be conducted in a manner that isolates the identified dust-generating source. This is achieved by placing dust samplers upwind and downwind of the source in question. The difference between these measurements is used to calculate the quantity of dust liberated by this source.

For a more mobile piece of equipment, such as a longwall shearer, a mobile sampling strategy must be used to isolate the dust generated by the equipment. Two sampling personnel would be required to travel with the shearer as it mines across the longwall face. One person would be located upwind of the shearer, while the second would be located downwind. These sampling personnel would maintain their respective distances from the shearer as it mines across the face. Figure 68 illustrates this mobile sampling strategy.

Figure 68 Mobile sampling used to quantify shearer dust

Controlling Respirable Dust in Longwall Mining Operations

Longwall operations have had difficulty in maintaining consistent compliance with the federal dust standard of 2.0 mg/m3.The practical measures are taken to control Respirable dust on intake roadways, belt entry, headgate entry (including the stageloader/crusher), shearer and shield in longwall mining operations. These include:

Control on Intake Roadways

  • Limit support activities during production shifts

  • Apply water or hydroscopic compounds to control road haulage dust

  • Use surfactants

Control from the Belt Entry

  • Belt maintenance

  • Wetting the coal product during transport

  • Belt cleaning by scraping and washing

  • Use of a rotary brush that cleans the conveying side of the belt (Figure 69)

  • Wetting of dry belts (Figure 70)

Figure 69 Rotary brush cleans the conveying side of the belt



Figure 70 Water sprays and belt wiper used to reduce dust from the nonconveying side of the belt as it returns

Control in the Headgate Entry, including the stageloader/crusher

  • Fully enclosing the stagerloader/crusher

  • Wetting the coal in the crusher and stageloader area (Figure 71)

  • Using scrubber technology in the stageloader/crusher area

  • Using a high-pressure water-powered scrubber (Figure 72)

  • Installation and maintenance of a goaf curtain (Figure 73)

  • Installation of a wing or cutout curtain between the panel side rib and the stageloader (Figure 74)

Figure 71 Enclosed stageroader/crusher and location of water sprays

Figure 72 High pressure water scrubber installed on top of crusher

Figure 73 Goaf curtain increases airflow down the face

Figure 74 Ventilation patterns around shearer without (left) and with (right) a cutout curtain



Controlling Shearer Dust

  • Face ventilation

  • Drum-mounted water sprays

  • Cutting drum bit maintenance

  • Directional water spray systems (Figure 75, Figure 76, Figure 77 and Figure 78)

  • Shearer deflector plates (Figure 79)

  • Crescent sprays (Figure 80)

  • Lump breaker spray manifold

  • Tailgate-side sprays (Figure 81)

Figure 75 Shearer-clearer directional spray system

Figure 76 Venturi sprays mounted on headgate splitter arm



Figure 77 Headgate splitter arm with flat-fan sprays mounted on goaf side of belting

Figure 78 Directional sprays mounted on face side of shearer body



Figure 79 Raised delector plate can enhance the effectiveness of the directional spray system

Figure 80 Crescent sprays located on shearer ranging arm



Figure 81 Spray manifold mounted on tailgate end of shearer body



Controlling Shield Dust

  • Canopy-mounted spray systems

  • Shield sprays on the underside of the canopy (Figure 82)

  • Air dilution

  • Unidirectional cutting sequence

Figure 82 Shield sprays located on the underside of the canopy

Controlling Respirable Dust in Continuous Mining Operations

Continuous mining operations accounts for 50 % of U.S. underground coal production and respirable dust control in the operations is a major challenge. As with any dust source, air and water are used to dilute, suppress, redirect, or capture dust. Ventilating air to a continuous mining section is the primary means of protecting workers from overexposure to respirable dust. Proper application of water spray systems, ventilation, and mechanical equipment (scrubbers) provides the best overall means of respirable dust control. Maintenance of scrubbers, water sprays, and bits are basic to any effective dust control strategy and must be routinely practiced. Suppression of dust is the most effective means of dust control. Suppression is achieved by the direct application of water, usually at the point of attack, to wet the coal before and as it is broken to prevent dust from becoming airborne. Once dust is airborne, other methods of control must be applied to dilute it, direct it away from workers, or remove it from the work environment. Redirection of dust is achieved by water sprays that move dust-laden air in a direction away from the operator and into the return entry or behind the return curtain. Capture of dust is achieved either by water sprays that impact with the dust in the air to remove it or by mechanical means (e.g. fan-powered dust collectors). Ventilating air dilutes and directs dust away from workers. Either blowing or exhausting ventilation is used on continuous mining sections. The practical measures are taken to control the dust on continuous mining machine, face ventilation, roof bolters, feeder-breakers and shuttle cars.

Continuous Miner Dust Control

  • Water spray systems. There are several types of water sprays available for use on continuous miners to control dust. Spray nozzle type, location, pattern, flow, and pressure are all factors to consider when designing a spray system. The type of spray used at a particular location depends on the desired application.

  • Flooded-bed scrubbers. Flooded-bed scrubbers capture dust-laden air from the cutting face, carry this air through ductwork on the miner, and pass the air through a filter panel that is wetted with water sprays (Figure 83)

  • Bit type and wear. Bit type and bit wear can adversely affect respirable dust concentrations. Routine inspection of bits and replacement of dull, broken, or missing bits improve cutting efficiency and help minimise dust generation. Bits designed with large carbide inserts and smooth transitions between the carbide and steel shank typically produce less dust and significantly worn bits without their carbide tips produce much more dust.

  • Modified cutting method. If roof rock must be cut, it is often beneficial to cut the coal beneath the rock first and then back the miner up to cut the remaining rock. This method of cutting leaves the rock in place until it can be cut out to a free, unconfined space, which creates less respirable dust (especially silica dust)

Figure 83 Components and design of a flooded-bed scrubber

Face Ventilation

  • Blowing face ventilation. The following best practices will reduce dust exposure on blowing ventilation sections:

  • The operator should be positioned in the mouth of the blowing line curtain with intake air sweeping from behind. The operator should not proceed past the end of the line curtain. If the operator must be on the return side of the curtain, some of the intake air should be bled over the line brattice to provide fresh air to the operator (Figure 84). Good communication with shuttle car operators is essential because visibility can be a problem depending on where the continuous miner operator is standing.

  • According to MSHA, when it is necessary for the operator to move from the clean air position (end of the curtain), the operator should allow the dust-laden air to clear the entry and stop the scrubber before moving.

  • According to MSHA, when aligning the continuous miner to square a face, the operator should position the machine and then return to the end of the curtain before coal cutting resumes. This reduces the potential for injury.

  • Brattice discharge velocities exceeding 800 fpm have better penetration to the face and thus better dilution of dust and methane. When brattice discharge velocities are less than 400 fpm, there is little difference in performance between blowing and exhausting ventilation.

  • Scrubber discharge must be on the opposite side of the line brattice to allow scrubber exhaust to discharge directly into return air.

  • The air quantity provided by the line curtain should be limited to 1,000 cfm over the scrubber capacity. Air quantities exceeding 1,000 cfm over the scrubber capacity can overpower the scrubber and push dust-laden air past the scrubber inlets. Therefore, MSHA typically requires that the airflow entering a cut be equal to or exceed the scrubber airflow by no more than 1,000 cfm and must be measured with the scrubber off.

  • Excess air velocity may be reduced by flaring out the line curtain at the end to lower the velocity of the air emerging from behind it or by pulling the line curtain back slightly to prevent overpowering the scrubber.

  • Experiments have shown that erecting a short line curtain during the slab cut shields the operator from the air jet emerging from a blowing duct.

Figure 84 Schematics of a blowing ventilation system

  • Exhausting face ventilation. The following best practices will reduce dust exposure on exhausting ventilation sections:

  • Figure Figure 85 shows a schematic of an exhaust ventilation system. Exhausting airflow allows for more flexibility than blowing, giving the operator more options to avoid dusty air. However, MSHA maintains that position A (opposite side of curtain) is preferred. As always, good communication between the continuous miner operator and shuttle car operators is essential for safe positioning.

  • An advantage of exhausting ventilation is that shuttle car operators are always positioned in fresh air.

  • Air quantity reaching the inby end of the line curtain should be equal to or slightly greater than the scrubber capacity to guard against recirculation of air.

  • MSHA regulations state that mean entry air velocity must be at least 60 ft/min when using exhaust ventilation systems.

  • The end of the exhaust curtain or tubing must be kept within 10 ft of the face when not using a scrubber to ensure that air reaches and effectively sweeps the face.

  • The operator should not proceed inby the end of the line curtain since this will expose the operator to dust-laden return air. If operator dust levels are too high, the first thing to check is whether the operator is standing parallel to or outby the end of the line curtain.

  • Scrubber exhaust must be on the same side of the entry as the line curtain to allow scrubber exhaust to discharge directly into return air.

Figure 85 Schematic of an exhaust ventilation system



Dust Control for Roof Bolters

Most roof bolting machines are equipped with MSHA-approved dry dust collection systems to remove dust during drilling. The following best practices can help reduce dust exposure to the bolter operator:

  • Maintaining the dust collector system

  • Cleaning the dust box

  • Using dust collector bags

  • Removing and replacing the canister filter

  • Cleaning the discharge side of the collector

  • Installing a sock on precleaners

  • Using “dust hog” bits

  • Positioning to avoid working downwind of the continuous miner

  • Wet drilling/mist drilling

  • Canopy air curtain

  • Routing miner-generated dust to the return

  • Working downwind of the bolter.

Dust Control in Intake Airways

The average concentration of respirable dust in intake air must be kept at or below 1 mg/m3 within 200 ft outby the working face. However, to maintain consistent dust control in the face area, MSHA recommends that intake concentrations be less than 0.5 mg/m3. Maintaining this concentration is not usually difficult, but requires attention from mine operators to address activities that can raise intake air dust levels. Typically, high levels of intake dust are sporadic and brief in nature due to activities in the intake entries that may take place over the course of a working shift. These sporadic activities include:

  • Delivery of supplies and/or personnel

  • Parking equipment in intake

  • Rock dusting

  • Scoop activity

  • Construction activity.

In addition, the belt entry can be used to bring intake air to the working faces and is a potential source of dust generation. If intake dust levels are high, the following steps can be taken to maintain dust levels to a minimum:

  • Good housekeeping practices will help keep intake entries free of debris, equipment, and supplies.

  • Supply delivery, scoop activity, stopping construction, and rock dusting should be dedicated to nonproduction shifts.

  • If haulage activities must take place during a production shift, the haulage roadways should be kept damp at all times. Since water will likely evaporate in the ventilation air, a hygroscopic salt or effective dust-allaying agent should be used. Keeping dust dampened in the main intake entries will limit dust entrained by activity in these entries.

  • Equipment should be parked in crosscuts to keep main airways clear of obstruction.

When belt air is used for face ventilation, dust generated in the belt area should be controlled. Controls at the belt head helped maintain low dust levels in the belt entry. Automated sprays were used to suppress dust at the section-to-main transfer point. A belt scraper equipped with sprays controlled dust by cleaning the outside surface of the belt after the coal had been transferred to the main belt.

Feeder-Breakers and Shuttle Cars

Dust measurements show that feeder-breaker operations can contribute a significant amount of respirable dust to belt entry air, which emphasises the need for dust controls at this location. Outby areas can be placed on a more stringent dust standard due to the presence of respirable silica dust. Following are some basic controls for these areas:

  • MSHA recommends hollow- or full-cone sprays at the feeder-breaker transfer point to wet and knock down coal and silica dust.

  • When shuttle cars unload, dust levels can be decreased by using automated sprays at the mouth of the feeder-breaker that activate during dumping to wet coal before it enters the crusher.

  • Throat sprays on the continuous miner will wet coal when entering the conveyor and lessen dust when transferred to and from shuttle cars. Redistributing a small portion of the water available on the continuous mining machine to the chain conveyor may be necessary to ensure that the loaded coal is wet enough to minimise dust reentrainment at the section loading point.

  • Shuttle cars should not be in a waiting position beneath check curtains.

  • Shuttle car operators should not be located in the direct discharge of the dust collector (scrubber) on the continuous miner.

  • When blowing ventilation is used, configure shuttle car runs to minimise the amount of time spent in return air.

Application Sites

The Respirable dust exposure limits and dust sampling requirements for inspectors and mine operators are included in 1977 Federal Mine Health Safety Act, subsequent amendments and MSHA recommendations. Detailed information can be found in MSHA’s web site http://www.msha.gov.

Technology Gaps/Needs

In light of the ongoing severity of the lung diseases in coal mining caused by the respirable dust, further improvements are required to control the Respirable dust in U.S. underground coal mining. it must be stressed that after control technologies are implemented, the ultimate success of ongoing protection for workers depends on continued maintenance of these controls. NIOSH researchers have often seen appropriate controls installed, but worker overexposures occurred because of the lack of proper maintenance of these controls. These may include:

  • Development of ventilated cutting drums. The drums should be designed to reduce the amount of dust from the cutting zone through water-powered dust capture tubes built into the hub of the shearer drum.

  • Development of suitable forms and their discharge systems through nozzles located in the shearer drum. Research study has shown that form can distribute a given quantity of moisture much more evenly over a large surface area. However, a high degree of mixing must take place between the foam and the cut coal for the foam application to be successful in reducing dust levels. Also, any chemical additive such as foam or wetting agents have the potential to disrupt the coal cleaning in the preparation plant depending on the type of cleaning used at the plant. This must be considered when selecting a water additive for dust control.

  • Development of high-pressure inward-facing drum sprays to confine the dust generated by the cutting drum to the face area.

  • It must be stressed that after control technologies are developed and implemented, the ultimate success of ongoing protection for workers depends on continued maintenance of these controls. It has often seen that appropriate controls installed, but worker overexposures occurred because of the lack of proper maintenance of these controls.


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