Textile Dyehouse Fire: NFPA 33 Spray Booth and Solvent Ventilation

Textile dyehouses work with flammable solvents such as ethanol, isopropanol and toluene. Spray booths, drying ovens and solvent storage fall under NFPA 33 (Spray Application Using Flammable or Combustible Materials) and NFPA 86. Solvent vapour rising from the fabric creates a Class I Div 1 hazardous area, meaning an explosive mixture is assumed to be present at any time. Projects need to address the three elements — booth, solvent and electrical — together; designing one well and neglecting another puts the whole facility at risk.

Spray Booth Design

• Airflow: 0.5 m/s (100 fpm) across the booth cross-section — the threshold that keeps vapour from accumulating in the booth.
• Enclosure: noncombustible metal (0.9 mm steel); the item most often skipped in site-built booths.
• Filter bank: traps overspray; weekly replacement should be mandatory.
• Fan: spark-resistant type with non-ferrous blades.
• Interlock: a fan failure must stop spraying immediately.

Solvent LEL and Ventilation

• Ethanol lower explosive limit (LEL): 3.3% (33,000 ppm)
• Toluene LEL: 1.1% (11,000 ppm)
• Operating limit: no more than 25% of the LEL — beyond this the booth is no longer at a normal operating point.
• Ventilation: 30 m³/min per m² of booth cross-section (equivalent to 0.5 m/s air velocity)
• LEL detector: alarm at 10%, automatic shutdown at 25% — on site, plants that bypass the second threshold are seen; this is wrong.

Sprinklers and Portable Extinguishers

• Booth sprinklers: NFPA 13-2025 Annex A.4.3.4.2 lists flammable liquids spraying as an Extra Hazard Group 2 (EH2) example; per Table 19.2.3.1.1 that means 0.40 gpm/ft² (16.3 mm/min) over 2500 ft² (232 m²). K115 (K8.0) sprinklers are used on site.
• Exhaust duct sprinklers: installed every 4 m inside the exhaust duct.
• Portable extinguishers: 20B:C CO₂ every 10 m.
• Dry chemical: automatic ABC protection for the booth — a line that operates even when no workers are present.

Class I Div 1 and Electrical

• Booth interior: Class I Div 1 (NFPA 70) — the most severe area.
• 1 m outside the booth: Class I Div 2.
• Beyond 3 m: unclassified.
• Lighting: explosion-proof LED, UL 844 listed.
• Static bonding: 1 ohm maximum resistance; most colleagues skip this measurement at commissioning.

Dyehouse ventilation with MEP Calc

MEP Calc sizes dyehouse booth airflow, LEL monitoring and makeup air per NFPA 33.

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