Detonation vs Deflagration: What Flame Arrestor Do You Need?
The single most consequential decision in flame arrestor selection is whether the protected system can experience a deflagration or a detonation. This article explains the detonation vs deflagration distinction, why flame speed and pressure rise differ so dramatically between the two, and how that difference dictates which flame arrestor — and which certification — is required.
Getting this wrong is not a matter of marginal performance. A deflagration-rated arrestor installed where a detonation can occur will simply fail, and the flame will propagate into the protected volume.
Deflagration: Subsonic Combustion
A deflagration is a combustion wave that propagates at subsonic speed relative to the unburned gas ahead of it. The flame is driven by heat transfer and molecular diffusion, not by a shock wave. Typical characteristics:
- Flame speed: a few metres per second (often 0.5–3 m/s in open systems, higher in pipes with turbulence).
- Pressure rise: modest — typically a few bar in a closed vessel, less in partially open systems.
- Propagation: the unburned gas is heated and compressed ahead of the flame, but not shock-compressed.
An ignition at the open end of a storage tank vent, or a flash inside a short pipe, produces a deflagration. This is the condition an end-of-line deflagration flame arrestor is designed and certified for.
Detonation: Supersonic Combustion with a Shock Wave
A detonation is a combustion wave coupled to a leading shock wave, propagating at supersonic speed. The shock compresses and ignites the gas simultaneously, so the flame travels faster than the speed of sound in the unburned mixture. Typical characteristics:
| Property | Deflagration | Detonation |
|---|---|---|
| Propagation regime | Subsonic | Supersonic |
| Flame speed | ~0.5–3 m/s (up to tens with turbulence) | ~1,500–2,500 m/s |
| Pressure rise | Modest (few bar) | Up to 20–30× initial pressure |
| Driving mechanism | Heat transfer / diffusion | Shock compression |
| Condition | Open ends, short paths | Confined, long paths with obstacles |
The pressure spike of a detonation is the key number. A detonation can generate a pressure wave up to 20–30 times the initial pressure — far beyond what a deflagration arrestor element is built to withstand. A deflagration arrestor subjected to a detonation will be mechanically disrupted and flame will pass through.
How a Deflagration Becomes a Detonation
In a pipeline or confined system, a flame that starts as a deflagration can accelerate. As it travels, turbulence generated by pipe walls and especially by obstructions (valves, bends, elbows) increases the burning rate, and a “pre-compression” effect builds the pressure ahead of the flame. Beyond a critical distance — the run-up length — the flame deflagration-to-detonation transition (DDT) occurs and the wave becomes a detonation.
This is why the deciding question is often distance. In a short, open vent, a deflagration is the realistic worst case. In a long pipeline — vapor recovery lines, flare headers, long transfer lines — the flame has room to accelerate, and a detonation arrestor is required.
Flame Arrestor Types and Where Each Applies
| Arrestor Type | Protects Against | Typical Application |
|---|---|---|
| End-of-line deflagration arrestor | External ignition at open end | Tank vent pipes, tank vent openings |
| In-line (pre-volume) deflagration arrestor | Deflagration in short pipe runs | Short interconnecting pipework |
| Detonation arrestor | Detonation (and deflagration) | Long pipelines, vapor recovery, flare lines |
| Combination PVRV + arrestor | Deflagration at tank vent | Tanks storing flammable liquids |
Certification: ISO 16852, ATEX, and the Service Condition
ISO 16852 defines test methods for flame arrestors against both deflagration and detonation, and a device is certified for the specific condition it passes. Two rules follow:
- A deflagration-certified arrestor must not be used where a detonation could occur.
- A detonation-certified arrestor is generally more robust and can often serve a deflagration duty (subject to capacity and pressure-drop checks), but the reverse is never true.
For EU installations, ATEX certification (2014/34/EU) is required in addition to the ISO 16852 type test. When in doubt about whether a system can develop a detonation, the safe engineering choice is the higher-rated device — but only after confirming that its pressure drop still allows the vent path to pass the required flow within the tank or system design pressure.
A Practical Selection Rule
Use this decision logic as a starting point, then confirm against ISO 16852 and the specific installation:
- Open tank vent, ignition at the open end: end-of-line deflagration arrestor.
- Short pipe between two points, no significant obstructions: in-line deflagration arrestor.
- Long pipe, or any pipe where a flame can accelerate (vapor recovery, flare, long transfer): detonation arrestor.
- Confined volume with ignition source and expected turbulence: detonation arrestor.
Wanan Technology supplies end-of-line and in-line deflagration arrestors and detonation arrestors certified to ISO 16852 and ATEX, and can advise on the correct type for your application. See the flame arrestor range or talk to our engineers.
Frequently Asked Questions
What is the difference between deflagration and detonation?
Deflagration is subsonic flame propagation driven by heat transfer, with typical flame speeds of a few metres per second and modest pressure rise. Detonation is supersonic propagation driven by a shock wave, with flame speeds of 1,500 to 2,500 m/s and pressure spikes up to 20 to 30 times the initial pressure. The two require different flame arrestor designs.
Which flame arrestor do I need for a storage tank vent?
For a storage tank vent (an open end), an end-of-line deflagration flame arrestor is typically sufficient, since an accidental ignition at the open end produces a deflagration. Detonation arrestors are required in pipelines where a flame can accelerate over distance into a detonation.
When is a detonation arrestor required?
A detonation (or in-line) flame arrestor is required in long pipelines and confined systems where a deflagration can accelerate to a detonation due to distance, obstructions, or pre-compression. Detonation arrestors are common on vapor recovery and flare lines, where flame run-up over a long pipe can cause a deflagration-to-detonation transition.
Does ISO 16852 cover both deflagration and detonation?
Yes. ISO 16852 defines test procedures for flame arrestors against both deflagration and detonation, and a device is certified for the specific condition it was tested for. A deflagration-certified arrestor must not be used where a detonation could occur, and vice versa. Always confirm the certification covers the actual service condition.
Can a detonation arrestor be used for a deflagration service?
A detonation-certified arrestor is generally more robust and can often be used in deflagration service, subject to the pressure-drop and capacity checks, but the reverse is not true. A deflagration arrestor must never be used where a detonation could occur. Selecting the higher-rated device where there is any doubt is the safer engineering choice.
Need help specifying equipment for your facility? Wanan Technology is an ASME U-Stamp certified manufacturer of sampling systems and tank safety equipment, supplying refineries, chemical plants, and storage terminals in 40+ countries including Sinopec, PetroChina, ExxonMobil, Lukoil, and BASF. Contact our engineering team for a technical review or download the product catalog.
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