Every blast room buyer asks about CFM and cartridge count. Far fewer ask the question that decides whether the collector is legal to run: is the dust you’re collecting combustible? If the answer is yes, the dust collector has to be built and installed to NFPA 652, 68 and 69 — and that changes the equipment, the location and the price. This guide explains what KSt means, which blasting dusts are a problem, and the four protection strategies Kresco builds into its CDC Series blast room dust collectors.
The abrasive is usually not the problem. The part is.
Sand, garnet, glass bead, aluminum oxide and steel grit all have a KSt of zero — they will not explode. What ends up in the dust collector, though, is a mix of abrasive fines and everything you blasted off the part: old paint, powder coat, primer, rust, oil residue, and the base metal itself. Blast aluminum castings, magnesium housings, zinc-coated steel or anything with an organic coating and the collector is now handling a combustible dust.
NFPA 652 puts the burden on the owner to run a Dust Hazard Analysis (DHA), which starts with a lab test of the actual dust from your actual process. The result is a KSt value and a Pmax, and those two numbers drive every protection decision downstream.
What KSt is
KSt (bar·m/s) is the deflagration index: how fast pressure rises when a dust cloud ignites in a standard test vessel. It is calculated from the thermal expansion of the gas (K), the speed of the flame front (S) and the temperature (T). Pmax is the maximum pressure the explosion reaches. Together they put the dust in one of four classes:
| Class | KSt (bar·m/s) | Pmax (bar) | Meaning |
|---|---|---|---|
| ST-0 | 0 | — | Not explosive |
| ST-1 | 1–200 | ~10 | Weak to moderate explosion |
| ST-2 | 201–300 | ~10 | Strong explosion |
| ST-3 | > 300 | ~12 | Very strong explosion |
The explosive particle size range is roughly 10 to 95 microns — exactly the fraction a cartridge dust collector is designed to capture and concentrate.
KSt values of dusts you’ll meet in a blast shop
Reference values from Kresco’s combustible dust guide, highest to lowest. Use them to decide whether to test, never in place of a test — particle size, moisture and mixture change the number.
| Dust | KSt | Class | Where it shows up in blasting |
|---|---|---|---|
| Magnesium | 508 | ST-3 | Mag castings, aerospace and motorsport housings |
| Aluminum | 415 | ST-3 | Any aluminum part, wheels, extrusions, castings |
| Powdered dye | 364 | ST-3 | Pigmented coatings |
| Phenolic resin | 269 | ST-2 | Composite tooling, laminates |
| Fiberglass | 216 | ST-2 | Boat hulls, FRP panels, wind blades |
| Wood | 205 | ST-2 | Timber-frame and log-home blasting, wood floor refinishing |
| Printer toner | 196 | ST-1 | Reference only |
| Corn starch | 163 | ST-1 | Starch and soda media (soft blasting) |
| Sugar | 154 | ST-1 | Reference only |
| Soy | 125 | ST-1 | Agricultural media |
| Wheat | 112 | ST-1 | Agricultural media |
| Oat | 87 | ST-1 | Agricultural media |
| Corn | 75 | ST-1 | Corn cob media — common in blast cabinets |
| Peat | 67 | ST-1 | Reference only |
| PVC | 46 | ST-1 | Plastic media, vinyl coatings |
| Charcoal | 10 | ST-1 | Reference only |
| Sand | 0 | ST-0 | Silica, garnet, slag, steel abrasives |
Two takeaways for blast rooms. First, aluminum is the one that catches shops out: an ST-3 dust from the most common non-ferrous part in any job shop. Second, corn cob and walnut shell media are themselves ST-1 — the abrasive is the fuel.
Where sparks come from
A dust explosion needs fuel, oxygen, confinement, dispersion and ignition. A dust collector supplies the first four by design. Ignition in a blast room typically comes from steel grit striking steel (sparks are visible in any grit room), static discharge on ungrounded hose or ductwork, a hot bearing on the blower, or a smoldering ember carried in from the room. Protection is about isolating and venting the collector so an ignition stays an event inside the collector rather than a fireball back in the building.
Four protection strategies (NFPA 68 / 69)
Which one applies depends on where the collector sits. Kresco builds all four.
1. Collector outside in a clear safety zone
- Standards: NFPA 652; NFPA 68 (venting); NFPA 69 12.2.4.4.1 (rotary airlock)
- Isolation on the inlet duct (passive flap valve or high-speed isolation valve)
- Explosion relief panels on the unit directed toward an unoccupied area
- Discharge to a safe, unoccupied location, or isolation on any return-air duct
- Close-clearance rotary airlock on the hopper discharge
2. Collector inside, vented outside through the wall
- Standards: NFPA 652; NFPA 68 6.8 (venting through discharge ducts); NFPA 69 12.2.4.4.1
- Same inlet isolation and rotary airlock
- Explosion vent mounted near an exterior wall and ducted through it with a properly sized relief duct
3. Collector inside with flameless venting
- Standards: NFPA 652; NFPA 68 6.9 (venting with flame arresting and particulate retention); NFPA 69 12.2.4.4.1
- Explosion vent discharges into a flameless vent (flame-arresting device) so the unit can sit on the shop floor
- Inlet isolation and rotary airlock as above
4. Collector inside with chemical suppression
- Standards: NFPA 652; NFPA 69 Chapter 10 (deflagration control by suppression); NFPA 69 12.2.4.4.1
- Chemical suppression system that detects the pressure rise and extinguishes the deflagration inside a unit built to withstand it
- Inlet isolation and rotary airlock as above
The accessory list, translated
- Explosion relief vent panels — calibrated burst panels that open at a set pressure and direct the flame front outward.
- Flameless vents — a relief vent with a flame-arresting mesh so it can discharge indoors.
- Passive isolation valve (ATEX 95 certified) — a flap in the inlet duct that slams shut on the pressure wave.
- High-speed isolation valves — actively triggered knife or pinch valves on the inlet.
- Blow-back damper — seals the intake duct so smoke, flame or pressure can’t travel back to the room.
- Abort damper — tied to a certified spark detector; redirects exhaust air to atmosphere the moment a spark is seen downstream.
- Spark detector + fire-extinguishing system — temperature sensors trigger a sprinkler inside the collector and shut the blower down to starve the fire of air.
- Rotary airlock — keeps the air seal on the hopper while metering dust out; counts as isolation on the discharge under NFPA 69.
- Broken-bag (broken-cartridge) detector, high-temperature probe and alarm, explosion-proof motor, grounding, silencer, bin/drum kit.
Inside the collector, Kresco’s optional nanofiber MERV 15 cartridges are flame-retardant media, and the DB6 filter bags used on baghouse units are knitted with conductive carbon fiber to bleed static to ground — both reduce the chance of ignition in the first place.
What this means when you’re buying a blast room
- Tell your supplier what you blast, not just the room size. “Steel weldments, mill scale, steel grit” and “aluminum castings, glass bead” are two different collectors.
- Get the dust tested if there is any non-ferrous metal or coating in the mix. A DHA is required under NFPA 652 for existing facilities as well as new ones.
- Decide the collector location early. Outside in a clear zone is the simplest and cheapest to protect; indoors means flameless vents or suppression.
- Budget for it. NFPA accessories are real money on a 20,000 CFM collector, and your insurer and AHJ will ask for them.
We size and spec Kresco blast rooms and dust collectors for U.S. customers — send room dimensions, parts and abrasive to Solvent Recycling Systems or call (847) 833-7010. Related reading: blast rooms and sandblast booths, blast media chart, and the blast room preventive maintenance checklist.
Source material: Kresco “Understanding Combustible Dusts and KSt Values” and “Dust Collector Explosion Protection” technical bulletins. This article is general guidance, not an engineering assessment; your DHA and local authority having jurisdiction govern.
