Development of a canopy

The U.S. National Institute for Occupational Safety and Health (NIOSH) conducted a study to determine the effectiveness of a filtered air delivery system to reduce respirable dust exposure of roof bolter operators in underground coal mines. When performing roof bolting operations, roof bolter operators may experience exposure to high levels of respirable dust during a working shift, especially when working downwind of the continuous mining machine. While drilling and installing roof bolts, this filtered air system supplies a clean curtain of air over the roof bolter operator by means of a plenum mounted beneath the canopy. This experimentally designed air curtain, adapted for roof bolting machines used in this study, is based upon previous design prototypes, computational fluid dynamics (CFD) simulations and trial-and-error testing. Both the prototype and final designs were tested in the full-scale mining galleries at the Office of Mine Safety and Health Research (OMSHR) in Pittsburgh, PA before being taken underground for field testing. Test results of the system in both laboratory and field studies show reductions in operator exposure to respirable dust when the operator is positioned directly under the protected zone of the canopy. Introduction Underground coal mine roof bolter operators are at risk of excessive exposure to respirable coal and silica dust, second only to continuous miner operators, based upon the U.S. Mine Safety and Health Administration (MSHA) personal dust samples (MSHA, 2009). Dust collectors on roof bolting ma­ chines, if maintained properly, effec­ tively minimize dust exposures during drilling and bolting activities. However, due to mining sequencing, there may be times during a working shift when roof bolter operators must work downwind of the continuous mining operation. When this scenario occurs, the dust from the continuous miner enters the roof bolting workplace, potentially exposing the operators to high levels of respirable dust. In addition, the bolting operation itself can contribute to higher respirable dust concentrations in the worker’s area if dust collectors are not maintained, adding to operator exposure. To lessen the operator’s dust exposure when these conditions occur, a plenum mounted under the roof bolter’s protective canopy has been developed to supply filtered air over the operator as bolting duties are performed. This filtered air is delivered to the plenum via tubing connected to a blower that is mounted on the roof bolting machine. The roof bolting procedure requires the operator to stand under a support canopy for protection from loose roof rock as holes are drilled into the mine roof for the installation of roof bolts (Fig. 1). This canopy is fixed to the drill boom and continually offers the operator protection from loose debris as the boom is repositioned for each new hole. Based upon time studies conducted for this study, nearly 66% of the operator’s drilling time is spent under this canopy during a complete cycle of bolting. The canopy, therefore, provides an ideal lo­ cation for installing a device to protect the operator from respirable dust. The underside of the roof canopy serves as a mount for such a device to deliver a filtered air supply over the operator during drilling activities. This device or canopy air curtain (CAC) serves as a plenum to blow a constant stream of fresh air over the breathing zone of the operator. The air supply will act as a “curtain” of fresh air for a zone of protection from the contaminated air around the bolter, regardless of the dust source (Fig. 2). Although a new application for roof bolting machines, air curtain technology for continuous miner operator cabs was originally developed under a U.S. Bureau of Mines contract in the 1970s by the Donaldson Company Inc. (1975). Before the advent of radio remotes for controlling the operation of continuous mining machines, the operators were located in an onboard control cab at the rear of the continuous miner. Close proximity of the cab to the cutter head subjected the operators to high levels of respirable dust during mining. In an attempt to re­ duce dust exposure to the operator, a contract was awarded to develop a device to provide a source of fresh air for these operators. This device was designed to be compatible with the mining equipment and offer protection from dust ex­ posure while minimally affecting operator safety, comfort and mobility within the cab area. The final working design consisted of a square plenum, blower and filter, and connec­ tive tubing. The plenum housing was made from fiberglass and used perforated plate and screen mesh as an air diffuser. The inlet port was tapered, as well as the area over the air diffuser. The surface of the air diffuser had dimensions of 43.2 x 43.2 cm (17 x 17 in.). Figure 3 shows the Donaldson air curtain design. Field testing of the air curtain in continuous miner cabs showed reductions in dust exposure from 30% to 75% depending upon the amount of time the operator spent in the protected zone. Figure 1 Roof bolter operator under roof canopy during roof drilling.