Introduction to Flame Arrestor

A Flame Arrestor is a device fitted to the opening of an enclosure or to the connecting pipe work in a system of enclosures and which allows gases, liquids, etc. to pass through it but prevents the transmission of a flame in order to prevent a larger fire or explosion. In process operations with combustible gases, flame arrestors help in mitigating the risk of spreading of fire and thus limit the impact of an explosive event. When used properly, flame arrestors can prevent catastrophic damage and loss of life. Anyone involved in selection and purchase of flame arrestors needs to understand how these products work, their advantages and their performance limitations. In this article we will try to cover the basics of the technology and terminology of flame arrestors and the types available.

A flame arrestor is also known as arrestor, deflagration arrestor and  flame trap.

Why Use a Flame Arrestor?

One of the greatest dangers involved with the transport or storage of flammable liquids or gases is that ignition of the flammable vapor may occur, resulting in fire or worse, an explosion. Whenever a flammable gas or vapour
is mixed with air/oxygen, there is the potential for an explosion. Accidental ignition of the flammable mixture will result in a flame that will travel through the unburnt mixture until the fuel is consumed by the reaction. In an enclosed space, such as a vessel or a pipe, the significant temperature increase of the mixture caused by the
combustion process will lead to a rapid increase in the volume of the gas mixture. The resulting increase in pressure will induce turbulent effects which will further accelerate the flame front. Failure to stop a flame can result in catastrophic damage to equipment, loss of production, injury to people and even loss of life and potentially large litigation costs.

Flame Generation and Types

If any flammable mixture of vapor or gas comes in contact with an ignition source, a flame front will develop. This flame will burn through the vapor or gas until:

1. The supply of fuel (vapor or gas) is consumed.
2. The heat necessary to sustain combustion is removed.
3. The oxygen concentration becomes either too high or too low to allow continued burning.

Deflagration

If a flame front is propagating at a speed less than the speed of sound in the vapor, it is known as deflagration. This is further classified in two types.

• Unconfined Deflagration

An unconfined deflagration occurs when there is an ignition of a flammable atmosphere outside a container or other process equipment. For example, a breathing or ventilation outlet from a tank storing gasoline may produce an unconfined cloud of flammable vapour in its immediate vicinity. Ignition sources such as a lit cigarette, a static electrical discharge or a lightning strike could ignite this vapour cloud and the resulting flame front may enter
the tank through the outlet.

• Confined Deflagration

A confined deflagration occurs when there is an ignition of a flammable atmosphere inside a pipeline, container or other process equipment. Typically this could occur in industrial or process plant. For example, many coal mines generate flammable and poisonous methane gas below ground which is pumped to the surface along a pipe and then burnt in a boiler for heating purposes. Problems with the boiler or the pumping system could ignite the pipe contents and the flame could travel back down the pipe resulting in an explosion below ground.

Detonation

A detonation occurs where a flame travels along a pipe, usually at supersonic velocities and is combined with a shock wave. Typically this occurs as a result of turbulence-induced flame acceleration caused by roughness in the pipe walls or interruptions such as bends, valves or changes in section of the pipe. It can also occur simply by allowing the flame to continue to accelerate along a pipe for a sufficient distance. A shock wave is characterized
by a step change in pressure and density through which the flame velocity changes from being subsonic to supersonic.

• Overdriven Detonation

If a flame front is propagating at a speed in excess of the speed of sound in the vapor, it is known as overdriven detonation. Overdriven detonation is a short lived phenomenon and usually occurs as the flame front is transitioning from a high speed deflagration to a detonation.

Flame Arrestors Working Principle

Flame arrestors operate on the principle of removing heat from the flame as it attempts to travel through narrow passages with walls of metal or other heat-conductive material.

Flame arrestors are passive mechanical devices that are mounted to a tank or in a process piping system. In normal operation, vapor mixture in the pipe are directed through the flame arrestor. A flame arrestor mainly consists of a housing, an element and connections to secure it to pipe work or equipment. The element is the device that quenches the flame and mainly is a form of “filter” that provides small apertures through which the process gas will flow but will prevent flame transmission. The flame front is broken down in the “filter” into smaller flamelets which are cooled by the large heat capacity of the element thus extinguishing the flame.

Materials used for the “filter” element include crimped metal ribbons, woven wire gauze, sintered materials and honey comb materials. Because of its construction, the element will cause a pressure drop or an obstruction to process flow. In order to mitigate this increased resistance to flow, the element area is usually larger than the cross sectional area of the pipe work. Larger elements also have a greater heat capacity.

Types of Flame Arrestors

All flame arrestors are designed to allow gases or liquids to pass through while preventing flames or sparks from creating an explosion or expanding into a larger fire. However, their range of style and size varies tremendously to fit each application.

End-of-Line Flame Arrestor

End-of-line flame arrestors are fitted to the end of a pipe line or exit to a vessel to prevent flames from entering, and not, as is sometimes believed, to prevent the flame exiting the pipe or vessel. Without a weather-hood, they may be mounted in almost any orientation, but inverted mounting is not recommended as this increases the risk of heat being trapped thus causing a flash back. With a weather-hood incorporated, they should be fitted in a conventional vertical orientation and be used outside exposed to rain and snow.

In-Line Flame Arrestor

In-line flame arrestors are fitted in piping systems to protect downstream equipment. The layout shown below is typical although it is also possible that the source of ignition could cause the flame to travel with the gas flow. If the flame could come from either direction then a bi-directional flame arrestor is required. In-line flame arrestors can be either deflagration or detonation arrestors depending on the conditions under which they are to be used. Pipe orientation is usually not a problem unless liquid is entrained in the gas flow and would tend to collect in the arrestor. In such situations, an eccentric flame arrestor housing may be fitted to allow collection and drainage of the liquid.

Pre-Volume Flame Arrestor

These are so called because they are designed to protect systems in which a flame may start within a container whose cross sectional area is somewhat larger than the flame arrestor element or the vent pipe and the desire is to prevent the flame leaving the container. They may be simply an element, an end-of-line arrestor or an in-line arrestor. Extreme care must be taken when considering such a situation as it is not possible to predict the conditions that the flame arrestor will have to handle because the volume of hot gases passing through the arrestor will exceed the volumes produced for conventional in-line arrestor flame testing. Although the conditions will tend to produce a confined deflagration it is possible that an arrestor that has been satisfactorily tested under confined deflagration arrestor conditions laid down in a product standard will not be satisfactory. Therefore, the only solution to ensure total confidence in the product specified is to test it under actual or simulated operational conditions.

Hydraulic Flame Arrestor

Liquid product flame arrestors trap some of the liquid flowing in a pipe so that the gases may bubble through it but any flame is extinguished. Hydraulic arrestors contain water whose level is automatically maintained. Similarly gases may bubble through it but any flame would be extinguished. This technique is particularly suited to a dirty gas flow with particulate matter entrained within it.