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Introduction

Although a dust control system is the best way to reduce dust levels in a workplace, it cannot control dust from secondary sources such as material spillage or leakage from a process or piece of equipment, or dust brought in through the ventilation system or through doors and windows. These secondary sources can contribute significantly to dust levels and can render the existing dust control system ineffective. This chapter describes some of the most important, but often overlooked, measures to aid in control of workplace dust levels.



Contributing Sources/Factors

Increases in workplace dust concentrations may result from-
  • Poor or inadequate maintenance of dust control systems and dust-producing equipment or processes
  • Poor or inadequate housekeeping procedures
  • Recirculation of previously emitted dust particulates
  • Recirculation of uncontrolled emissions from dust collectors
Note: The discussion provided here assumes that adequate dust controls are installed on all major dust sources.



Administrative Measures

Administrative measures provide a second line of defense against dust exposure. Instituting such measures also indicates management commitment to dust control. The following steps are recommended:
  • Establishing a companywide dust control policy endorsed by upper management.
  • Forming a dust control task force made up of plant managers, health and safety manager, production manager, etc. The purpose of the task force is to-
-Conduct dust surveys and identify high dust-level areas
-Assign priorities to areas requiring dust controls
-Suggest proper dust control techniques
-Prepare timetables for completion of dust control projects/activities
-Estimate costs
  • Educating and training plant personnel. A short training course can be established to familiarize plant personnel with such items as-
-Company and division dust policies
-Government standards and safe levels
-Reasons why dust surveys are conducted and ways to measure dust
-Measures the company is taking to reduce dust levels and improve working conditions
-Ways plant personnel can help
-Types of personal protection available
  • Maintaining an awareness of dust control. Information can be collected and exchanged regarding ongoing dust control programs in all divisions of the company or in related industries and government research organizations.

Number of Hours on Each Job for Each Employee to Keep Exposure Levels Less Than 2 mg/m

Cumulative Exposure  = T1C1 + T2 C2 . . . TnCn

Ttotal

Where:
T    =   time
C   =   concentration
Time Spent at Each Job
  Job 1 Job 2 Job 3 Job 4 Employee
  Conc. Conc. Conc. Conc. Cumulative
Employee 1.0 mg/m 0.5 mg/m 1.2 mg/m 4.5 mg/m Exposure mg/m






W 0 2 4 2 1.85
X 0 4 2 2 1.67
Y 4 1 1 2 1.84
Z 4 1 1 2 1.84

Periodic Dust Housekeeping Checklist

___ All mobile equipment (trucks, front-end loaders, dozers, etc.) clean.

___ Motors and switch gear clean of excessive dust, oil, slurry, and debris.

___ All operating department floors and working surfaces to be vacuumed or washed and free of scrap or debris.

___ Tools, bars, shovels, etc. stored in racks.

___ Hoses washed down on reel installed and maintained at such points as slurry pumps, stone belt transfer points,
       etc.

___ Safety shower operational, available, and clean.

___ Safety eyewash bottle, first-air kit, and stretcher checked.

___ Dust accumulations around process equipment at a minimum; spills promptly cleaned up.

___ Fire extinguishers checked and available.

___ Belt-conveying systems free of spills and buildup, particularly at transfer and loading points.

___ Building siding and roofs properly maintained and free from broken or loose sheets.

___ Building doors and windows clean and in good repair at all times.

___ Locker rooms neat, clean, and in sanitary condition.

___ Offices neat and orderly.

___ Bulletin boards available and properly maintained.

___ Unpaved roadways and parking areas treated to minimize dust.

___ Yards and fields mowed frequently.

___ Plant storm sewers and open drainage ditches clean.

___ Parts inventory stored in orderly fashion.

___ Traffic lanes in shop areas clearly marked and free of scrap.

___ Abandoned process equipment and machinery promptly removed from the plant.




Preventive Maintenance Program

A preventive maintenance (PM) program is the key to reliable and efficient operation of any dust control equipment or system. Although PM programs require time and money, investment can pay for itself through improved morale, increased equipment and system reliability, increased equipment and hardware life, and reduced cleanup costs and system downtime. When instituting a PM program, the following points should be considered:
Effects of Bag Hopper Overflowing on Worker Exposure
Effects of Bag Hopper Overflowing on
Worker Exposure
  • Conduct PM programs on all dust control system hardware and components, as well as dust-producing sources, during plant shutdowns or as recommended by the equipment manufacturer.
  • Carry all necessary spare parts in sufficient quantities for dust control systems.
  • Give high priority to patching holes, caulking and sealing cracks, and maintaining dust seals.
  • Inspect and adjust all belt conveyors and their skirting rubber and dust seals.
  • Inspect belt conveyor idlers and nonmoving idlers. Remove and replace missing or broken idlers.
  • Inspect all belt conveyor training idlers. Adjust them as necessary so the conveyor belt does not travel laterally.
  • Shut and clamp all access and inspection doors before any operation begins. Schedule adequate time for workers to perform routine cleanup at work stations.
  • Rotate periodic cleanup among crews.
  • Inspect all dust seals and repair or replace.
  • Inspect belt scrapers on belt conveyors and adjust. Replace worn-out components.
  • Measure velocity and static pressures weekly.
  • Check for plugged ductwork and clean immediately. If this problem occurs repeatedly, redesign the ductwork.
  • Develop safeguards to prevent overflowing bins.
  • Follow the preventive maintenance program for dust collectors, fans, and motors described in chapter 4.
  • Inspect nozzles and other components periodically. Replace worn-out nozzles as needed.
  • Follow the preventive maintenance program for pumps and compressors described in Chapter 3.



Proper Operating Procedures

No dust control system can perform reliably unless it is operated properly. To achieve maximum efficiency, the following measures are suggested:
Increace in Dust Exposure When Using Compressed Air to Clean Clothing
Increase in Dust Exposure When Using Compressed Air to Clean Clothing
 
  • Educate operators on startup and shutdown procedures.
  • Instruct operators that all dust control systems should be in operation before any processing equipment is started.
  • Eliminate the use of compressed air jets to clean accumulated dust from the equipment or clothing and substitute a vacuum cleaning system.
  • Use a vacuum cleaning system to clean spills and dust accumulations. Avoid brooms and shovels.
  • Use pressure water, where applicable, to clean equipment during plant shutdown or as necessary.
  • Increase in Dust Exposure When Using a Broom
    Increase in Dust
    Exposure When Using
    a Broom

  • Check the speed of belt conveyors and slow them down, if possible, to reduce dust circulation.
  • Install an alarm to sound when a dust collector stops operating.
  • Seal off obsolete working areas and remove unused equipment.




Managerial Measures

A plant manager, production superintendent, or production foreman can play an important role in implementing dust control measures. The following measures can enhance the effectiveness of nonengineering controls:
  • Adjust worker schedules so total daily exposure to harmful dust does not exceed threshold limit values.
  • Adjust production schedule, where possible, to prevent or reduce secondary dust sources in the area. For example, empty a dust hopper at off times when no workers are in the area.
  • Provide or arrange for appropriate respirators to be stored at key locations.
  • Implement proper and continuous use of respirators, where necessary.



Outside Dust Source Control

Most minerals processing facilities have haul roads, loading and unloading facilities, and stockpiles. Although these dust sources are located outside the processing facility/building, dust can enter through doors, windows, air intakes, and other openings. The following measures are suggested:
Effects of Bulk Loading Outside on Worker's Exposure Inside the Mill
Effects of Bulk Loading
Outside on
Worker's Exposure
Inside the Mill
  • Treat the surfaces of haul roads regularly to reduce dust formation/generation due to vehicle traffic.
  • Pave the haul roads, if cost permits.
  • Vacuum or wash the paved roads periodically.
  • Enclose all loading and unloading facilities and equip them with dust controls.
  • Spray active stockpiles periodically to reduce dust spread due to wind.
  • Cover and/or spray all inactive stockpiles to eliminate or reduce dust spread.
  • Locate the building's air intakes in such a way to minimize outside dust infiltration.
  • Use automatic bin level indicators to shut off feed to bin before overflow occurs.



Recirculation of Uncontrolled Emissions

A dust collection system can remove large volumes of air from a room or building. Unless this air is replaced, the room or building can come under negative pressure, or partial vacuum. Outside air brought into the building in a controlled fashion is known as make-up air.

If provisions for make-up air are not made, outside air will enter through doors, windows, and other openings in an uncontrolled fashion and may-
Building Air Inlets and Outlets - Good
  • Bring particulates or contaminants into the building
  • "Starve" the existing dust collection system
  • Create high-velocity cross drafts through doors, windows, and openings, which may be undesirable
  • Affect the efficiency of natural draft stacks or chimneys
  • Create unwanted drafts on workers or equipment
  • Increase plant heating costs
Building Air Inlets and Outlets - Poor
Building Air Inlets and Outlets
Make-up air may be filtered and heated or cooled. In northern climates, considerable heating costs may be incurred if make-up air is heated. To reduce plant heating costs, "cleaned" exhaust gases from dust collectors may be brought into the building to serve as make-up air. This recirculation of exhaust gases can reduce plant heating costs, but it can also bring in dust if the process is not properly designed, installed, and operated.

Most industrial health agencies do not recommend recirculation of exhaust gases if the dust is hazardous to health. Incorrect operation or poor maintenance - both frequent occurrences-can result in return of contaminated air into the workplace. However, under the following conditions, the recirculation of exhaust gases may be permitted.

Installation of a dust collector or air-cleaning system of adequate efficiency so that exit concentrations do not exceed the allowable CR values. CR values are calculated as follows:


CR = 1 [ TLV - Co ] QT x 1



2 QR K

Where:

CR  = dust concentration in exit air (i.e., "cleaned" air) from the collector, before mixing with room air
QT  = total air volume exhausted from the area, ft/min
QR  = volume of air recirculated, ft/min
K    = mixing factor, usually varying from 3-10, with 3 being good mixing
TLV = threshold limit value of the dust
CO  = dust concentrations measured in workers' breathing zone with exhaust gases discharged outside
  • Installation of a secondary air-cleaning system of equal or greater efficiency than the primary system or provision for a reliable monitoring device to obtain and analyze a representative sample of recirculated air. Such a monitoring system should be fail-safe in the event of power failure, environmental contamination, or poor maintenance.
  • Installation of a warning signal to indicate malfunction of the secondary air-cleaning system or above-limits dust concentrations.
  • Provision for immediate bypass of recirculated air to the outdoors or complete shutdown of the dust-generating process when the warning signal goes off.
  • Provision for proper distribution of cleaned air to allow it to mix with ambient air.
  • Avoidance of high discharge velocities to prevent disturbance of the accumulated dust, which can create a local miniature dust storm.
  • Recirculation of hazardous materials with low TLV's is not recommended.