A class d fire extinguisher is the only tool rated for combustible metals like magnesium, sodium, and titanium. Water makes these fires worse - much worse. Here is how the right dry powder agent stops the reaction and keeps your facility FDNY-compliant.
Key Takeaways
- Class D extinguishers fight combustible metal fires only - magnesium, sodium, potassium, titanium, and zirconium
- Water is dangerous on metal fires - it reacts violently, producing hydrogen gas and spreading the fire
- Sodium chloride dry powder is the most common Class D agent - it smothers the fire by forming a crust over the burning metal
- NFPA 10 requires Class D units where combustible metals are present - machine shops, foundries, and chemical plants
- A&J Fire Extinguisher supplies and inspects Class D units in NYC - same-day service, flat-rate pricing, FDNY-certified
What Is a Class D Fire?
Class d fires involve combustible metals that burn at extremely high temperatures. Common metals include magnesium, sodium, potassium, titanium, and zirconium. These fires behave differently from ordinary combustible fires because the metal itself becomes fuel.
When combustible metals ignite, they burn at temperatures exceeding 5,000 degrees Fahrenheit. Standard fire extinguishers cannot put them out. In fact, using water or the wrong agent makes the situation worse.
Why metal fires are different:
- Magnesium burns so hot it can split water into hydrogen and oxygen, feeding the fire
- Sodium reacts explosively with water, producing hydrogen gas that ignites instantly
- Titanium combustion is self-sustaining once started, even without external oxygen
- Metal fires produce their own oxygen through chemical decomposition
This is why the National Fire Protection Association created a separate fire class for metals. If your facility works with these materials, you need the right safety equipment on hand.
What Is a Class D Fire Extinguisher?
A class d fire extinguisher uses a specialized dry powder agent designed to smother combustible metal fires. Unlike ABC dry chemical extinguishers that interrupt the chemical reaction, Class D agents work by forming a thick crust over the burning metal. This crust cuts off oxygen and absorbs heat.
The most common agent is sodium chloride powder, which melts and forms a solid crust. Other agents include:
- Copper powder - for lithium and lithium-alloy fires
- Graphite powder - for lithium fires in confined spaces
- Sodium carbonate - for sodium, potassium, and sodium-potassium alloy fires
Each agent targets specific metals. Using the wrong powder can be ineffective or dangerous. Always match the agent to the metal hazard in your facility.
How dry powder agents work:
- The extinguisher discharges powder under low pressure
- Powder coats the burning metal surface completely
- The powder melts and fuses into a solid crust
- The crust seals the metal from oxygen
- Heat absorption cools the metal below ignition temperature
- The fire is out and the crust prevents re-ignition
This controlled application is key. Unlike standard extinguishers that blast pressurized content, Class D units use a gentle, sweeping motion. The operator must apply enough powder to completely cover the burning metal.
When Do You Need a Class D Extinguisher?
NFPA 10 requires Class D fire safety equipment wherever combustible metals are present in quantities that could sustain a fire. Common locations include:
- Machine shops - where magnesium, titanium, or aluminum are machined
- Foundries and metal casting - molten metal operations
- Chemical plants - sodium, potassium, and reactive metal processing
- Aerospace manufacturing - titanium and magnesium components
- Recycling facilities - scrap metal that may include unknown reactive metals
- Laboratories - research using alkali metals
If your NYC facility falls into any of these categories, FDNY inspectors expect to see a properly placed Class D unit during annual inspections. Missing equipment means violations and potential shutdowns.
OSHA 29 CFR 1910.157 also requires appropriate portable fire extinguishers for the specific hazards present. A general-purpose ABC extinguisher does not satisfy the Class D requirement where combustible metals are present.
Class D vs Other Fire Extinguisher Types
| Feature | Class D (Dry Powder) | Class ABC (Dry Chemical) | CO2 | Water |
|---|---|---|---|---|
| Target | Combustible metals | Ordinary/liquids/electrical | Flammable liquids | Ordinary combustibles |
| Agent | Sodium chloride, copper, graphite | Monoammonium phosphate | Carbon dioxide | Water |
| Application | Gentle sweep, full coverage | Pressurized blast | Direct discharge | Direct stream |
| Metal Fire Safe? | Yes | No - can scatter burning metal | No - does not smother | DANGEROUS - causes explosion |
| Re-ignition Risk | Low (crust prevents) | High (metal stays hot) | High (no smothering) | Extreme (hydrogen gas) |
The table above shows why having the wrong equipment is worse than having no equipment at all. Applying water to a sodium fire produces hydrogen gas, which ignites explosively. Using an ABC extinguisher on magnesium can scatter burning metal particles across the room.
Types of Class D Dry Powder Agents
NFPA 10 and extinguishing agents manufacturers classify Class D agents by the metals they are rated to suppress:
Sodium Chloride (NaCl) - Most Common
- Rated for magnesium, sodium, potassium, and sodium-potassium alloys
- Available in 30 lb and 50 lb handheld units
- Also available in 150 lb and 350 lb wheeled cart units for large facilities
- Most popular choice for general metal fire protection
Copper Powder - For Lithium
- Specifically designed for lithium and lithium-alloy fires
- Copper powder conducts heat away from the metal and forms a protective crust
- Required in battery manufacturing and lithium processing facilities
Graphite Powder - Confined Space Lithium
- For lithium fires where copper powder cannot reach
- Effective in tight spaces and enclosed equipment
- Used in battery research labs and aerospace
WARNING: ABC dry chemical is NOT Class D. Monoammonium phosphate (ABC powder) is not rated for metal fires. The pressurized discharge can scatter burning metal particles. Never substitute an ABC unit when Class D is required.
How to Use a Class D Fire Extinguisher
Fighting class d fires requires a different technique than standard extinguisher use. The powder must completely cover the metal to form an effective crust.
Step-by-step:
- Alert others and call 911 - metal fires can escalate fast
- Approach from upwind - metal fires produce toxic fumes and flying sparks
- Pull the pin and remove the nozzle from the retainer
- Aim at the base of the fire from a safe distance
- Apply powder in a gentle, sweeping motion - do NOT blast the burning metal
- Build up a thick layer covering the entire burning surface
- Extend coverage 6 inches beyond the fire edge to prevent spread
- Do not disturb the crust for at least 30 minutes after the fire is out
- Evacuate if the fire grows beyond the extinguisher capacity
Critical safety warnings:
- Never use water on a combustible metal fire
- Never use CO2 on a metal fire - it provides no smothering crust
- Do not break the crust after discharge - oxygen can re-ignite the metal
- Wear full PPE including face shield and heat-resistant gloves
- Class D extinguishers have limited range - stay as far back as possible
Inspection and Maintenance
Class D fire extinguishers require the same inspection schedule as other portable extinguishers under NFPA 10:
Monthly Visual Check
- Verify the extinguisher is in its designated location
- Confirm the pressure gauge reads in the operable range
- Check that the nozzle is clear and unobstructed
- Inspect for visible damage, corrosion, or tampering
- Verify the operating instructions are legible
Annual Professional Inspection
A certified technician must perform a full inspection every 12 months. For businesses in NYC, A&J Fire Extinguisher provides FDNY-certified annual inspections that cover all fire extinguisher types, including Class D units. Call (718) 852-2762 to schedule same-day service.
Internal Maintenance Schedule
- Dry powder types: Internal inspection and powder replacement every 6 years
- Hydrostatic testing: Every 12 years (empty cylinder, pressure test, refill)
- Replacement: If the unit shows corrosion, damage, or the powder has compacted
Choosing the Right Class D Unit
When selecting a class d fire extinguisher, consider these factors:
Metal hazard type
- Match the agent to your specific metal (sodium chloride for most, copper for lithium)
Fire size potential
- 30 lb handheld for small operations (30-45 seconds discharge)
- 50 lb handheld for medium facilities
- 150 lb wheeled cart for large foundries and industrial plants
- 350 lb wheeled for heavy industrial operations
Facility layout
- Place units within 50 feet of the metal hazard (NFPA 10 guidance)
- Ensure clear access - do not block with equipment or inventory
- Consider multiple units for large or multi-level facilities
Compliance requirements
- FDNY certificate of fitness may be required for certain metal operations
- OSHA requires annual inspection documentation
- NYC Fire Code may specify placement and quantity based on occupancy type
Need help selecting the right Class D unit? Contact A&J Fire Extinguisher at (718) 852-2762. Our Brooklyn-based team can assess your metal hazards, recommend the correct units, and handle installation - all in one visit.
Frequently Asked Questions
Class D extinguishers are designed exclusively for fires involving combustible metals such as magnesium, sodium, potassium, titanium, and zirconium. They must not be used on ordinary combustible, flammable liquid, or electrical fires. Using a Class D agent on a non-metal fire can be ineffective. If your facility has multiple fire hazards, you need the appropriate fire extinguisher type for each class.
Class A extinguishers fight ordinary combustible fires (wood, paper, cloth). Class B covers flammable liquid fires (gasoline, oil). Class C is rated for energized electrical equipment. Class D handles combustible metal fires specifically. Each class requires a different extinguishing agent. Many buildings need multiple types. An ABC extinguisher covers classes A, B, and C but does not protect against metal fires - Class D units are separate.
Only Class D dry powder agents approved for the specific metal should be used. Sodium chloride powder works for magnesium, sodium, and potassium. Copper powder is rated for lithium fires. Graphite powder handles lithium in confined spaces. Water, CO2, foam, and ABC dry chemical are all dangerous on metal fires. The class d extinguishing agent must suppress the fire by forming a smothering crust - no other method achieves this safely.
Baking soda can help smother a small chimney fire by cutting off oxygen, but it is not a rated or reliable fire safety tool. For chimney fires, call 911 immediately and use a chimney fire extinguisher stick or close the damper if safe to do so. Baking soda is sodium bicarbonate - it works on small grease fires in the kitchen but should never replace proper fire extinguishers. A&J Fire Extinguisher in Brooklyn carries chimney-rated equipment and can inspect your fireplace setup.
Yes. Even small amounts of combustible metals like magnesium shavings or sodium scraps can sustain a fire. NFPA 10 requires Class D protection wherever combustible metals are present in ignitable quantities. Machine shops cutting small magnesium parts still need proper Class D units. The risk is not just the metal - it is the reaction when standard extinguishers are used incorrectly. A&J Fire Extinguisher can evaluate your facility and recommend the right protection for your specific metal hazards.