Highway milling machines with water spray systems control the airborne dust generated when grinding and removing pavement.
Description:
Highway milling machines use a rotating drum with teeth to grind and remove asphalt or concrete pavement prior to repair or replacement. The machines often employ a conveyor to move debris into a truck for transport to disposal or recycling facilities. Removing and conveying asphalt or concrete pavement generates a large amount of dust that may contain crystalline silica and creates a hazard for everyone in the vicinity.
Water spray systems are the most common method of controlling dust generated by highway milling machines. These controls use a pump to deliver water to the milling surface and to the conveyor where it combines with dust and silica particles and reduces airborne levels. Adding moisture to the material will also reduce the amount of dust produced downstream.
If properly designed, maintained and used, water spray systems may significantly reduce exposures near milling machines. This may reduce exposures enough that a respiratory protection program isn’t required. Continuous use may exceed the OSHA Permissible Exposure Limit, even with the use of wet dust suppression, however.
14 spray nozzles; 12 on the drum and 2 on the conveyor
Two pumps, adjustable up to 30 gallons per minute
Model
Horsepower
Working Speed, feet per minute
Operating Weight*, pounds
Water Capacity, gallons
BM 2000/50
469
0 to 112
65,257
713
BM 2000/60
590
0 to 98
71,650
872
*Operating weight with a full fuel tank
Risks Addressed:
Milling asphalt and concrete pavement is a high dust activity that, in the absence of controls, would place workers at risk of lung disease, cancer, chronic obstructive pulmonary disease (COPD), lung scarring and silicosis with prolonged exposure to silica. Silicosis is an incurable, sometimes fatal disease.
The ACGIH recommends that airborne concentrations of insoluble or poorly soluble Particles Not Otherwise Specified (PNOS) be kept below 3 mg/m3 for respirable particles (less than 10 micrometers diameter) and 10 mg/m3 for inhalable particles (less than 100 micrometers diameter). The OSHA permissible exposure limit (PEL) for Particles Not Otherwise Regulated (PNOR) is 5 mg/m3 for the respirable fraction and 15 mg/m3 for total dust.
The NIOSH-recommended exposure limit (REL) for silica is 0.05 mg/m3 as a time-weighted average concentration for up to a 10-hour workday during a 40-hour workweek. This is one-half of the OSHA standard when the dust is pure silica, but still twice the ACGIH-recommended threshold limit value (TLV) of 0.025 mg/m3.
How Risks are Reduced:
A fine water spray or mist is delivered to the drum and conveyor, during or immediately after milling, where it combines with airborne particles created during the breaking of asphalt or concrete pavement causing the now larger particles to settle. Adding moisture to the material will also reduce the amount of airborne dust and silica generated during other activities such as loading or transporting. Silica and dust in the asphalt and concrete materials are only hazardous if inhaled and are not hazardous for skin contact. As long as the dust does not become airborne, the hazard is reduced. The extent to which these particular systems are effective in meeting occupational exposure limits has not been evaluated by scientists or independent parties.
Using water will visibly suppress dust, but the extent that it reduces the small, respirable particles is unclear, without additional testing. Although exposure is not reduced to zero, substantial reduction is expected. This is dependent on the amount of water used, the water spray pattern, how effectively it prevents suspension of particles in the air, the nature and amount of work and the extent to which workers are exposed to resuspended dust after it dries. While published sampling data on these specific tools is very limited, evidence indicates exposure to respirable silica can be significantly reduced through the use of wet methods.
In evaluations conducted during milling of asphalt and concrete pavement with wet dust suppression, NIOSH collected 21 personal air samples for those working on or around the milling machines. The sampling results indicated an average respirable dust concentration of 0.89 mg/m3 and respirable silica (quartz) concentration of 0.05 mg/m3, equal to the NIOSH REL and twice the ACGIH TLV.
In a similar evaluation, Linch reports personal sampling results ranging from 0.22 to 2.9 mg/m3 for respirable dust and from less than the detection limit to 0.36 mg/m3, seven times the REL and 14 times the TLV, for respirable silica.
Effects on Productivity:
Reducing airborne dust levels reduces vehicle and site cleanup times and avoids exposing other workers, members of the public, adjacent property, cars and building occupants, which can increase liability and time-consuming disputes. Improved worker comfort is a result of reduced airborne dust which may in turn result in less fatigue for the worker and greater productivity. In some cases, particularly where highway milling is intermittent, the use of water spray systems may be adequate to reduce the need to wear a respirator, and the need for an employer respiratory protection program.
Additional Considerations:
In addition to dust supression, water also plays a critical role as a coolant in these milling machines and reduces wear. Water flows adequate for cooling may or may not be adequate to reduce respirable dust.
The use of water controls may result in wet and slippery ground and walking surfaces. During cold weather this may lead to the formation of ice and an increased risk of slips, trips and falls. Milling debris that is not removed from the work area while wet may become airborne once dried, posing an inhalation hazard to anyone in the area. Maintaining a work area free of debris and excess water reduces the risk of these hazards.
The use of diesel and particularly gasoline-powered equipment poses the risk of carbon monoxide exposure, especially in areas where airflow is reduced. Steps to control exposure are important because the gas is invisible, odorless and tasteless. Poisoning by carbon monoxide can occur quickly indoors, but working outdoors does not ensure operators won’t be overcome. Small, inexpensive personal monitors should be worn by the operator to warn of unacceptable exposures. Equipment should not be left idling to cut down on carbon monoxide and to conserve fuel.
Milling machines and related equipment frequently generate sound levels that are greater than 90 decibels, the OSHA Permissible Exposure Limit (PEL), and hazardous. Hearing protection should be worn when using milling machines unless an industrial hygienist has conducted noise monitoring and indicated that hearing protection is not required.
As is the case with any construction equipment, users should follow manufacturer safety recommendations and comply with any applicable local, state or federal regulations.
To calculate the return on investment (ROI) for your specific application, please visit our Return on Investment Calculator. While a specific ROI example has not been developed for this particular solution, the ROI Calculator provides a useful tool and guidance on how to generate your own on investment analysis.
