By DHA Report Group · Updated October 20, 2026
If your facility crushes, grinds, stores, or handles sulfur, NFPA 655 was the standard written for you. It has been folded into NFPA 660. The sulfur requirements now live in Chapter 23, Sulfur, applied along with the fundamentals that cover every combustible dust facility.
Where NFPA 655 went
In NFPA 660, sulfur has its own chapter, Chapter 23. The fundamentals that used to come from NFPA 652, including the dust hazard analysis (DHA) requirement, sit in Chapters 1 through 10 of the same document. You apply the fundamentals plus Chapter 23.
Who this applies to
Any operation that handles sulfur, whether as a solid that gets crushed, ground, stored, and conveyed, or in molten form.
Why sulfur gets its own chapter
Sulfur ignites more easily than most combustible dusts, and the standard calls out a few things that make it different:
- Low ignition temperatures. NFPA 660 cites about 374°F (190°C) for a sulfur dust cloud and about 428°F (220°C) for a dust layer. Mixing sulfur with inert solids does not raise its ignition temperature.
- Very low ignition energy. Sulfur dust can be ignited by very small sparks, which makes static control a real concern.
- Hydrogen sulfide. Agitating liquid sulfur releases hydrogen sulfide, which can create a flammable atmosphere and ignites with even less energy than the dust. Molten sulfur should be moved with pumps or inert gas, not compressed air.
- Detection. Infrared and near-infrared detectors do not work on sulfur fires, so detection has to be chosen with that in mind.
- Housekeeping. The standard’s reference depth for a deflagrable sulfur layer is about 1/16 in. (1.7 mm), thinner than for wood.
Your DHA is where these get worked through for your equipment and your layout.
What carried over
- A DHA is required, performed or led by a qualified person. NFPA 660 notes a person can be qualified for one dust type and not another, so ask about real sulfur experience.
- The DHA is reviewed at least every five years and whenever your process changes. See DHA revalidation.
What to do with a DHA written to NFPA 655
NFPA 660 ties DHA reviews to two triggers: at least every five years, and whenever your process changes.
- When your next review is due, by either trigger, write it to NFPA 660.
- Old section references to NFPA 655 will not match NFPA 660, so updating them belongs in that review.
- No DHA yet: start with one written to NFPA 660.
Get a DHA written to NFPA 660
New or an update from NFPA 655. Fill out a 15-minute intake and get a quote built from your own facility.
Keep reading
- Combustible Dust Testing Explained: KSt, Pmax, MEC, and MIE
- Combustible Dust Housekeeping Under NFPA 660
- Is My Dust Combustible? How to Find Out Before Your DHA
Questions about NFPA 655 and NFPA 660
Is NFPA 655 still a separate standard?
No. NFPA 655 was consolidated into NFPA 660. Its sulfur requirements now sit in Chapter 23.
Which chapter of NFPA 660 covers sulfur?
Chapter 23, Sulfur, applied along with the fundamentals in Chapters 1 through 10, including the DHA requirement.
What is the ignition temperature of sulfur dust?
NFPA 660 cites about 374°F (190°C) for a sulfur dust cloud and about 428°F (220°C) for a dust layer.
Why is hydrogen sulfide a concern with sulfur?
Agitating liquid sulfur releases hydrogen sulfide, which can form a flammable atmosphere and ignites with extremely little energy.
Do I need a new DHA because NFPA 655 was replaced?
NFPA 660 ties DHA reviews to two triggers: at least every five years, and whenever your process changes. When your next review is due, write it to NFPA 660 and update the old section references.