Flame Arrestors: Stopping Flame Propagation in Piping

Flame Arrestors: Stopping Flame Propagation in Piping is the study of a passive safety device that allows flammable gas or vapour to pass through a pipe while stopping any flame front from travelling through it, quenching the flame inside narrow passages sized below the gas group maximum experimental safe gap (MESG). The device has no moving parts and consumes no energy. It works purely on heat transfer and geometry, absorbing enough energy from the advancing flame that the reaction cannot sustain itself on the protected side.

The Quenching Principle

A flame propagates because the burning zone heats the unburnt gas ahead of it to its ignition temperature. Force that gas through a bundle of very small channels and two things happen: the large metal surface area drains heat out of the flame faster than the reaction can generate it, and the narrow gap prevents the radical chain reactions from crossing. Below a critical channel width, the flame is extinguished. That critical width tracks the MESG, the largest gap between two flanges that will still prevent an internal explosion from transmitting to the surrounding atmosphere in the standard test apparatus. Elements are engineered with apertures well under the MESG of the gas they must handle, which is why the correct gas group must be known before selection.

Gas Groups and MESG

Gases are ranked by how easily a flame passes a gap. A smaller MESG means the gas transmits a flame through a narrower gap, so hydrogen and acetylene are the hardest to stop and demand the tightest elements. The table below lists representative MESG values and the matching hardware classifications. Numbers vary slightly by reference; use the group, not a single decimal, when specifying.

An arrestor certified for propane (IIA) will not reliably stop a hydrogen (IIC) flame. Matching the element to the process gas group is the single most important selection decision, and it links directly to hazardous area classification under ATEX, IECEx and zones.

Deflagration Versus Detonation

The two combustion regimes place very different demands on the device. A deflagration is a subsonic flame front where pressure and flame travel are loosely coupled and peak pressures are modest. A detonation is supersonic, with the flame locked to a leading shock wave and transient pressures many times higher, especially in the unstable or overdriven state that occurs near the deflagration-to-detonation transition point.

• Deflagration arrestors have a lighter housing and are certified for a limited flame run-up. They must be installed within the tested distance of the ignition source.
• Detonation arrestors use a heavier cage and element assembly to survive the shock, and are qualified for both stable and unstable detonation. They suit long pipe runs where a flame can accelerate.

End-of-Line Versus In-Line

End-of-line arrestors sit at an open vent to atmosphere, such as a storage tank breather, and only face flame from outside. In-line arrestors are flanged into piping and must stop a flame arriving from either direction. Many in-line units carry an endurance burn or short-time burning rating, meaning a stabilised flame can sit on the element for a defined period without breaking through, which matters where re-ignition is likely. These devices work alongside pressure relief hardware such as a rupture disc and pressure and vacuum vents rather than replacing them.

Where They Are Used

• Tank vents: atmospheric and low-pressure storage tanks with flammable contents.
• Vapour recovery: loading racks and terminals returning displaced vapour, a classic detonation-arrestor duty.
• Flare and vent headers: preventing flashback from the flare tip into the collection system.
• Blanketing and inert gas systems: protecting the nitrogen supply from a tank fire.

The Main Maintenance Concern: Fouling

The dominant failure mode is not the explosion event but plugging. The same narrow passages that quench a flame trap polymer, dust, ice, corrosion product and insect debris. Fouling does two dangerous things at once. It restricts flow, which can pull a tank into vacuum or over-pressure because the vent can no longer breathe, and it can degrade the element so that protection is compromised. A blocked arrestor is therefore a double hazard. Sizing must account for the flow demand set by the relief case; see the reasoning in pressure relief valve sizing. Practical upkeep includes:

• Scheduled differential-pressure checks or visual inspection of the element.
• Removal and cleaning on a defined interval, matched to the fouling rate of the service.
• Temperature sensors on burn-rated units to detect a stabilised flame sitting on the element.
• Confirming the element and gaskets are reinstalled with no bypass gap.

Tracking these intervals across many vents is exactly the kind of recurring, safety-critical task a CMMS handles well. Book a Fabrico demo to see how inspection schedules and asset history stay auditable.

Frequently Asked Questions

Can one flame arrestor cover any gas?

No. The element aperture is sized against the gas group MESG. A unit rated for IIA propane will not stop an IIC hydrogen flame, so the process gas group must be confirmed before selection.

What is the difference between a deflagration and a detonation arrestor?

A deflagration arrestor stops a subsonic, lower-pressure flame and must sit close to the ignition source. A detonation arrestor is built to survive the supersonic shock of a fully accelerated flame and suits long pipe runs.

Why is a plugged flame arrestor dangerous rather than just inconvenient?

Plugging blocks normal breathing, which can over-pressure or collapse a tank, and it can mask or degrade the element so that flame protection is no longer assured. It is both a process and a safety hazard.

How often should flame arrestors be inspected?

There is no universal figure. The interval is set by the fouling rate of the service, using differential pressure trends and inspection history, with dirtier or polymerising streams needing far more frequent cleaning.