Flame arrester is a device used to prevent the propagation of flame fronts in process piping. Flame arresters block the propagation of flame by quenching the flame passing through it. Failure to stop a flame propagation can result in extensive damage to personnel,equipment, environment and loss of production.
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How Flame Quenching is achieved
The flame quenching is accomplished by breaking the flame in to very small 'flamelets'. The heat of the flame is absorbed by the flame element which has a large surface area and provides immediate heat transfer reducing the temperature of the flame front below the flash point of the vapour.
The second method involves reducing the space between the bank assembly crimps. All gases have a defined gap through which a flame will not pass. This is called MSEG (Maximum Experimental Safe Gap). To quench a flame and prevent its onward movement, the spacing between the crimp assembly is reduced below the MSEG of the gas or vapour being handled.
Types of Flame Arresters
The various types of flame arrestors are:
- End-of-Line (Vent-to-Atmosphere) deflagration type
- In-line deflagration type
- In-line detonation type
End-of-Line deflagration type
A deflagration is an explosion propagating at subsonic velocity. End-of-line flame arrestors are deflagration type flame arrestors. They are used in gases with sub-sonic flame speeds. Most designs use a single element of crimped wound metal ribbon that provides the heat transfer needed to quench the flame.
An end-of-line flame arrestor is used where the potential ignition source is located outside the vessel. End-of-Line arrestors are commonly installed on atmospheric pressure storage tanks, process vessels and transportation containers.
In-line deflagration type
In-line deflagration flame arrestors are designed for confined flame propogation. The distance between potential ignition source and the location of the deflagration flame arrester should not exceed the L/D ratio (pipe length/pipe diameter), for which the device was approved. As per EN ISO 16852 L/D ratio shall be limited to ≤ 50 for deflagration flame arresters of explosion groups IIA and IIB3 (hydrocarbon/air mixtures) and to ≤ 30 for explosion group IIC (hydrogen/air mixtures).
In-line detonation type
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A detonation is an explosion propagating at supersonic velocity. It entails increased compression of the gases by shock waves in front of the flame. Detonations require fuel–oxidant mixtures that are sufficiently reactive for the combustion zone to propagate at supersonic speeds. Detonations in pipes can develop from deflagrations, which after a flame path of about 100 D can undergo deflagration to detonation transition (DDT) and then form an overdriven detonation that eventually ends in a stable detonation. Detonation flame arrestors are used where the L/D ratio exceeds the values stated above or where the location of ignition source is not known.
Inspection and Maintenance of Flame Arrestors
Flame Arrestors have a tendency to clog up over a period of time and require regular inspection and cleaning. There is a need to regularly monitor the pressure drop across the element which will give an indication of the extent of fouling of the flame arrester element and can determine the frequency of cleaning.
Additional resources
Additional information on Flame Arrestors can be found at the following links
Amal Flame Arrestors - Safety Systems UK Ltd.
Elmac Engineering - Flame Arrestors
Applicable Industry Codes and Standards
- BS EN 12874 - Flame arresters. Performance requirements, test methods and limits for use (This code has been withdrawn in Feb 2011).
- BS EN ISO 16852 - Flame arresters. Performance requirements, test methods and limits for use.
- API RP 2028 - Flame Arresters in Piping Systems.
- UL 525 - Standard for Flame Arresters.