505.0 Product-Conveying Systems
Exception: Ducts conveying vapor or fumes having flammable constituents less than 25 percent of their Lower Flammability Limit (LFL) shall be permitted to pass through other spaces.
- Combustible concentration reduction
- Oxidant concentration reduction
- Deflagration suppression
- Deflagration pressure containment [NFPA 91:184.108.40.206]
Contaminated air shall not be recirculated to occupied areas unless contaminants have been removed. Air contaminated with explosive or flammable vapors, fumes, or dusts; flammable or toxic gases; or radioactive material shall not be recirculated.
- Where ducts pass through fire barriers
- Where a collection system installed on the end of the system is protected with an automatic extinguishing system
- Where the duct system is protected with an automatic extinguishing system
- Where ducts have been listed with interrupters
- Where necessary to facilitate the control of smoke pursuant to the applicable NFPA standards [NFPA 91:4.2.9]
|Class 1||-||Ducts conveying nonabrasives, such as smoke, spray, mists, fogs, noncorrosive fumes and gases, light fine dusts, or powders.|
|Class 2||-||Ducts conveying moderately abrasive particulate in light concentrations, such as sawdust and grain dust, and buffing and polishing dust.|
|Class 3||-||Ducts conveying Class 2 materials in high concentrations and highly abrasive materials in low concentrations, such as manganese, steel chips, and coke.|
|Class 4||-||Ducts conveying highly abrasive material in high concentrations.|
|Class 5||-||Ducts conveying corrosives, such as acid vapors.|
Systems conveying particulate matter shall be designed by employing the constant velocity method. Systems conveying explosive or radioactive materials shall be pre-balanced through duct sizing. Other systems shall be permitted to be designed with balancing devices such as dampers. Dampers provided to balance airflow shall be provided with securely fixed minimum-position blocking devices to prevent restricting flow below the required volume or velocity.
|NATURE OF CONTAMINANTS||EXAMPLES||FEET PER MINUTE DESIGN VELOCITY (feet per minute)|
|Vapors, gases, smoke||Vapors, gases, and smoke||Any|
|Fine light dusts||Cotton lint, wood flour, litho powder||2500|
|Dry dusts and powders||Fine rubber dust, molding powder dust, jute lint, cotton dust, shavings (light), soap dust, leather shavings||3000|
|Average industrial dusts||Grinding dust, buffing lint (dry), wool jute dust (shaker waste), coffee beans, shoe dust, granite dust, silica flour, general material handling, brick cutting, clay dust, foundry (general), limestone dust, packaging and weighing asbestos dust in textile industries||3500|
|Heavy dusts||Sawdust (heavy and wet), metal turnings, foundry tumbling barrels and shake-out, sandblast dust, wood blocks, hog waste, brass turning, cast-iron boring dust, lead dust||4000|
|Heavy or moist dusts||Lead dust with chips, moist cement dust, asbestos chunks from transite pipe cutting machines, buffing lint (sticky), quick-lime dust||4500|
For SI units: 1 foot per minute = 0.005 m/s
* Systems that are handling combustible particulate solids shall be in accordance with NFPA 654.
Approved nonmetallic hoods and duct systems shall be permitted to be used for Class 5 corrosive systems where the corrosive mixture is nonflammable. Metal hoods used with Class 5 duct systems shall be protected with an approved corrosion-resistant material. Edges of hoods shall be rounded. The minimum clearance between hoods and combustible construction shall be the clearance required by the duct system.