A field study of a roof bolter canopy air curtain (2nd generation) for respirable coal mine dust control

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A field study of a roof bolter canopy air curtain (2nd generation) for respirable coal mine dust control

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A field study of a roof bolter canopy air curtain (2nd generation) for respirable coal mine dust control


A  ۲nd generation roof  bolter canopy air curtain (CAC)  design was tested  by  National Institute  for Occupational Safety and Health (NIOSH)  at a  Midwestern underground coal   mine. During the study, the roof  bolter never operated downwind of  the continuous miner. Using a  combination of  personal Data Rams (pDR)  and gravimetric samplers, the dust control efficiency of the roof  bolter CAC was ascer- tained. Performance evaluation was determined using three methods: (1) comparing roof  bolter operator concentrations underneath the CAC to roof  bolter concentrations outside the CAC, (2) comparing roof  bol- ter operator concentrations underneath the CAC to the concentrations at the rear of the bolter, and finally, (3) using the gravimetric data directly underneath the CAC to correct roof  bolter operator concentrations underneath the CAC and comparing them to the concentrations at the rear of the bolter. Method 1 dust control efficiencies ranged from    ۵۳٫۹%  to 60.4%.  Method 2 efficiencies ranged from    ۱۵۰٫۵% to 52.2%, and Method 3 efficiencies ranged from 40.7% to 91%. Reasons for  negative and low  dust control efficien- cies  are  provided in this paper and include: incorrect sampling locations, large distance between CAC and operator, and contamination of intake air  from line curtain. Low dust concentrations encountered during the testing made it difficult to discern whether differences in concentrations were due to the CAC or due to variances inherent in experimental dust measurement. However, the analyses, especially the Method 3 analysis, show that the CAC can  be  an  effective dust control device.

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  1. Introduction

Canopy air  curtains (CAC) were originally developed for use  on the cabs  of continuous miners to protect miners from underground coalmine respirable dust in  the mid-1970s by  the U.S. Bureau of Mines (USBM).  Once  developed, field  testing continuous miners at different underground coal  mine sites demonstrated CAC dust control efficiencies ranging from 23% to  ۶۹% [۱].  In  the 1980s, the National Coal  Board  modified the USBM CAC and field  tested it   on   a  boom-type  heading  machine.  Results of  their  testing demonstrated dust control efficiencies of 35%–۶۸% in blowing ven- tilation and 40%–۸۷% in exhausting ventilation [2]. This  field  test- ing  demonstrated the ability of  the CAC to  successfully protect miners from respirable coal  mine dust.

The elimination of the cab on continuous miners occasioned the development for  roof  bolter operators. Laboratory testing of  the roof  bolter CAC has  shown that the CAC can  be  an  effective res- pirable coal  mine dust control for roof  bolter operators. Laboratory test results demonstrated  dust control efficiencies ranging from14% up  to  ۷۵% [۳–۵]. Unfortunately, there is limited information on  their effectiveness for  controlling respirable coal  mine dust in actual  operating conditions at  underground  coal   mining  sites. Two  underground tests of the CAC on  roof  bolters demonstrated dust control efficiencies of 35% and 53% before problems occurred with operation of the CAC [4]. This study is the first  to conduct field testing of the roof  bolter CAC of sufficient duration to  collect an adequate amount of  data  allowing analysis to  demonstrate its effectiveness for roof  bolter operators.

Very  few  studies of  CAC have been completed outside of  the USBM and NIOSH. A radial air  curtain has  been developed by  the College of Mining and Safety Engineering at Shandong University of  Science and Technology and is  fully  described in  a  Computa- tional Fluid Dynamic (CFD) study of the device [6–۸]. However, this device is radically different and is not  a CAC which provides per- sonal protection to  an  individual mine equipment operator. This device is  a  tube that attaches to  the blowing ventilation tubing and consists of  slots to  allow air  to  radially emanate from the

device. It provides airflow that creates a wall  of air that essentially traps dust between the airwall and the face,  protecting machinery operators who maybe outby the airwall. An  exhaust ventilation tubing is required to  remove the trapped dust at the face.  These studies focused on  the CFD analysis of  the airflows showing its effectiveness. Field  studies were conducted to  show their curtain performance. However, there was  minimal discussion of its  effec- tiveness for personal protection. While the device provided protec- tion to  mine equipment operators using airflow, the methodology of protection is very  different from that of the CAC. Another version of an air curtain was  designed to be installed on a longwall shearer body. This curtain created a wall  of air  which separated the venti- lation airflow into two channels. One kept dust contaminated air at the face  while the other kept the clean air  in  the walkway. CFD analysis and field  measurement comparisons were conducted to show its effectiveness [9]. While effective, this methodology is also different from the CAC.

Since  the implementation of the new respirable coal  mine dust limit from 2.0  to  ۱٫۵ mg/m3, roof  bolter CACs are  becoming more commonplace in  underground coal  mines as  a  dust control tool to  prevent roof   bolter operator overexposure to  respirable  coal mine dust [10].  J.H. Fletcher & Co. has  been instrumental in deliv- ering an effective design which incorporates the filter, blower, and canopy plenum  seamlessly into  the  design of  the roof   bolter, resulting in a successful operational roof  bolter CAC.

A field  study was  conducted by the NIOSH to test the effective- ness of the roof  bolter CAC for  respirable coal  mine dust control. The  study was   conducted at Prairie State Energy’s underground coal   mine; the Lively  Grove   Mine.   The  Lively  Grove   Mine   is  a room-and-pillar mine containing coal  from the Herrin #6 seam. The  mine produces approximately 7 million tons of coal  per  year to  the adjacently located power plant. Testing was  conducted on a roof  bolter which operated in  entries 7–۱۳ in  a 13-entry main. The roof  bolter is manufactured by J.H. Fletcher & Co. and is listed as serial #: 2015-306. The mine employed a blowing face  ventila- tion system to  the roof  bolter machine during bolting operations. However, during this testing the roof  bolter never operated down- wind of the continuous miner.

The CAC system is integrated into the roof  bolter machine with the hydraulically driven fans  and filter mounted on the roof  bolter body and the plenum, which provides air over  the operator, incor- porated into the roof  bolter canopy. The fans  are  connected to the canopy via  ۱۰٫۲-cm diameter hose. The  left  and right side  of the roof  bolter each had a  CAC system in-place, which operated the entire time during roof  bolter operation.

The shape of the canopy/plenum used at the mine site  is shown in Fig. 1. This canopy is the 2nd generation design from J.H. Fletch- er’s original slotted CAC. The original slotted CAC had dust control efficiencies ranging from 14.2% to 24.5% in the laboratory [5]. This

۲nd generation CAC is  an  improvement upon NIOSH’s  original design that uses uniform airflow across the plenum. The  uniform filtered airflow provides protection to  the roof  bolter operator by flowing directly  over   the operator resulting in  displacement of air  contaminated by respirable dust from the operator’s breathing zone. The  uniform filtered airflow also  provides a  column of  air that prevents any  entry ventilation airflow contaminated with res- pirable coal  mine dust from penetrating. The  ۲nd generation CAC also,   implements  recommendations from the  NIOSH  computa- tional fluid  dynamics (CFD) evaluation conducted on  the original design which recommended staggered slots or nozzles if perimeter outlets are  to  be  used [5]. This  new design incorporates staggered perimeter nozzles to  prevent infiltration of contaminated air  into the CAC domain or the protection zone. The  protection zone con- sists of an  equally spaced pattern of holes providing airflow over the roof  bolter operator at a  lower velocity than the perimeter holes.

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