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August 2001 |
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Contents
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| Main Page | |
| Manufacture of Asphalt Roofing Products | |
| Occupational Exposure to Asphalt Fumes | |
 Guidelines for Protecting Workers |
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| References | |
| Abbreviations | |
| Glossary | |
| Appendix | |
This section describes available engineering controls, work practices, and personal protective equipment (PPE) used to reduce worker exposure to asphalt fumes in the roofing manufacturing industry. Engineering controls include process substitution, isolation (enclosures), and general and local exhaust ventilation. Some of these methods may be used for other purposes such as energy conservation or compliance with other regulatory requirements. Information is provided on available engineering controls and work practices to reduce worker exposure to asphalt fumes for operations and jobs identified in Section 4.3.
The following engineering controls are used to reduce worker exposure to asphalt fumes during the delivery, handling, and storage of hot asphalt.
Typically, a variety of tanks are used to contain hot asphalt in asphalt roofing manufacturing plants. These include storage tanks for delivered asphalt, saturant asphalts, and coating asphalts to be used in the production process; polymer and stabilizer mixing tanks in which asphalt coatings are made for conventional asphalt and modified bitumen roofing products; smaller-scale heating tanks for specialty operations such as laminating; and finished product tanks in plants that produce roofing asphalt for sale or shipment elsewhere.
Studies to determine the effectiveness of the following controls are necessary. Available engineering controls for reducing asphalt fume exposures include closed tanks. Asphalt fumes generated in these tanks may be vented directly to either the outside atmosphere or to one of several types of fume capture devices. Typical capture devices that can reduce the concentration of asphalt fumes and dusts released from these tanks or from blowing stills, coaters, and saturators include, but are not limited to, the following:
Reducing the concentration of asphalt fumes in the space above the hot asphalt also reduces the concentration of asphalt fumes emitted from the storage or processing tank through any openings (e.g., vents) that may be present. Asphalt is usually delivered to a plant by pipeline, truck, or rail. When pumping asphalt from the delivery vessel into the storage tank, receiving lines are attached to the storage tank below the liquid asphalt level (either by "bottom loading" or "submerged fill"). This practice helps reduce the emission of asphalt fumes during a loading operation.
The asphalt in the storage tanks is heated before its use in the manufacturing process. Excess heated asphalt is recirculated rather than returned to the storage tank to allow for lower storage temperatures of the asphalt. This procedure reduces the concentration of asphalt fumes emitted from the tank. Air sweeps* of storage tanks further reduce the concentration of asphalt fume in the space above the asphalt to a fraction of the lower explosive limit. Trumbore [1992] reported that recirculating excess asphalt to the start of the process and using air sweeps in the tank(s) lower the asphalt fume concentrations in storage tanks by 50% to 75%. Current asphalt fume concentrations average less than 25% of the lower explosive limit in all storage tanks where these controls are used.
Figure 5-1. Total enclosure of saturator, wet looper and coater. (Source: EPA[1988].)
Complete enclosure, coupled with adequate exhaust ventilation is a commonly used engineering control for saturators. Typically, the enclosure includes not only the saturator but also the wet looper and, in some instances, the coater as well. Available fume-capture devices are the same as those discussed in Section 5.1.1 (mist eliminators, high-velocity air filters or fiber bed filters, electrostatic precipitators, and incinerators and regenerative thermal oxidizers).
In some plants, physical or operating constraints may make complete enclosure of the entire saturator-wet looper-coater series impractical. In these instances, canopy hoods with adequate local exhaust ventilation (LEV) can be used for the entire series of process units or for one process area such as the coater. Figure 5–1 depicts typical hood configuration.
The procedures for venting a totally enclosed coater are similar to those for saturators. For coaters with canopy hoods, the procedure for rethreading† a break in the line requires the operator to wear proper PPE including a respirator (see Appendix) when needed. The exhaust fans should remain in operation during the rethreading process.
Another option for plants using saturators with both spray and dip processes is to rely exclusively on the dip process. The reduction in asphalt fume exposures in plants using the dip process has not been evaluated or quantified, although dip-only saturators are likely to generate lower airborne concentrations of asphalt fumes. The strong trend in the industry away from spray-process saturators has been generated by the need to reduce fire, explosion, and burn hazards associated with the spray processes. The switch to dip-only saturators has also been influenced by a variety of other factors, including the age and remaining useful life of the existing equipment; the cost and incremental exposure reduction benefits of a dip-only saturator; and the comparative costs and exposure reduction benefits of other available control options such as increased exhaust flow rates, improved door seals, reduced apertures in existing enclosures, and improved hood designs in existing LEV systems.
The emission of asphalt fumes from modified bitumen impregnation vats is controlled primarily with canopy hoods and adequate LEV. In many instances, these hoods extend over the cooling vats as well as the impregnation area.
The capture of asphalt fumes at the coater is best achieved using either (1) full enclosure of the coating process (together with the saturator and wet looper) or (2) canopy hoods. In either case, adequate exhaust ventilation capacity is necessary to achieve complete capture of the roofing asphalt fumes generated by the coater. In some cases, extending the hood or canopy over the granule application section (which basically follows the coater) may be necessary to capture fumes emitted by the hot web after it has left the coater area.
The removal of asphalt fumes by all of the in plant exhaust ventilation systems should be routed to capture devices before discharge to the outside air. Examples of these capture devices include
Section 5.1 gives more complete descriptions of these capture devices [EPA 1988]. All of these exhaust streams should be vented to the outside atmosphere after passing through the capture device.
Follow these practices to minimize asphalt fume exposure to workers outside the coater enclosure:
Higher exhaust rates due to excessive openings can lower the capture efficiency of the ventilation systems. To minimize the potential for fume exposures, all doors should be sealed and remain closed during operation of the line. The size of the apertures where the sheet enters and leaves the saturator should be minimized to reduce the potential for exposure to fumes and to ensure inward air flow from the workplace by the negative pressure in the enclosure. Adequate makeup air to the plant facility is important to ensure optimal capture velocity of the exhaust ventilation systems.
Assuming that an adequate configuration for the enclosure and proper exhaust ventilation is in place, the greatest potential for asphalt fume exposure during the coating process is during maintenance and repair work on the coater. Exposures to asphalt fumes can occur when workers must enter the enclosure during the manufacturing operation to correct a break in the operation of the line (occurs primarily from breakage of the felt sheet). Workers may need to enter the enclosure up to three times in an 8hr shift, and they usually spend 10 to 20 min correcting the problem. Follow these practices to ensure that fume exposure to the worker entering the enclosure is minimized:
The exhaust ventilation rate in the enclosure must be sufficiently high to exhaust the asphalt fumes from the entire enclosure in the event of a break in the felt. Maintaining negative pressure in the enclosure is essential to prevent leakage of asphalt fumes into the work areas and to minimize fume concentrations inside the enclosure during the rethreading process. For formal maintenance during shutdowns, the equipment should be cooled and drained so that the potential for asphalt fume exposure is minimal.
In some plants, operational or plant configuration (layout) constraints preclude enclosure as a control option for coaters. These constraints are more likely to be found in older facilities where congested initial design or subsequent manufacturing modifications have created space restrictions. Therefore, enclosures cannot be installed without restricting access to process equipment or normal line operation. In these instances, asphalt fumes generated during the coating process can be collected by a canopy hood with adequate LEV.
Typical canopy hood dimensions vary from 5 to 8 ft in width and normally exceed the mat width by 2 to 3 ft. The length in the machine direction is typically 6 to 14 ft. Exhaust ventilation flow rates from canopy hoods located above the coater range from 4,000 to 8,000 cfm. The vertical distance from the felt line to the hood opening should be minimized for optimal capture of the fume. Enclosure of the sides perpendicular to mat flow varies by facility. Insets in the canopy hood to block internal area and increase capture velocity at the hood periphery have proved effective in increasing fume capture efficiency. Cross air currents in the work area should be minimized to prevent interference of fume capture at the canopy hood.
After the saturation and coating processes, the asphalt is no longer heated and asphalt fume generation decreases as the product cools. Typically, some release of asphalt fume occurs as the impregnated felt leaves the coater area; however, the felt sheet is immediately covered by a layer of granules and mineral-parting agents. The deposit of this mineral coating onto the felt sheet helps in the cool-down process of the asphalt and decreases the release of asphalt fume.
The application of the mineral-parting agents generates the release of inorganic mineral dust. The dust contributes to the airborne concentration of particulates in the mineral application and cooling process areas of the plant. LEV is generally installed at granule applicators, slate drums, dust drums, back dust applicators, and transfer rolls to reduce dust exposure. The amount of asphalt fume captured is probably determined by the proximity of the LEV system to where mineral granules are applied to the asphalt on the felt sheet. Owens Corning [1993a,b] reported that VOC emissions from the cooling section of the process are greatly reduced when there is effective dust collection before cooling. All manufacturers use local dust collection with capture in a bag-type dust collector. Dust collection flow rates vary significantly because of differences in plant configurations; they range up to 30,000 cfm.
Some general dilution ventilation occurs in the manufacturing buildings as a result of additional air being exhausted from the cooling sections of the manufacturing line. Although general ventilation of the manufacturing buildings varies widely in roofing manufacturing, it contributes to reducing exposures to asphalt fume. The types of ventilation systems used range from gravity ventilators (which provide makeup air to the process) to power ventilators (which exhaust large air volumes from the process building).
After the cooling process, the temperature of the product is lowered to near ambient temperatures and asphalt fume generation is minimal. Asphalt fume exposures may occur during the application of both sealant asphalt to shingle products and laminating asphalt to laminated shingle products [Owens Corning 1993a,b].
These low-mass asphalt applications are applied from Star Wheel applicators or by means of asphalt extruders. LEV is installed at the point of asphalt application. Asphalt application temperatures are generally in the range of 250 to 400 °F (120 to 205 °C), but the sealant asphalt is cooled almost instantaneously by water, air, or soap solutions. Laminating asphalt is cooled by water baths or other means to eliminate product sticking. Because of the relatively small amounts of asphalt used in these processes and rapid cooling, fume generation is probably minimal; however, no data have been reported on the fume exposure that may occur during this process.
Good work practices and PPE should be used to prevent injury from combustion, explosion, or contact with hot asphalt. Before entering any enclosed area in a manufacturing plant, the area must be vented to remove asphalt fumes and ensure that adequate oxygen is available for workers. The equipment to be repaired must also be cooled to reduce the potential for burns. Maintenance work in potentially hazardous areas such as asphalt tanks, saturators, and coaters should be restricted to those personnel who are adequately trained and required to be in the area. Proper use of PPE, including respirators (see Appendix), should be mandatory.
PPE that may be used during these operations includes the following:
Appropriate PPE will reduce both dermal and inhalation exposures to asphalt fumes.
Effective work practices play an important role in preventing asphalt fume exposures and reducing the risks of acute and chronic health effects. For example, all operational instruments and fume control equipment must be properly maintained to ensure that they are effective in reducing asphalt fume exposures. Where respiratory protection is provided, all applicable OSHA requirements should be followed in accordance with a written respirator program, including use of NIOSH-approved respirators, training, fit-testing, and medical approval, as well as proper inspection, cleaning, maintenance, repair, and storage of respirators [29 CFR‡ 1910.134].
*Air sweeping is the practice of exhausting the vapor space above an asphalt storage tank rather than allowing the tank to vent through working and breathing losses only. This practice is very effective at lowering the vapor concentrations above the liquid to only a fraction of the lower explosive limit. The volume of air exhausted from the vapor space is typically 60 to 100 cubic feet per minute (cfm). This exhaust stream is either filtered to remove condensable particulate or incinerated for volatile organic compounds (VOCs) and particulate destruction.
†Rethreading occurs at startup and following a line break. Whether at startup or following a line break, the process is similar. Line operators pull/feed the web (paper or fiber glass) through the machines and rolls on the line. All machines are stopped or slowed by a jog position during this process. If the rethreading process requires operators to feed the web through the saturator or coater, standard PPE for exposure to hot asphalt is recommended—that is, respirators, safety glasses, leather work gloves, and a long-sleeved shirt or leather/Kevlar forearm/upper-arm sleeves.
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