DHA Combustible Dust Testing Strategy Based on NFPA 660

Combustible Dust Testing for a DHA Is Not a One-Size-Fits-All Test Package

When a facility begins a Dust Hazard Analysis (DHA), one of the first questions is often: What combustible dust testing do we actually need? The answer is not necessarily to order every available dust test. A more effective approach is to use a staged testing strategy in which each test answers a specific engineering question identified by the DHA.

NFPA 660, Standard for Combustible Dusts and Particulate Solids, establishes the current framework for identifying and evaluating combustible-dust fire, flash-fire, and explosion hazards. The standard consolidates the fundamental combustible-dust concepts previously addressed by NFPA 652 and the legacy commodity-specific dust standards into a unified framework.

For most facilities, an efficient testing strategy follows this logic:

Representative Sample  →  Explosibility Screening  →  Explosion Severity  →  Scenario-Specific Ignition Testing

But there is another important branch:

No-Go Explosibility Result  →  Fire / Combustibility Evaluation

No-Go for dust explosibility does not automatically mean No Hazard.

A dust that does not propagate a dust-cloud deflagration under a screening test can still burn, smolder, ignite on a hot surface, self-heat, or create another significant fire hazard. That distinction is fundamental to developing a defensible DHA.

Step 0 — Make Sure You Are Testing the Right Dust

Before selecting a test, first ask: Does the laboratory sample actually represent the dust that creates the hazard in the process?

This question can be more important than the choice of test itself. Particle size, moisture content, composition, contamination, oxidation state, and process history can significantly affect dust explosibility and ignition sensitivity. A coarse raw-material sample taken from a bag, for example, may not represent the much finer dust collected in a baghouse after grinding, conveying, screening, or attrition.

A sampling strategy should consider representative process locations such as:

• Dust collectors and baghouses
• Cyclones
• Mills and grinders
• Screens
• Conveying systems
• Transfer points
• Hoppers and bins
• Packaging equipment
• Fugitive dust accumulation areas
• Recycle streams
• Fines generated through attrition

Particle-size distribution and moisture content should also be documented when they can materially affect interpretation of the test results.

The objective is not necessarily to create the finest dust physically possible. The objective is to test a sample that represents the credible process condition or credible worst-case condition being evaluated by the DHA.

Step 1 — Determine Whether the Dust Is Explosible: Go/No-Go Screening

When the explosibility of a dust is unknown, an efficient starting point is typically a dust explosibility screening test, commonly referred to as a Go/No-Go test.

The purpose is straightforward: Can a dispersed cloud of this dust propagate a qualifying deflagration under the specified test conditions?

ASTM E1226 is commonly used for combustible-dust explosibility screening. HazBasis uses the pressure response and applicable screening criteria to determine whether the tested sample produces a Go or No-Go result.

If the Result Is “Go”

A Go result indicates that the tested dust is capable of propagating a dust-cloud deflagration under the test conditions. The next questions become:

• How severe could the explosion be?
• How easily can the dust ignite?
• What dust concentration can support an explosion?
• Can hot surfaces ignite the dust?
• Could electrostatic discharges ignite it?
• Can oxygen reduction prevent an explosion?
• What protection systems are required?

These questions lead to additional testing selected around the specific DHA scenario and protection strategy.

If the Result Is “No-Go”

A No-Go result indicates that the tested sample did not produce a qualifying propagating dust-cloud deflagration under the screening conditions. But the assessment should not automatically stop there.

NFPA 660 separately addresses fire-hazard potential as well as explosibility. When the fire or combustibility characteristics of a powder are unknown, a suitable fire-hazard evaluation should be selected based on the material and process conditions.

A No-Go dust may still:

• Ignite and burn as an accumulated layer
• Propagate combustion across accumulated powder
• Smolder
• Produce burning particles
• Ignite from hot equipment
• Self-heat during bulk storage
• Create localized fires in collectors, bins, dryers, or conveyors

This leads to the No-Go fire-hazard branch discussed later in this article.

Step 2 — If the Dust Is Explosible, Determine Explosion Severity: Kst and Pmax

Once explosibility has been established, the next step is often dust explosion severity testing. The principal parameters are:

Pmax — Maximum Explosion Pressure

Pmax represents the maximum pressure generated during standardized dust-cloud deflagration testing.

(dP/dt)max — Maximum Rate of Pressure Rise

This measures how rapidly pressure develops during the explosion.

Kst — Dust Deflagration Index

Kst normalizes the maximum rate of pressure rise to the test vessel volume, allowing the deflagration severity of different dusts to be compared.

ASTM E1226 is commonly used to determine these parameters. Testing across a series of dust concentrations is used to identify the conditions producing the governing Pmax and maximum rate of pressure rise rather than relying on a single arbitrary concentration.

Why Kst and Pmax Matter to a DHA

These values are important inputs when evaluating or designing:

• Explosion venting
• Explosion suppression
• Explosion isolation
• Dust collector protection
• Process enclosure protection
• Silos and bins
• Cyclones
• Mills
• Filters
• Pneumatic conveying systems

For example, determining the required explosion vent area for a dust collector normally requires credible explosion severity data. However, Kst alone does not tell you how easily the dust ignites.

Severity and sensitivity answer different questions. Kst/Pmax describe explosion severity; ignition tests describe how readily a credible ignition source can initiate the event.

Step 3 — Select Ignition-Sensitivity Tests Based on the DHA Scenario

Once a dust has been determined to be explosible, the next stage should generally be scenario-driven testing. Not every project requires every ignition-sensitivity test.

Minimum Ignition Energy — MIE

Question answered: How little spark energy can ignite the dust cloud?

MIE is particularly important when electrostatic charging can occur during:

• Powder transfer
• Pneumatic conveying
• Mixing and blending
• Filling and emptying
• Filtration
• FIBC handling
• Plastic container use
• Flexible hose use
• Personnel interaction with powders

ASTM E2019 is commonly used for MIE testing. MIE data helps the DHA determine whether electrostatic discharges are credible ignition sources and how much emphasis should be placed on bonding, grounding, conductive equipment, personnel grounding, FIBC selection, and other electrostatic controls.

Use MIE when the DHA question is primarily about spark or electrostatic ignition sensitivity.

Minimum Explosible Concentration — MEC

Question answered: What is the lowest tested concentration of suspended dust capable of propagating a deflagration?

MEC is typically reported in mass of dust per unit volume of air. Testing can support evaluations involving dust-cloud concentration, ventilation, enclosure dust loading, pneumatic conveying, process containment, concentration monitoring, and risk assessments involving dispersed dust clouds.

ASTM E1515 or EN 14034-3 is commonly used depending on the project basis. Laboratory MEC should not simply be treated as a universal operating limit because real industrial dust clouds can be heterogeneous and transient. Appropriate uncertainty and engineering margin should be considered.

Minimum Autoignition Temperature of a Dust Cloud — MAIT / MIT Cloud

Question answered: At what hot-environment temperature can a dispersed dust cloud ignite?

This is especially relevant around dryers, ovens, heaters, hot ducts, bearings, motors, process equipment, heated air streams, and other elevated-temperature surfaces or environments. ASTM E1491 is commonly used for dust-cloud autoignition testing.

The result can support the selection and evaluation of equipment temperature limits and the DHA hot-surface ignition assessment.

Minimum Ignition Temperature of a Dust Layer — LIT

Question answered: What hot-surface temperature can ignite the tested dust layer?

A dust layer behaves differently from a suspended dust cloud. Dust accumulated on hot equipment can insulate the surface, retain heat, smolder, and eventually ignite. Relevant locations include motors, bearings, dryer surfaces, ductwork, light fixtures, process vessels, electrical equipment, and other surfaces where dust can accumulate.

ASTM E2021 is commonly used for LIT testing. Layer thickness matters, so the laboratory value should not simply be copied into an equipment specification without considering layer thickness, applicable code criteria, and an appropriate safety margin.

Limiting Oxygen Concentration — LOC

Question answered: Below what oxygen concentration will the tested dust cloud no longer sustain a qualifying deflagration?

LOC becomes particularly important when inerting or oxygen reduction is being considered as the explosion-prevention strategy. Applications can include mills, dryers, reactors, powder-processing vessels, pneumatic conveying systems, closed material-handling systems, and highly reactive powders.

ASTM E2931 or EN 14034-4 is commonly used depending on the project basis. The measured LOC is a laboratory hazard property—not automatically the operating oxygen setpoint. A practical operating limit must incorporate engineering margin and consideration of analyzer accuracy, mixing, process variability, response time, alarms, interlocks, and failure scenarios.

Step 4 — If the Dust Is No-Go, Evaluate the Fire Hazard

No dust explosion does not mean no combustible-dust fire hazard.

NFPA 660 distinguishes fire-hazard potential from dust-cloud explosibility. A particulate solid can fail an explosibility screening test yet still ignite and propagate combustion as accumulated material.

Therefore, when a Go/No-Go screening test produces a No-Go result, the next question should be: Can this material still create a credible fire hazard under the actual process and storage conditions?

Burn Rate / Fire Propagation Testing

One option for evaluating fire/combustibility behavior is UN Test N.1 — Test Method for Readily Combustible Solids, or another appropriate method based on the project objective. This evaluates whether combustion propagates through the material rather than whether a suspended dust cloud explodes.

A powder can therefore be No-Go for dust-cloud explosion but still combustible as an accumulated powder.

What About Self-Heating?

For some processes—especially those involving bulk storage, drying, elevated temperatures, oxidation, reactive powders, biomass, organic powders, or long residence times—another question becomes important: Can the material generate heat internally and undergo self-heating or spontaneous ignition?

Self-heating testing may therefore be appropriate when the DHA identifies credible scenarios involving:

• Silos
• Hoppers
• FIBCs
• Drums
• Bulk piles
• Dryers
• Heated powder
• Long storage periods
• Aerated powder
• Materials susceptible to oxidation or decomposition

Self-heating is not answered by a Go/No-Go dust explosibility test. Likewise, Kst, Pmax, MIE, or MEC does not tell you whether a bulk powder can slowly self-heat. The testing program must follow the physical hazard scenario.

A Practical NFPA 660 Combustible Dust Testing Decision Matrix

DHA Question	Recommended Test	Typical Engineering Use
Can the dust cloud explode?	Go/No-Go Explosibility Screening	Initial hazard identification
How severe can the explosion be?	Kst, Pmax, (dP/dt)max	Venting, suppression, isolation, and enclosure protection
Can a small spark or static discharge ignite it?	MIE	Electrostatic hazard assessment and ignition source control
What dust concentration can support an explosion?	MEC	Concentration, ventilation, and prevention evaluation
Can a hot environment ignite the cloud?	MAIT / MIT Cloud	Hot surface and equipment temperature assessment
Can accumulated dust ignite on a hot surface?	LIT	Equipment-temperature and dust-layer assessment
Can oxygen reduction prevent the explosion?	LOC	Inerting system design
Does a No-Go material still burn?	Fire/Combustibility or UN N.1 Testing	Fire hazard evaluation
Can bulk material self-heat?	Self-Heating Testing	Storage, dryer, silo, FIBC, and bulk powder fire evaluation

Do You Need Every Test Before Performing a DHA?

Usually, no. In many cases the DHA itself helps determine what additional testing is necessary.

Scenario 1 — New Dust Collector

A new dust collector handles a powder with no available explosibility data.

Logical testing focus: Go/No-Go → Kst/Pmax → MIE and/or MAIT/LIT as required by the ignition-source assessment

If explosion protection is required, Kst and Pmax become particularly important.

Scenario 2 — Powder Transfer With Significant Static Charging

The dust is already known to be explosible, but electrostatic charging is a major concern.

Logical testing focus: MIE

The highest-value additional test may be MIE rather than automatically repeating a full suite of dust tests.

Scenario 3 — Nitrogen-Inerted Milling System

The engineering basis depends on preventing ignition through oxygen reduction.

Logical testing focus: LOC

LOC data, together with the process-control basis, can support the inerting strategy.

Scenario 4 — No-Go Dust Stored in Bulk

The material does not propagate a dust-cloud deflagration, but significant quantities are stored in a silo or container.

Logical testing focus: Combustibility + self-heating

The remaining concern may be fire propagation or self-heating rather than Kst or MIE.

This is why combustible dust testing should be tied to the DHA hazard scenario rather than treated as a standard package.

Why Testing the Wrong Sample Can Produce the Wrong DHA

Even the correct ASTM method can produce misleading engineering conclusions if the wrong sample is tested. Dust properties can change substantially with:

• Particle size
• Moisture
• Composition
• Oxidation
• Process temperature
• Contamination
• Supplier formulation
• Milling
• Drying
• Attrition
• Separation

For example, testing coarse pellets from an incoming raw material container may tell you very little about the fine dust collected downstream from a grinder.

A defensible DHA should document what was sampled, where it was collected, how it was prepared, and why that sample represents the hazard being evaluated.

Avoid the “Kst-Only” Approach to Combustible Dust Safety

Kst is probably the most widely recognized combustible dust property. It is also one of the most frequently misused.

A Kst value tells you about the rate of pressure rise and deflagration severity of the tested dust. It does not, by itself, tell you:

• The minimum ignition energy
• The minimum explosible concentration
• The hot-surface ignition temperature
• The dust-layer ignition temperature
• The limiting oxygen concentration
• Whether the material self-heats
• Whether electrostatic discharge is a credible ignition source

The better DHA question is not simply “What is the Kst?” but “What properties do we need to know to evaluate the credible fire and explosion scenarios in this process?”

Recommended Combustible Dust Testing Workflow

1	Understand the material and process
2	Develop a representative sampling strategy
3	Determine dust explosibility — Go/No-Go
4A — If GO	Determine explosion severity — Kst, Pmax, and (dP/dt)max
5A	Select scenario-specific testing: MIE, MEC, MAIT/MIT Cloud, LIT, LOC, or other tests as required by the process
6A	Use the test data in the DHA to evaluate prevention and mitigation safeguards
4B — If NO-GO	Evaluate remaining fire-hazard potential
5B	Select burn-rate / combustion-propagation, layer ignition, self-heating, or other fire testing as appropriate
6B	Document the technical basis and conclusions in the DHA

Combustible Dust Testing Should Support Decisions, Not Just Produce Numbers

The purpose of combustible dust testing is not to build a laboratory report containing as many numbers as possible. The purpose is to provide the data needed to make defensible engineering decisions.

A good testing program should help answer questions such as:

• Does this material present a dust explosion hazard?
• What credible fire hazards remain if it is not explosible?
• Does the dust collector require explosion protection?
• How should venting or suppression be designed?
• Is explosion isolation required?
• Is static electricity a credible ignition source?
• What equipment surface temperatures are acceptable?
• Is inerting technically feasible?
• Are existing safeguards adequate?
• What data should be incorporated into the DHA?

That is why HazBasis approaches combustible dust testing and Dust Hazard Analysis as connected activities rather than independent services. Laboratory data should feed directly into the hazard analysis, and the hazard analysis should define what laboratory data is actually needed.