The flame arrester is a device used in a different piping system to stop and defuse propagating flame fronts.
Also called the Flame Arrestor or just arrester; it simply allows the passage of gas but stops flames to pass through. Thus avoiding risks of major fire explosions.
Discovered in 1815 by Humphry Davy it soon becomes the norm of basic fire safety across major industries.
Used in applications such as fuel pipelines, vents of fuel storage tanks, the air intake of marine engines and the exhaust system of almost all I.C Engine.
Not just it stops the propagation of a flame; but it also stops the spread of an open fire or already occurred explosion.
They are typically used where the risk of a fire or explosion is high, or an existing fuel tank is venting directly to the atmosphere.
When properly applied flame arresters can significantly reduce possible damage to personnel, environment, equipment, and loss of time and money.
Fitted to the opening of an enclosure or pipe connections. Under the normal condition, it allows the passage of gases or vapors to flow but restricts flame or fire if ignition or explosion takes place.
Since changes in the temperature, gas composition and pressure; affects the performance of any flame arrester.
It is always advised to limit the use of flame arrestor to places; for which it is designed and tested for with flame burning in the presence of air and not oxygen.
How Does Flame Arrester Work? – Answered
In simple words, a flame arrester quenches or stop flame by breaking it into different smaller pieces or flamelets.
In the process, these flamelets lose part of their heat and thus cool down. This is done by removing heat by conduction through walls and wire mesh.
Absorbing self-heat from flame fronts it finally able to quench fire; by dropping the burning gas below its auto-ignition temperature.
A flame arrester is composed of a number of flame filters that help quench the fire. These flame filters are nothing but corrugated wire mesh that provides small apertures through which gas mixture can pass through.
Now when the flame front pass-through these apertures; it slows down making it possible for the corrugated wire mesh filters to take away the heat.
You can consider the whole process as of gradual heat absorption; till the flame stops allowing only the gas mixture to pass through.
In other words, here the rate of heat diversion from the medium is more than what been added by the combustion process.
The material used for its construction is stainless steel, ductile iron, stainless steel, PTFE elements, and Hastelloy.
Why Flame Arrestor Is Required In First Place?
All major industries including refining, shipping, petrochemicals, oil exploration; chemical, paper, production, pharmaceutical, and power generation involve storing and handling flammable mixtures.
Now one of the greatest dangers involving storage and transportation; of flammable substances is the risk of explosion or fire.
Accidental ignition of this gas mixture results in an explosion or Deflagration. The term deflagration stands for those explosions that propagate at subsonic velocity.
If not checked this will quickly spread to all connected pipes and systems. This will then lead to a rapid increase in its volume; thus accelerating flame fronts.
Not just it would be hard to control but will burn through the unburnt mixture; moving along till the fuel is fully consumed.
Failure to stop such an event can have turbulent after-effects; leading to loss of production, life, manpower, money and brand image.
The types of flame arrestor used for the purpose depends largely on; kind of explosive atmosphere, high-risk area, what really be to be protected, operating procedure and most important potential ignition sources.
To improve further safety it is advised to only use flame arrester; that is designed and tested for that particular condition.
Different Stages Of Flame Propagation
A flame has different stages of development as it starts and propagates through the pipeline. Studying them helps us find the best arrester type for the purpose.
These stages are quite distinct in terms of velocity and pressure developed. One can understand that by the following example:
“When a flame propagates in gas along the pipeline. The heat of the flame expands the gas upfront. This adds more heat and generates a pulse of pressure; thus speeding the combustion reaction.”
This process then feeds on itself generating more pressure and velocity until it achieves the dynamic state of stability. These stages of flame propagation are listed as following:
- Low-Pressure Deflagration: It is the state just after ignition in which the flame propagates below subsonic speed with minimal pressure increase. This initial state for flame propagation develops within a very short length of pipe. As the flame propagates further its intensity increases with speed reaching up to 200 m/s.
- High-Pressure Deflagration: This is the state found after 6 to 10 meters from the point of ignition. Here the pressure increase significantly but the speed is still subsonic at about 300 m/s. It then lasts for a fair amount of distance followed by transition state.
- The Transition State: When the propagating flame front passes sonic velocity it undergoes a transition state. Here the forward pressure built so far suddenly converts into a shock wave. What really happens is the developed pressure when reaching a limit to 700 kPa; it expands with an explosion when came in contact with the flame front.
- Unstable Detonation: When flame propagates even further down the pipe, it experienced increased compression pressure followed by a shock wave. Here, the propagation speed of the flame front is supersonic. It is only temporary because here velocity and pressure are dependent on the shockwave energy. Now since it dissipates soon the state doesn’t last forever.
- Stable Detonation: Beyond a certain point i.e 40 to 60 meters from ignition source; the shock wave’s effects responsible for the unstable detonation becomes much weaker. Here the flame has finally reached the state of stable detonation. Here the propagating speed is supersonic but with little variation in both volume or pressure.
Flame Arrester vs Detonation Arrester – What’s The Difference?
Flame or detonation arrestors are used in a system to protect against unwanted combustion process.
A combustion process can take place in two main types and their subtypes; stabilized burning and explosion.
Now since arrestor of any kind is used mainly to protect against the chance of explosion. It can further be two subtypes i.e detonation and deflagration.
Here, the term deflagration stands for an explosion that propagates at subsonic speed. If this process occurs in the presence of open-air its called atmospheric deflagration.
Similarly, if it occurs within a confined volume and is initiated by an internal heat source such as I.C engine; its called Prevolume deflagration.
Now if it occurs within a pipeline expands at subsonic speed; its termed as inline deflagration. It is these three types of fires fire arresters are installed for; although scientifically even detonation arrester is a kind of fire arrestor.
On the other hand, detonation is the sudden expansion of gas with an explosion at supersonic speed followed by a shock wave.
It too can be of two types; stable detonation and unstable detonation. A detonation is basically unstable during and just after its transition from deflagration to stable detonation.
It is considered stable when it propagates within a system or pipeline; without much variation in volume or pressure.
It is these two detonation types for which detonation arresters are been used. In short, every detonation arrestor is a kind of fire arrestor but not vice versa. Plus its advantage is that it can be installed anywhere in the system.
Different Types of Flame Arrestor
A flame arrester can be best classified based on its way of installation or application; for example, boats, tanks, vent stacks, etc.
Depending upon installation a flame arrester is of three main types; End of line, In-Line, and Pre-Volume flame arrestors.
1 ) End of Line Type
An end of line flame arrester is of deflagration type; used to protect in Vent-to-Atmosphere settings. Propagating below the subsonic speed exposed to open air.
These types of flame arrestors are mainly used in the venting system of tanks. It allows free venting operation in combination with fire prevention.
Typically fitted to the end of a pipeline it prevents the flame from coming in our exit. Most end line designs use Crimped Metal, perforated plates or wire gauges to quench the flame through quick heat transfer.
Conventionally these arrester types prevent the fuel oil tanks to explode; in the event, its vent is stuck by a lightning strike.
These arrestors can be mounted in any required direction. Although, inverted mounting is not recommended as the risk for blowback.
When exposed to weather such as rain or snow; they must incorporate a weather hood and be mounted vertically.
At last, an end-of-line flame arrestor is preferred over others; when the potential ignition source is outside the tank, pipeline or vessel.
2 ) In-Line Flame Arrester
An in-line flame arrester can be of both deflagration or detonation type. These are fitted in different piping systems to protect subsequent downstream equipment.
In case the risk for an explosion is there from either side; a bidirectional in-line arrester is required to be installed.
Unless liquid is entrained in the gas flow; there is no issue of the orientation of either pipe or the flame arrester. Since these are designed for confined flame propagation at the sub and supersonic speed; it first reduces its speed and then quenches the fire.
Here as both the speed and heat of the flame are high; it either has parallel plates construction or wire gauges in packs for faster heat transfer.
In a typical arrangement, it has an exposed side, protected side and the arrester element to quench the fire.
For this phenomenon, the underlying process can be either of the two; vapor destruction systems or vapor recovery systems.
Now since it can be of either detonation or deflagration type. The selection is done based on the distance between; the arrester and potential source of ignition.
3 ) Pre-Volume Type
When the possible source of ignition is within the tank itself or a pipeline with a larger cross-section area; the type of flame arrester used is called a pre-volume arrester.
Simply these are much better arresters that are used to avoid the spread of fire from one system or tank to another.
Although here the propagation speed is usually below subsonic speed the pressure and heat generated are too much to be handled by a typical deflagration arrester.
Thus special care and consideration need to be needed regarding its design. Making it slightly superior to normal fire arresters.
While in normal inline or end of line arresters they are made to match the expected conditions. A pre volume in-line or end of line arrester needs to be double the strength of expected condition.
What Is A Flame Arrestor On A Boat?
If you have a boat that features an inboard boat motor you already have a flame arrester attached to it.
The role of this arrester is to protect the fuel tank against the engine backfire; thus protecting you and your boat from unwanted fire or explosion.
Fitted on each carburetor it avoids ignition of gasoline vapors from engine backfire. Maybe why its often also termed as backfire flame arrester.
In the U.S it is not just required to have backfire arrester installed in each boat; but mandatory to have one approved by U.S. Coast Guard.
This makes it illegal to have boats with outdated or below standard arresters to be installed on the inboard motor. You can’t even have holes in the mesh or pates.
Making it compulsory to have only “SAE-1928” or “UL 1111” marked arresters. One should regularly clean the mesh; to improve the flow thus improving overall efficiency.
You can use different chemicals such as brake cleaner, K&N kit to clean. Once done make sure it’s tightly fastened to the carburetor.
Choosing An Effective Flame Arrester – Things To Consider
For optimum protection, it is best to buy an appropriate flame arrestor for the purpose. Now to select the appropriate flame arrester; one needs to follow the following instruction from the manufacturer:
- Analysis of gas or vapor: The gas mixture is to be analyzed for their type, molecular weight, and density. This helps select the correct flame arrester design and materials for the purpose.
- Service life: Based on the type of material used and gas mixture the service life of an arrester varies. One needs to take into this consideration before finalizing the buying decision.
- Flowrate: Sufficient information should be provided by the manufacturer to calculate flowrate through arresters in volumetric terms. These can be calculated based on construction type; type of tanks, arrester dimensions, pressure and filling rate for the tank.
- Pressure ranges: The maximum and minimum working pressure range under which it can quickly quench fire under all conditions. It depends largely on having the correct flame arrester element.
- Type: The type of flame arrester required for the purpose ex: inline, end of line or pre volume type depends much on whether the expected fire will be for short-term or in the form of detonation.
- Temperature range: The temperature range for a flame arrestor provides the maximum and minimum temperatures at which it works. This allows for selecting the best design for the operating conditions.
- Orientation & Pressure Drop: It refers to the intended orientation preferred for the arrester type. The pressure drop is information many times used along with the temperature range. This allows us to know the maximum pressure to which if a fire erupts will be easily quenched by the arrester.
- Connection type: Whether its a pipe, machinery or flame arrester it’s important to know of all associated parts; their position and function. Providing this information the manufacturer helps you successfully achieve regular maintenance and housekeeping.
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