Flame Arrester

What is a Flame Arrester

A flame arrester, also written flame arrestor and historically called a flame trap, is a device or form of construction that allows free passage of gas but interrupts or prevents the passage of flame. It is a passive, no-moving-parts safety component: there is nothing to actuate, no signal to wait for, and no power required. Its job is to sit in a gas path and guarantee that if a flame ever reaches it from one side, that flame cannot continue to the other side where a flammable atmosphere, often a storage tank full of vapor, is waiting to be ignited.

The physics is heat transfer, not mechanical blockage. The arrester contains an element, a tightly packed bundle of very narrow parallel channels, most commonly formed by winding a crimped metal ribbon against a flat ribbon so that the corrugations create thousands of small triangular passages. When a flame front enters these passages it is split into many thin flamelets, each in close contact with a large area of metal. The metal conducts heat away faster than the combustion reaction can replace it, so the burning gas is cooled below its auto-ignition temperature and the flame extinguishes. Gas continues to flow, but flame does not.

The engineering history runs directly back to Sir Humphry Davy. Asked in 1815 to solve the firedamp explosions killing miners in the coal pits of north-east England, Davy found that a fine metal wire gauze would cool a flame attempting to pass through it enough to prevent ignition of the explosive mixture outside. His safety lamp, first trialled at Hebburn Colliery in January 1816, enclosed its flame in a wire-gauze chimney and is the first practical flame arrester. Every modern arrester element is an industrial refinement of that single observation: a flame in contact with enough cold metal area will quench.

The reason a separate device is needed is that flammable vapor systems cannot be kept flame-free by procedure alone. Storage tanks breathe in and out as they fill, empty, and respond to day-night temperature swings, and that breathing vent is open to the atmosphere where lightning, static, a nearby fire, or a tool spark can present an ignition source. Vapor recovery lines, flare headers, and inert-gas blanketing systems all connect multiple vessels through piping that can carry a flame from one to another. The arrester is a passive last line of defense that works alongside active safeguards such as a combustible gas detector, making an ignition event local rather than catastrophic.

Four engineering questions decide whether a given arrester is the right one: which gas it must stop (its explosion group and MESG), what kind of flame event it must survive (deflagration or detonation), how long a stabilized flame might sit on it (short-time or endurance burning), and how much pressure drop the system can tolerate at full vent flow. The rest of this guide works through each of these in turn, because an arrester that is wrong on any one of them is not a partial solution; it is a device that will let the flame through on the day it matters.

Arrester Types and Placement

Flame arresters are classified on two independent axes: the type of flame event they are tested to stop, and where in the system they are installed. Getting either axis wrong is the most common and most dangerous selection error, because the device will pass its visual inspection and bench test yet fail to stop a real flame. The table below summarizes the four primary categories before each is explained.

Deflagration is an explosion that propagates at subsonic velocity, roughly 1 m/s to 1,000 m/s, generating moderate overpressure from a few millibars up to several bar. Detonation propagates supersonically, above 1,000 m/s and up to about 2,000 m/s, and is coupled to a shock wave that produces extremely high local pressure, several hundred bar in a confined pipe. The distinction is not academic: a deflagration arrester quenches a relatively gentle flame, whereas a detonation arrester must also survive the mechanical impulse of the shock wave, which is why detonation units have heavier housings and an integral shock absorber upstream of the element.

The link between the two is run-up distance. A deflagration ignited in open or short piping stays a deflagration, but the same deflagration ignited in a long run of confined pipe accelerates as it travels, transitioning through an overdriven, unstable detonation into a stable detonation. In propane-air mixtures a stable detonation reaches on the order of 1,800 m/s (about 5,900 ft/s) with peak pressures in the hundreds of psi. The practical consequence: if there is meaningful straight pipe between the possible ignition point and the arrester, a deflagration-only arrester is unsafe and a detonation arrester is required. An unstable detonation, occurring right at the deflagration-to-detonation transition, is actually the worst case, with the highest transient pressure, so a true detonation arrester is qualified for both stable and unstable detonations.

End-of-line arresters mount on the open vent flange of a tank or vessel and protect against a flame flashing back from the atmosphere into the tank, the classic lightning-strike-on-a-vent scenario. They face an unconfined deflagration. In-line arresters install within piping; a deflagration in-line unit is valid only up to a manufacturer-specified maximum length of unprotected pipe downstream, beyond which the flame could accelerate to detonation before reaching it. A detonation arrester can be installed anywhere in flammable vapor piping because it is qualified for the fully developed event. Hydraulic arresters use a liquid seal rather than a metal element to break the gas path, and suit dirty or polymerizing vapors that would foul a crimped-ribbon element, such as digester and landfill gas.

Explosion Groups and the Quenching Gap

The single most important number in flame arrester selection is the Maximum Experimental Safe Gap, or MESG. It is the widest gap between two parallel surfaces through which a flame of a given gas-air mixture will not propagate when ignited under standardized test conditions. MESG is a physical property of each gas, and it sets the upper bound on the quenching gap that an arrester element may have: the channels inside the element must be narrower than the MESG of the worst-case gas so that heat is stripped from the flame before it can re-establish on the protected side.

Gases are grouped by MESG into the IEC and ATEX explosion groups used across all hazardous-area equipment. Group IIA gases have a wide MESG and are the easiest to arrest; Group IIC gases have the narrowest MESG and are the hardest. A critical rule follows from this ranking: an arrester certified for a more difficult group also protects against all easier groups, so a IIC-rated unit is automatically valid for IIB and IIA service, but never the reverse. The table below gives the group boundaries and representative gases with their approximate MESG values.

Group IIA covers propane, methane, most solvents, gasoline vapor, and the alcohols, with MESG above 0.9 mm. These are the easiest flames to quench, and most tank-farm hydrocarbon service falls here. Group IIB, from 0.5 mm to 0.9 mm, includes ethylene, hydrogen sulfide, and town gas; the European standards further subdivide it into IIB1, IIB2, and IIB3 sub-bands so that an arrester can be matched more precisely without paying for full IIC capability. Group IIC, at MESG of 0.5 mm and below, is the most hazardous, covering hydrogen and acetylene, and demands the tightest, most expensive element.

The engineering trap is that the arrester must be chosen for the most aggressive gas the line can ever carry, not the gas present during normal operation. A line normally handling propane (IIA) but cross-connected to a hydrogen header during regeneration must use a IIC arrester. Specifying to the routine gas leaves a latent failure: the device looks correct, passes inspection, and lets the flame straight through the first time the harder gas appears. Note also that MESG shifts with temperature and pressure, and the standard tests assume roughly atmospheric conditions, 80 kPa to 160 kPa and minus 20 degrees C to plus 150 degrees C under EN ISO 16852, so elevated-temperature or enriched-oxygen service requires explicit re-qualification rather than the catalog rating.

Element and Housing Materials

A flame arrester is two materials problems in one device. The arresting element must resist the process vapor and any condensate while conducting heat well enough to quench the flame, and the housing must contain the explosion or detonation pressure without distorting the calibrated gaps inside. The two are specified independently, because the corrosion duty on the wetted element and the mechanical duty on the pressure-bearing housing are different.

316 and 316L stainless steel is the default element material. It pairs adequate thermal conductivity with broad corrosion resistance against water, steam, hydrocarbons, and most organic solvents, and it can be drawn and crimped into the thin ribbon that forms a precise, reproducible quenching gap. The low-carbon 316L grade is preferred where the element is welded into a cartridge, because it resists the intergranular corrosion that can attack sensitized weld zones. For the large majority of tank-farm and refinery vapor service, a 316L element is the correct and economical choice.

Hastelloy C and Alloy 20 elements, both nickel alloys, are specified when the vapor or its condensate is chloride-bearing or strongly acidic, conditions that pit and crack stainless steel. Hastelloy C-276 resists wet chlorine, hydrochloric acid carryover, and ferric chloride, while Alloy 20 targets sulfuric acid service. These alloys cost several times more than stainless and are harder to fabricate, so they are reserved for the genuinely corrosive streams in chemical and pharmaceutical plants rather than applied as a blanket upgrade. Aluminium elements appear in weight-sensitive or marine duties where the vapor is benign.

Housings follow the pressure duty. Carbon steel, ductile iron, and cast aluminium serve general deflagration service; 304 and 316 stainless are used outdoors and in corrosive atmospheres; and Duplex stainless, Hastelloy C, or Alloy 20 housings are reserved for severe combined corrosion-and-pressure duty. A detonation arrester housing additionally carries the shock load, so it is heavier and includes a shock-absorber section. The table below maps common vapor streams to a sensible element and housing starting point; always confirm against the manufacturer's corrosion chart for the specific concentration, temperature, and condensate chemistry before ordering.

Key Specification Parameters

A flame arrester data sheet looks short next to an instrument data sheet, but every line on it is load-bearing. Seven parameters drive the selection decision: certified explosion group, flame-event rating, burning rating, nominal size and connection, pressure drop at rated flow, temperature and pressure window, and certification scope. Each is explained below.

Certified explosion group states the hardest gas group the unit is tested for, IIA, IIB with its sub-bands, or IIC. This must be at least as severe as the worst-case gas in service, per Chapter 3. Flame-event rating states whether the unit is qualified for deflagration only, or for detonation including both stable and unstable detonations. A deflagration-only unit on a line that can detonate is a latent failure regardless of its group rating.

Burning rating describes how long a stabilized flame can sit on the element after ignition without breaking through. A short-time burning rating qualifies the unit only for a brief tested interval, so it must be fitted with one or more temperature sensors on the unprotected side of the element that trip an emergency shutdown valve when the metal heats up. An endurance burning rating, tested under EN ISO 16852 for up to two hours, qualifies the element to hold a continuous flame for that period without instrumentation. Endurance units cost more but suit unattended vents where a sensor-and-shutdown loop is impractical.

Pressure drop at rated flow is where venting capacity and flame safety collide. The element is a flow restriction, and it narrows further as it fouls or ices, so the drop at the maximum required vent flow must stay inside the tank or relief-device allowance, often a small fraction of the set pressure of the companion safety relief valve. Manufacturers publish flow-versus-pressure-drop curves and sizing software; crimped-ribbon and large-crimp elements are chosen precisely because they give high flow at low drop. Undersizing chokes emergency venting and can over-pressure the vessel, the same failure a fouled element causes.

Temperature and pressure window bounds the rating: EN ISO 16852 type tests assume 80 kPa to 160 kPa and minus 20 degrees C to plus 150 degrees C, and many arresters are rated for an operating gas temperature only up to about 60 degrees C in standard form, with high-temperature variants extending to 150 degrees C or 220 degrees C. Service outside the tested window invalidates the certificate. Certification scope lists the standards and approval marks the unit carries, summarized below.

• EN ISO 16852: the international and European performance standard for flame arresters, defining test methods and limits for use; the reference document for ATEX-scope devices.
• ATEX 2014/34/EU: the EU directive making the arrester an approved protective system for explosive atmospheres, with the CE mark and type-examination certificate.
• UL 525: the North American standard for end-of-line and detonation flame arresters, with deflagration and detonation parts.
• USCG / FM: US Coast Guard approval for marine and tank vent service, and FM Approvals for insurance-driven scope, common on North American specifications.
• IEC 60079: the explosive-atmospheres series that defines the gas explosion groups (IIA, IIB, IIC) the arrester rating maps onto.

One subtlety underlies all of these: an arrester certificate is valid only for the installation geometry it was tested in. The certified maximum unprotected pipe length, the allowed orientation, and the connection sizes are part of the rating. Re-piping around an arrester, or relocating it further from the ignition source than the certificate allows, silently voids the protection.

Selection Decision Factors

To turn the preceding five chapters into a specific model, follow the decision sequence below. Most selection failures are not single wrong steps; they come from deciding the size or price before the safety parameters are pinned down. These eight steps form a reusable RFQ template.

1. Worst-case gas and explosion group: List every flammable gas the line can ever carry, including cross-connections and upset cases, and pick the smallest MESG among them. Specify the arrester to that group, IIA, IIB sub-band, or IIC, never to the routine gas alone.
2. Flame event, deflagration or detonation: Map the upstream piping. Open or short vents take a deflagration arrester; any long confined run that lets a flame accelerate takes a detonation arrester qualified for stable and unstable detonations. Confirm the certified maximum unprotected pipe length.
3. Placement, end-of-line or in-line: Decide whether the threat is an external flame entering an open vent (end-of-line) or an internal flame travelling through connected piping (in-line). This sets the body style and connection geometry.
4. Burning rating and instrumentation: Choose short-time burning with temperature sensors and an emergency-shutdown valve for attended, monitored lines, or endurance burning (up to two hours per EN ISO 16852) for unattended vents where instrumentation is impractical.
5. Element and housing materials: Select 316L stainless for general service, Hastelloy C or Alloy 20 for chloride or acid vapors, or a hydraulic liquid seal for dirty polymerizing gas, per Chapter 4. Match the housing to corrosion plus pressure duty.
6. Size and pressure drop: Size the element from the maximum required vent flow so that pressure drop stays within the tank or relief-device allowance, using the manufacturer flow curves. Verify the drop with the element partially fouled, not just clean.
7. Connections and temperature window: Specify flange standard and rating (for example DN80 PN16 or ANSI 3 inch 150#), confirm the gas and ambient temperature stay inside the tested window, and select a high-temperature variant where needed.
8. Certification scope: Require the certificate that lists the certified group, flame-event and burning ratings, validated installation geometry, and the marks the project needs (EN ISO 16852 plus ATEX, UL 525, USCG, or FM).

One last dimension is routinely overlooked: serviceability and inspection. A flame arrester is only protective if its element is clean and intact, yet elements foul with dirt, insects, polymer, ice, and corrosion product, which both chokes venting and can degrade the quenching gap. Specify a design that allows the element to be removed, inspected, and cleaned or replaced without cutting the line, plan a scheduled inspection interval, and keep spare cartridges on hand. PROTEGO (FLAMEFILTER element, DR and DA series), Emerson Enardo (DFA series), Elmac Technologies (SGE-IB and DFB series), Cashco, Protectoseal, and Shand and Jurs all offer type-tested ranges with serviceable cartridge designs, making them reliable starting points for a specification.