Class K & Class D Fire Extinguishers
When ABC Will Make It Worse — Cooking Oils and Combustible Metals
A standard ABC dry-chemical extinguisher fights most fires. It will catastrophically fail on two specific classes — and using one anyway can kill you. Class K (cooking oils, NFPA 10 §6.6) demands a wet-chemical extinguisher; Class D (combustible metals — magnesium, lithium, sodium, titanium) demands an agent matched to the specific metal. Here’s why, what to deploy, and where each is required.
Why classes K and D need their own extinguishers
The five fire classes (A, B, C, D, K) aren’t arbitrary. Each represents a fundamentally different combustion chemistry, and each demands an extinguishing agent that addresses THAT chemistry. ABC dry chemical works on three of them (ordinary combustibles, flammable liquids, energized electrical) because the agent interrupts the free-radical chain reaction common to those fires. ABC fails on K and D because the underlying chemistry is different.
- Class K — cooking oils and fats. Fire is a deep, hot oil pool. Auto-ignition temperature 600-700°F for typical vegetable oils. The fire’s persistence comes from the hot mass of oil — knocking down the flame without cooling does nothing.
- Class D — combustible metals. Magnesium, sodium, potassium, lithium, titanium, zirconium. Each metal burns at much higher temperature than a typical fire, and most react violently with water and/or CO₂. The agent must NEVER react with the metal.
Class K — wet chemical for cooking-oil fires
Agent
Aqueous potassium-acetate, potassium-citrate, or potassium-carbonate solution. Listed under UL 154 (now part of UL 711). Discharged as a fine mist — NOT a stream — to avoid splattering hot oil.
How it works — saponification
The alkaline potassium salts react with the fatty acids in the burning oil to form a soap-like foam (saponification). The foam blanket cuts off oxygen AND cools the oil below auto-ignition. Reflash protection is the key advantage over dry chemical; the oil cools below 600°F before the foam breaks down.
NFPA 10 §6.6 placement rules
Required within 30 ft (travel distance) of any commercial cooking equipment that uses combustible cooking media. Most jurisdictions require it whenever a Type I hood + automatic suppression is required by NFPA 96. Includes deep fryers, woks, char-broilers, range tops, and tilt skillets.
Mandatory placard
NFPA 10 §6.6.2 requires a placard at the extinguisher: "In case of an appliance fire, use the automatic-suppression system. Then, if necessary, use the Class K-rated portable fire extinguisher." The fixed system (Ansul R-102, Pyro-Chem, Amerex KP series) discharges first; the portable handles residual hot spots only. Operating the portable while the fixed system is also engaged risks blowing burning oil out of the appliance.
Class D — metal-specific dry agents
Class D extinguishers come in several agent types — each matched to specific metals. Selecting the WRONG agent for the metal can intensify the reaction.
Sodium chloride (Met-L-X)
Pressed-salt agent. Effective on sodium, potassium, magnesium swarf, sodium-potassium alloys. Heat fuses the salt into a crust over the burning metal, smothering it. Ansul’s Met-L-X is the canonical product. Apply at the edge of the burning mass and work in.
Copper (Copper-X)
Copper-powder agent. Specifically formulated for lithium. Coats the lithium surface and smothers without reaction.
Sodium-bicarbonate (Na-X)
For lithium, magnesium, sodium-potassium, zirconium. Forms a crust through CO₂ release and bicarbonate decomposition.
Graphite (G-Plus, Lith-X)
For lithium, zirconium, sodium, magnesium. Graphite acts as a heat sink and oxygen-displacement layer. Lith-X is the legacy lithium product.
Sizing + placement (NFPA 10 §6.6 + §6.5)
- Class K travel distance: 30 ft maximum. Mount near every cooking appliance station, not just the entrance to the kitchen. Above floor level (handle ≤ 48 in per ADA).
- Class D travel distance: 75 ft maximum (NFPA 10 §6.5). For a typical machining or process cell, that effectively means one Class D within the cell.
- Class D quantity: NFPA 10 §5.5.4 sizes by metal type and the largest expected quantity of metal. For magnesium swarf at a CNC cell, 30 lb of agent typical. For a sodium-handling lab, larger.
- Backup means: sand or vermiculite buckets are recommended for Class D as a low-tech backup. NFPA 484 (Combustible Metals) gives industrial process detail.
Inspector field check
Class K
- Within 30 ft travel distance of every cooking appliance using combustible oil/fat.
- Mounting height — handle ≤ 48 in (ADA reach).
- Mandatory placard present + legible.
- Annual tag current; 5-year hydrostatic sticker present.
- Crew training documented — use AFTER the fixed system runs.
Class D
- Within 75 ft travel distance of any combustible-metal hazard.
- Agent type matches the specific metal — verify against the SDS or process design.
- Backup sand bucket / vermiculite container nearby.
- NO WATER signage in the area.
- Operator training documented — Class D specific, not a general fire-extinguisher class.
- Service company is licensed for Class D — many vendors aren’t.
Frequently Asked Questions
Why does an ABC extinguisher fail on a kitchen grease fire?
When is a Class K extinguisher actually required?
What about a sticker that says "Use of Class K Required"?
Why are Class D extinguishers metal-specific?
Is lithium-ion battery fire a Class D fire?
How are Class K + D extinguishers tested and inspected?
References
1. NFPA 10 (2022): Standard for Portable Fire Extinguishers §5.5, §6.6, §8.3.
2. UL 711: Rating and Fire Testing of Fire Extinguishers.
3. UL 154: Carbon-Dioxide Fire Extinguishers + UL 711-D for Class D.
4. NFPA 96 (2024): Ventilation Control + Fire Protection of Commercial Cooking Operations.
5. Manufacturer specs — Ansul Met-L-X, Ansul Lith-X, Amerex Class D, Buckeye Class K.
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