This standard outlines comprehensive guidelines for the design, installation, and operation of local exhaust ventilation (LEV) systems in facilities producing asbestos-containing products. It aims to control airborne asbestos fibers by detailing specifications for hood design, ductwork, dust collection, and routine maintenance, ensuring occupational safety. The standard serves as a critical resource for engineers, health and safety professionals, and plant supervisors working in asbestos manufacturing environments.
Overview
This standard outlines comprehensive guidelines for the design, installation, and operation of local exhaust ventilation (LEV) systems in facilities producing asbestos-containing products. It aims to control airborne asbestos fibers by detailing specifications for hood design, ductwork, dust collection, and routine maintenance, ensuring occupational safety. The standard serves as a critical resource for engineers, health and safety professionals, and plant supervisors working in asbestos manufacturing environments.
Audience
Contents
Structure
This section defines the extent of the standard, focusing on inspection, testing, and upkeep of local exhaust ventilation systems used in asbestos product manufacturing. It highlights mandatory weekly and annual inspections to maintain system efficiency and safety. A yearly inspection checklist includes verifying the integrity of hoods and ducts, checking for air leaks, inspecting filter elements and cleaning mechanisms, ensuring lubrication, and confirming fan and drive conditions. Airflow and fan inlet pressures are measured against design values. Typical exhaust rates for various machines and duct thickness requirements are also provided along with SI units used.
This section addresses the principles behind LEV systems specifically for asbestos dust control. It emphasizes capturing dust at its source to reduce airborne fiber exposure, specifying minimum capture velocities (0.5 to 2.5 m/s), volumetric flow rate calculations based on hood area and capture velocity, and minimum duct velocities (≥ 15 m/s) to prevent dust settling. Hood design should position capture devices close to the emission source and partially or fully enclose it for optimal efficiency.
Key considerations for hood design include ensuring sufficient air velocity to entrain dust (captor-type) or maintaining negative pressure (enclosure-type) to prevent leakage. Openings should be minimized to reduce unnecessary airflow. Hoods must be placed close to dust sources and shaped to direct dust particles efficiently into the hood. The section also provides formulas for calculating required airflow based on hood opening area and capture velocity.
Ducts should be circular and constructed from mild steel with smooth interiors to minimize airflow resistance and dust accumulation. Minimum air velocities are specified (17.5 m/s, increasing to 22.5 m/s for metal-containing dust). Duct thickness varies with diameter, and installations should feature generous bend radii, connections from the top or side only, access doors for maintenance, test points for airflow verification, and adequate structural support.
This part details exhaust air volumes for various machinery and outlines preferred dust collector types, primarily fabric filter collectors, which can be intermittent or continuous. Cyclones are not recommended alone due to low efficiency but may serve as pre-filters. Filter area calculations ensure proper filtration speed and minimal pressure drop. Systems must include dust hoppers with bagging-off outlets using polyethylene bags designed to minimize dust leakage during handling.
This section covers exhaust systems characterized by very high capture velocities (50–60 m/s) but low air volumes (0.3 to 7 m³/min), suitable for hand tools and machining operations. Hoods or nozzles are custom-fitted and positioned very close to dust sources. Small-bore flexible hoses connect to fixed steel ducting. Operator positioning must avoid standing between hood and dust source to ensure safety. Formulas and example calculations for volumetric flow rates are provided.
The section describes balancing devices such as fixed balancing cones and adjustable slide dampers (which must be locked after setup). Airflow measurement techniques using 10 mm diameter test holes covered when not in use are explained. Duct design considerations to maintain adequate velocity, minimize pressure loss, and enable maintenance access are reiterated. Typical airflow velocities for various operations are tabulated along with airflow calculation formulas.
Inspection protocols include weekly operational checks and a comprehensive annual examination by qualified personnel. The annual checklist verifies hood and duct integrity, absence of air leaks, filter condition, wear on cleaning mechanisms, lubrication status, fan and drive condition, and airflow and pressure measurements. The section also recommends permanent gauge installations near test points and highlights the importance of regular maintenance for system effectiveness.
This section outlines requirements for air volumes based on hood design and proximity to dust sources that influence collector and fan sizing. It summarizes typical exhaust rates for various machines. Fine dust collection methods include fixed extraction systems with filtered air discharge and dust collection in hoppers equipped with bagging outlets. Polyethylene bags are used for dust containment with designs that facilitate easy changing and minimize dust escape. Disposal protocols for other waste types are referenced.
Guidance on determining air volumes needed for effective dust capture is provided, emphasizing hood design and source proximity. It includes tables of typical exhaust rates and airflow velocities for different operations. Capture velocities vary by operation type, with transport velocities in ducts maintained around 20 m/s.
Recommendations include using circular mild steel ducts with smooth interiors and minimum air velocities of 17.5 m/s (up to 22.5 m/s for metal dust). Thickness specifications depend on duct diameter. Layout advises gradual bends, top or side connections only, access doors and test points for maintenance, and robust support to ensure stability.
Operational best practices include starting the exhaust system before machine operation to ensure suction at hoods, keeping the system running during short stoppages to prevent dust escape, and running exhaust for at least two minutes after machine shutdown. Filter cleaning should be performed manually or automatically during breaks. Dampers used for balancing must be locked to prevent unauthorized adjustments. Weekly inspections using light beam methods verify hood positioning and system function.
Frequently Asked
The recommended face velocities for capturing asbestos dust vary according to hood type and operation. For close-proximity capture such as milling or turning, an ideal face velocity is approximately 10 m/s. If this is not feasible, booth-type hoods should maintain face velocities between 1.75 and 2.5 m/s. Minimum inlet velocities at enclosure openings should not be less than 0.75 m/s to prevent dust escape. Additionally, duct transport velocities should be maintained at or above 20 m/s to avoid dust settling within the ductwork.
Exhaust hoods must be designed to maximize dust capture by positioning them as close as possible to the dust source. Captor-type hoods rely on sufficient air velocity at the hood opening to entrain dust, typically between 0.5 and 2.5 m/s, while enclosure or booth-type hoods maintain negative pressure inside to prevent dust leakage, with velocities around 0.3 to 1.0 m/s. Hood openings should be kept as small as practical to minimize airflow volume while ensuring effective capture. The overall system includes the hood, ducting, dust collector, and exhaust fan arranged in sequence to optimize dust control.
Fabric filter collectors are the preferred choice for asbestos dust control, available in intermittent or continuous operation types. Intermittent filters are suitable when the system can be shut down for cleaning, while continuous filters clean sleeves via shaking or reverse air jets without stopping operations. Cyclones should not be used as the sole dust collector due to poor efficiency with asbestos fibers but may serve as a pre-filter to remove larger particles before the fabric filter. Dust collector units should be designed with accessible hoppers and exhaust air discharged outside the building to ensure safe handling and maintenance.
Local exhaust ventilation systems should undergo weekly inspections to verify hood positioning, check for air leaks, assess filter hopper status, and confirm the proper function of cleaning mechanisms and pressure gauges. Additionally, a comprehensive inspection and testing by a competent individual must be conducted at least once annually. Any defects or issues identified during inspections require immediate correction, and detailed maintenance records should be maintained to ensure ongoing system reliability and effectiveness.
Ducting should be fabricated from smooth, corrosion-resistant mild steel in a circular cross-section to facilitate dust transport and ease of cleaning. The ducts must be sized adequately to maintain air velocities between 15 and 20 m/s to prevent fiber settling. Installation layouts should minimize bends and abrupt changes, using gradual bends with radii at least 1.5 times the duct diameter. Horizontal runs should be avoided when possible, or velocities maintained to prevent deposition. Access doors should be installed regularly for inspection and cleaning, and joints must be airtight to prevent fiber leakage. Exhaust outlets must be positioned away from air intakes and occupied areas, ensuring safe dispersal of fibers.
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