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49 Cards in this Set
- Front
- Back
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Approaches and Examples of the Major Control Approaches
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Administrative controls
Engineering controls- PPE Source Modification Substition Process Change Isolation Ventilation Process controls isolating techniques isolating equipment isolating employees |
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Approaches and Examples of the Major Control Approaches - Administrative controls
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Administrative controls- Management involvement, training of employees, rotation of employees,
air sampling, biological sampling, medical surveillance |
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Approaches and Examples of the Major Control Approaches -Engineering controls-
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Engineering controls- Process change, substitution, isolation, ventilation, source modification
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Approaches and Examples of the Major Control Approaches - PPE
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• PPE - Gloves, aprons, rubberized clothing, hard hats
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Approaches and Examples of the Major Control Approaches - Source Modification
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Source modification - Changing a hazard source to make it less hazardous (e.g, wetting dust
particles or lowering the temperature of liquids to reduce off-gassing and vaporization) |
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Approaches and Examples of the Major Control Approaches - Substitution
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Substitution - Substituting a less hazardous material, equipment, or process for a more hazardous
one (e.g., use of soap and water in place of solvents, use of automated instead of manually operated equipment) |
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Approaches and Examples of the Major Control Approaches - Process Change
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Process change -Changing a process to make it less hazardous (e.g., paint dipping in place of
paint spraying) |
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Approaches and Examples of the Major Control Approaches - Isolation
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Isolation - Separating employees from hazardous operations, processes, equipment, or
environments (e.g., use of control rooms, physically separating employees and equipment, barriers placed between employees and hazardous operations) |
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Approaches and Ex. of the Major Control Approaches - Ventilation
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Ventilation - Two fundamental approaches: general exhaust (dilution
of air contaminants) and local exhaust (of air contaminants) |
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Approaches and Ex. of the Major Control Approaches - Process Controls
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Process controls - Continuous processes typically are less
hazardous than intermittent processes |
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Approaches and Ex. of the Major Control Approaches - Isolating techniques
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Isolating techniques- Storage of hazardous materials (e.g., use of
ventilated storage cabinets for chemicals, size of storage container) |
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Approaches and Ex. of the Major Control Approaches - isolating equipment
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Isolating equipment (e.g., physical isolation of valves and pump
seals, barriers around equipment) |
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Approaches and Ex. of the Major Control Approaches - isolating employees
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Isolating employees (e.g., use of closed control rooms, isolation
booths, supplied-air islands) |
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Types of Ventilation
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Types of Ventilation
• Local exhaust ventilation (LEV) • General exhaust ventilation (GEV) • Natural ventilation |
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Natural Ventilation
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Natural Ventilation
• Wind • Thermal convection • Economical but limited usefulness • No mechanical air movement (fans) |
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General Exhaust Ventilation (GEV)
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General Exhaust Ventilation (GEV)
• Removal of contaminants by moving entire air mass. – Also known as “dilution” ventilation – Comfort ventilation – Reduce stale air (CO2, biological agents, odors, heat) |
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Limitations of GEV
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Limitations of GEV
• Inadequate mixing leads to build-up of contaminant in stagnant pockets • Direction of air flow causes contaminants to be carried past occupants • Large releases over short time periodventilation system can’t handle spikes • Seasonal changes in HVAC operationvariable air flow |
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General Exhaust Ventilation GEV Feasibility
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GEV Feasibility
• Low toxicity gas/vapor contaminants • Low fire risk • Minor, decentralized sources • Uniform release of contaminants • No exposed workers near source • Not recommended for particulates • Toxicity may require large volumes of air • Velocity/rate of evolution usually high for high contaminant concentration |
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GEV Location of Inlets/Outlets
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GEV Location of Inlets/Outlets
• Critical to ensure good mixing • Avoid placing inlets directly across from outlets • Air movements from clean to dirty |
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Ventilation System Components
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Ventilation System Components
• Supply – Conditioned outside air – Includes replacement (make-up) air • Return (recirculating) – Conditioned air drawn back into HVAC system – Contaminant can accumulate if inadequate • Exhaust – General – Local |
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Replacement (Make-up) Air
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Replacement (Make-up) Air
• Conditioned air that replaces exhausted air • Design for 10% excess make-up air • Inadequate make-up air results in negative pressure • Exhaust systems can’t function without adequate make-up air |
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Recirculating Air
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Recirculating Air
• Recirculation of exhaust air after cleaning • Lower energy costs • Not for toxic contaminants • Suitable air cleaner |
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Industrial Ventilation Applications
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Industrial Ventilation Applications
• Condition air (heat, cool, %RH) • Prevent fire & explosion • Control atmospheric contaminants • Natural versus mechanical |
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Ventilation “Laws”
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Ventilation “Laws”
• Conservation of Energy – Pressures (SP,VP,TP) – Pressure differentials > air movement toward negative pressure – SP – (either negative or positive) potential energy in the system that starts the air moving. – VP – is due to air moving through the system which represents kinetic energy. |
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Ventilation “Laws”
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Ventilation “Laws”
• Conservation of Mass (flow) – Constant volume of air • Volumetric Flow Rate Q=VA Q=volumetric flow rate (cfm) V=average velocity (fpm) A=cross-sectional area (ft2) |
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Air Flow Characteristics
• Blowing versus Exhausting: |
Air Flow Characteristics
• Blowing versus Exhausting: – Rule of thumb • Can blow much further than you can exhaust • Hoods need to be very close to contaminants |
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Local Exhaust Ventilation (LEV)
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Local Exhaust Ventilation (LEV)
• Removal of air at/near the source of the contaminant |
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LEV Advantages
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LEV Advantages
• Remove relatively small volume air (lower energy costs) • Worker exposures are reduced • Less need for air purifying system (smaller scale, cost) • Can use to reclaim product |
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Components of a LEV System
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Components of a LEV System
• Hood • Ductwork • Air cleaning device • Fan • Stack |
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LEV Components: Hoods
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LEV Components: Hoods
• Capture/Contain contaminant • Design to allow worker access • Flange improves LEV performance • Maximum enclosure |
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Capture Velocity
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Capture Velocity
• Velocity required to move specific contaminants into hood – Sufficient to overcome interferences |
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Capture Distance
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Capture Distance
• Distance at which the hood can capture contaminants |
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Basic Types of Hoods
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Basic Types of Hoods
• Capture hoods (push-pull, slot) • Receiving hoods (hot process canopy) • Enclosures (lab hoods, spray booths) |
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Hood Design/Contaminant Issues
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Hood Design/Contaminant Issues
• Physical State • Temperature of process • Generation of contaminant • Velocity and direction of released contaminant • Toxicity |
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Hood Design Considerations
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Hood Design Considerations
• Locate hood close to contaminant source • Move air away from workers’ BZ • Align hood w/contaminant “throw” • Crossdrafts, locations • Worker/process access |
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Crossdrafts/Turbulence
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Crossdrafts/Turbulence
• Supply air • Doors, windows • Moving equipment • People |
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LEV Components: Ductwork
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LEV Components: Ductwork
• Move air • Distribute air • Contaminated air under negative pressure |
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Transport Velocity (Minimum Duct Velocity)
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Transport Velocity (Minimum Duct
Velocity) • Minimum velocity required in ductwork to keep particulates suspended in airstream • Dusts (> 2000 fpm) • Insufficient velocity results in buildup in ducts, eventual obstruction |
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Energy Losses
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LEV Components: Ductwork • Move air • Distribute air • Contaminated air under negative pressure Transport Velocity (Minimum Duct Velocity) • Minimum velocity required in ductwork to keep particulates suspended in airstream • Dusts (> 2000 fpm) • Insufficient velocity results in buildup in ducts, eventual obstruction Energy Losses • Resistance/friction of air against walls of duct • Turbulence from change in direction or velocity • Hood entry losses • Energy losses result in pressure drop in system • Big fans cost $$$$ |
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Friction Losses in Duct System
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Friction Losses in Duct System
• Round ducts: – Cross-sectional surface area – More resistant to collapse – Better for particulates • Less friction with – Larger ducts – Smooth curves in system |
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Air Distribution
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Air Distribution
• Testing & balancing • SP balance; can’t be adjusted by worker • Dampers/blast gates: more flexible but subject to tampering |
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LEV Components: Air Cleaning Devices
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• Particulates:
– Examples: filter, cyclone, baghouse, electrostatic precipitators, water spray • Efficiency = Amount collected / Amount entering • Vapors/Gases: – Examples: wet scrubber, activated charcoal, cooling (condensation), flare |
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LEV Components: Exhaust Fans
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LEV Components: Exhaust Fans
• Located at roof • Not inline or at source • Can have convective air flow even if not operating • Can run in reverse direction |
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Selection of Fans & Motors
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Selection of Fans & Motors
• Capacity (depends on resistance of system) • Efficiency (cost to run) • Noise/vibration • Maintenance (ease of inspection & maintenance) |
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LEV Components: Stacks
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LEV Components: Stacks
• Reentrainment issues • Exhaust velocity • Location • Stack height (10 ft above roof line) |
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Testing Ventilation Systems
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Testing Ventilation Systems
• Smoke tubes-air movement patterns, pressure relationship • Trace gases-sulfur hexafluoride • Pitot tube/inclined manometer-velocity pressure • Anemometer-velocity (fpm) |
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Measurements
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Measurements
• Design specifications vs. performance • Face velocity-average several readings • Duct measurements: – 7 to 8 duct diameter from interferences – Readings vary with location in duct – Pitot traverse |
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LEV Systems/Visual Clues
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LEV Systems/Visual Clues
• Duct shape, type, condition • Elbows • Dampers • Air cleaner • Diffuser/grills |
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Ventilation Standards & Regulations
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• OSHA 29 CFR 1910.94
– Industrial Operations • Abrasive blasting • Grinding, polishing, buffing • Spray finishing • Open surface tanks • Welding, cutting, brazing (.252) • Flammable/combustible liquid storage (.106) • Dip tanks with combustible/flammable liquid (.108) • Others: asbestos, lead, Chemical Hygiene |
