FM Global DS 7-78: Fire Protection of Industrial Exhaust Systems

Industrial exhaust systems carry flammable vapour, fumes, dust or particulate; deposits that accumulate in the duct become fuel, and a fire can spread quickly along the ductwork. Plastic (PP, PVC, FRP) ducts and plastic liners can carry a fire on their own. FM Global DS 7-78 Industrial Exhaust Systems (October 2021, interim revision October 2024) gives the rules for duct materials, sprinkler protection inside and outside the duct, air velocity, drainage and maintenance.

Scope (§1.0)

• Industrial exhaust systems and chimney liners that convey fumes or particulates to atmosphere or to treatment equipment: plating, printing, ovens, painting, mining, metallurgy, chemical, pulp and paper and food processing.
• Process piping is not covered.
• For commercial cooking exhaust in restaurants and cafeterias, only the quarterly inspection and cleaning recommendation (§2.4.1.2) applies; the other provisions of the data sheet do not.
• Where an occupancy- or equipment-specific data sheet gives different guidance, that data sheet governs.

Construction and Location (§2.1)

• Ducts and equipment are metal or noncombustible. If plastic ducts or plastic liners must be used, they should be FM Approved; if approved materials are not available, they are protected per the data sheet (§2.1.1).
• The duct takes the shortest, most direct route to atmosphere; waste emission equipment is located outdoors where it is accessible for manual firefighting (§2.1.2–§2.1.3).
• Penetrations of fire barriers are avoided; where unavoidable, a fire damper of equivalent rating is fitted at the penetration. If a damper cannot be used, the duct is enclosed in a shaft of equivalent rating for 7.6 m on each side of the barrier (§2.1.2.2).
• Insulation is noncombustible or FM Approved (§2.1.1.1).

Protection Inside the Duct (§2.2.2)

• Sprinklers or water spray nozzles go inside ducts with a cross-section of at least 516 cm² or a diameter of at least 254 mm where combustible deposits thicker than 1.5 mm are expected throughout, or where the duct is combustible (non-approved plastic or plastic-lined metal) (§2.2.2.2).
• Locations: within 0.9 m of the intake, of the entrance and exit of any intermediate equipment or building, and of any change in direction of 90° or more; at 3.7 m centres in horizontal ducts and 7.4 m in vertical ducts, including the top of vertical ducts (§2.2.2.3).
• Design: at least 57 L/min (15 gpm) per sprinkler with 8 sprinklers operating, 74 °C quick-response, K ≥ 5.6 (K80) (§2.2.2.5). A rating up to 27 °C above the internal ambient is acceptable; extended coverage sprinklers are not used (§2.2.2.6–§2.2.2.7).
• FM Approved corrosion-resistant sprinklers or nozzles with a spray angle above 140°; in extremely corrosive environments, 75 L/min per nozzle per Table 2.2.2.8 (§2.2.2.3F, §2.2.2.8).
• Inside combustible equipment and intermediate devices, 90 L/min (25 gpm) (§2.2.2.11).
• Indoors, ceiling sprinklers per DS 3-26 for the occupancy; where the occupancy needs no sprinklers but the ducts are large enough to sustain a self-propagating fire, sprinklers directly above the ducts at 3.7 m spacing, 8 × 57 L/min (§2.2.2.1).

Outdoor Protection (§2.2.3)

• Internal protection of outdoor ducts and equipment follows the indoor rules (§2.2.3.1).
• Combustible outdoor ducts and equipment get automatic external protection when they are in an inaccessible or congested area, are made of highly combustible thermoplastics such as polypropylene or polyethylene, or expose a combustible wall or roof (§2.2.3.2.1).
• External water spray at 10 mm/min (0.25 gpm/ft²) over the exposed surface, at 3.7 m vertical centres (§2.2.3.5–§2.2.3.6). For low-combustibility thermosets such as FRP, fixed monitors may be used under specific conditions (§2.2.3.3).

Hose Allowance and Drainage (§2.2.6–§2.2.7)

• A minimum 946 L/min (250 gpm) hose allowance for all designs, plus additional water where fixed monitors are used (§2.2.6.3).
• Where sprinklers are installed inside a duct, low-point drains prevent duct collapse, sized per Table 2.2.7.1 assuming a 4% slope (§2.2.7.1). Where hazardous residue is present, containment for 20 minutes of actual discharge (§2.2.7.4).

Air Velocity and Interlocks (§2.3)

• Duct velocity is chosen to keep vapour or particulate from settling; recommended values depend on the contaminant (Table 2.3.1.7): vapours, gases and smoke usually 5–10 m/s, welding fumes 10–12.7 m/s, average industrial dust 17.8–20.3 m/s, heavy dusts 20.3–22.9 m/s, heavy or moist dusts 22.9 m/s and up (§2.3.1.7).
• Systems conveying ignitable vapour are kept below 25% of the lower explosive limit (LEL) in normal operation, with an audible alarm at 25% LEL (§2.3.1.9, §2.3.4.1).
• A low-airflow alarm at 95% of design velocity (§2.3.4.2).
• Spark-producing operations are not manifolded with systems handling combustible particulates, and incompatible materials (e.g. oxidizers and organics) are not mixed in ductwork (§2.3.1.1–§2.3.1.2).
• Fire dampers at equipment inlets and outlets; the discharge at least 1.8 m from combustible construction and 7.6 m from unprotected openings of nearby buildings (§2.3.1.3, §2.3.1.6).
• Readily accessible inspection and cleaning ports at junctions, turns and intermediate runs (§2.3.2).
• Metal duct parts are bonded and grounded (§2.6.2).

Inspection and Maintenance (§2.4)

• Ongoing monitoring of filters; monthly temperature checks of fan and motor bearings.
• Quarterly visual inspection of ducts, equipment and fire protection components (commercial cooking included). Deposits thicker than 3 mm are removed with non-sparking methods; after one year of records the frequency can be adjusted with the FM field engineer.
• Annual (or per manufacturer) testing of fans, motors, controls and interlocks, duct integrity and dampers; infrared scans of motors and a grounding check every 1–3 years.

Quick Checklist

• Ducts metal/noncombustible or FM Approved plastic
• In-duct sprinkler triggers (size, deposits, combustible duct) checked
• Sprinkler layout: 0.9 m, 3.7 m horizontal, 7.4 m vertical; 8 × 57 L/min, 74 °C quick-response
• External protection of 10 mm/min on combustible outdoor ducts
• 946 L/min hose allowance and low-point drains
• Duct velocity per Table 2.3.1.7, 25% LEL alarm, low-airflow alarm
• Quarterly inspection with a 3 mm deposit limit

Frequently Asked Questions

Does DS 7-78 apply to commercial kitchen exhaust?

Only in part. Under DS 7-78 §1.0, for commercial cooking in restaurants and cafeterias only the quarterly inspection and cleaning recommendation (§2.4.1.2) applies; the other provisions, including in-duct sprinklers, do not. Kitchen exhaust is covered by NFPA 96 and local regulations.

When are sprinklers needed inside a duct?

Under DS 7-78 §2.2.2.2, in ducts of at least 516 cm² cross-section or 254 mm diameter where combustible deposits thicker than 1.5 mm are expected throughout, or where the duct is combustible (non-approved plastic or plastic-lined metal). The design uses 57 L/min from each of 8 sprinklers, 74 °C quick-response, K ≥ 5.6 (K80).

What air velocity should an exhaust duct have?

DS 7-78 does not give a single minimum velocity; Table 2.3.1.7 recommends ranges by contaminant: usually 5–10 m/s for vapours and gases, 17.8–20.3 m/s for average industrial dust and 20.3–22.9 m/s for heavy dusts. The design velocity is based on the resistance expected at the time of filter change or cleaning (§2.3.1.7).

How often are ducts inspected and cleaned?

DS 7-78 §2.4.1.2 calls for a quarterly visual inspection; deposits thicker than 3 mm are cleaned using non-sparking methods. After one year of records the frequency can be adjusted with the FM field engineer so that deposits do not exceed 3 mm between inspections.

What hose allowance applies to exhaust system protection?

A minimum of 946 L/min (250 gpm) for all protection designs, with additional water where fixed monitor nozzles form part of the protection (§2.2.6.3).

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