LFL Archives - Confined Space Entry Training

Blog Post #10 – In this post, we will examine the second category listed in the Federal OSHA definition for a hazardous atmosphere, which is an atmosphere with airborne combustible dust at a concentration that meets or exceeds its LFL (which stands for Lower Flammable Limit). LFL represents the minimum quantity of combustible dust that must be suspended in the air within the space to be able to cause a flash fire or dust explosion if ignited. In these conditions, the airborne dust is considered to have met or exceeded its lower flammable limit, or LFL.

OSHA’s 5-foot Rule of Thumb for Dust Measurement

The LFL of combustible dusts depends on many factors, including the type of material that makes up the dust, the size of the dust particles, the moisture content in the dust particles, and the density of the dust particles within the airspace.

There are instruments available that can measure the amount of combustible dust in the air to determine if it meets or exceeds its Lower Flammable Limit (LFL). However, for airborne dust measurement purposes, OSHA’s definition of a hazardous atmosphere due to airborne combustible dust refers to the use of the “5-foot rule of thumb”. That means that if the combustible dust suspended in the air is so dense that the entrant cannot see more than five feet from the end of their nose, there is a high risk of a flash fire or dust explosion.

In cases where this condition exists, or is likely to exist, inside of a confined space at any time during entry operations, it must be classified as a permit-required confined space.

Examples of Combustible Dusts

Here are examples of some, but not all, of the various types of airborne combustible dusts that could be present inside of some confined spaces during entry operations: Aluminum powder, dust from animal feed, coal dust, corn starch, cosmetic powders, dry fertilizer dust, dust from dry food and spices (coffee, tea, flour . . .), grain dust, powdered metal, plastic dust, powdered chemicals, rubber dust, sugar dust, textile fibers, and wood dust.

How Dust Fires and Explosions Occur

Combustible dust flash fires, a process called deflagration, is a rapid combustion process that produces a flame front which can quickly spread through a dust cloud if the concentration of combustible dust in the air is dense enough. And in some cases where the airborne dust fire is contained within a confined space, pressure quickly builds and an explosion can occur.

Here is a brief overview of the steps leading up to a combustible dust flash fire and explosion inside a confined space:

A cloud of combustible dust is created in the air within a confined space, either through a process like grinding, mixing or other material handling activities, or by a settled dust being disturbed by other work activities such as cleaning with compressed air.

A small portion of dust within the cloud is then ignited by a spark, flame, or other ignition source. Even static electricity must be considered as an ignition source.

If the airborne combustible dust cloud is dense enough, the burning dust particles spread to others nearby, and those in turn ignite other airborne particles, resulting in a fire quickly spreading through the dust cloud. This process is called deflagration.

The rapid spread of burning dust particles not only causes temperature inside the confined space to rapidly rise, it also creates a pressure wave that increases the internal pressure within the confined space.

In the right conditions, the pressure wave can cause an explosion, leading to severe damage to the structure of the confined space.

The burning dust cloud can continue to burn and expand. And the percussion from additional explosions may lead to more combustible dust inside the space being introduced into the air, adding even more fuel to the fire.

When a dust fire or explosion occurs, it can obviously lead not only to structural damage, but possible injury or death to anyone nearby. See an example of a dust explosion in this YouTube video showing a grain elevator explosion.

Classify All Confined Spaces with Potential for Airborne Dust Fires or Explosions as Permit-required Confined Spaces

So, consider the confined spaces present at your workplace, and take care to identify those with an actual or reasonable potential for a dense cloud of airborne combustible dust developing during entry operations, as those spaces will have to be classified as permit-required confined spaces.

Here are examples of some, but not all, types of confined spaces that, depending on materials they have contained, could produce heavy levels of airborne combustible dust during entry operations:

Conveyor systems
Cyclone separators
Duct work and air-handling systems
Dust collectors and bag houses
Grain storage bins, elevators, and silos
Hoppers and chutes
Mixers and blenders
Ovens and furnaces
Powder processing equipment such as mills and pulverizers
Truck and rail-car hoppers and tanks

In our next blog post (#11), we will examine the third criteria of a hazardous atmosphere that could be present inside of a confined space (oxygen deficiency or enrichment). In the meantime, please provide your feedback and questions to this blog post about airborne combustible dust at levels exceeding its lower flammable limit, or LFL, in the comments section below.

And as always, I hope you will take a moment to spread the word about our confined space training blog by share a link to this post with others in your network so they can benefit from this information. Thanks – Curtis

Hazardous atmosphere flammable gas vapor or mist greater than 10% LFL or LEL

Blog Post #9 – In previous blog posts, we discussed how to identify confined spaces, and provided several examples of such spaces. Next we will examine the factors that determine whether or not each confined space identified at your workplace must be classified as a permit-required confined space, starting with a hazardous atmosphere that contains a flammable gas, vapor, or mist in excess of 10% of its lower flammable limit, or LFL.

OSHA Definition of a Permit-required Confined Space

The Federal OSHA definition of a permit-required confined space for general industry can be found in 1910.146, paragraph B, titled definitions, and in 1926.1202, also titled definitions, for the construction industry. That is where OSHA lists the four types of hazards that require a confined space be classified as a permit required confined space. Those four types of hazards are:

The space contains an actual or potential hazardous atmosphere;

The space contains a material with the potential to engulf the entrant;

The space is configured to trap or asphyxiate the entrant; or;

The space contains some other recognized serious safety or health hazard.

What is a Hazardous Atmosphere?

The Federal OSHA definition for “hazardous atmosphere”, the first classification listed above, is also found in 1910.146, paragraph B, definitions, for general industry, and in 1926.1202, definitions, for the construction industry.

There are five different types, or classifications, of hazardous atmospheres, listed in these two OSHA definitions. Those classifications, which are nearly identical in the two standards, are:

(1) – Flammable gas, vapor, or mist in excess of 10 percent of its lower flammable limit (LFL);

(2) – Airborne combustible dust at a concentration that meets or exceeds its LFL;

(3) – Atmospheric oxygen concentration below 19.5 percent or above 23.5 percent;

(4) – Atmospheric concentration of any substance for which a dose or a permissible exposure limit is published in subpart G, Occupational Health and Environmental Control, or in subpart Z, Toxic and Hazardous Substances, of this part and which could result in employee exposure in excess of its dose or permissible exposure limit;

(5) – Any other atmospheric condition that is immediately dangerous to life or health.

The balance of this particular blog post focuses on the first category of a hazardous atmosphere listed above, which is a flammable gas, vapor, or mist greater than 10% of its lower flammable

Greater Than 10 Percent Lower Flammable Limit (LFL)

OSHA includes this type hazard because they do not want employers to wait until conditions are ripe for a flash fire or explosion to occur inside of a confined space before taking precautionary measures. Atmospheric conditions inside a confined space that contain a low level of flammable gas, vapor, or mist could quickly change for the worse. Therefore, OSHA regulations classify a flammable gas, mist, or vapor present inside a confined space at levels over 10% of its LFL as a hazardous atmosphere. Once that level is reached inside a confined space, or if there is a reasonable probability that it will develop within the space during entry operations, that space must be classified as a permit-required confined space.

Lower Flammable Limit (LFL) Explained

So, what is LFL? It stands for Lower Flammable Limit. The LFL is the minimum amount of a particular flammable gas, vapor, or mist that must be present in the atmosphere to ignite and burn. If you introduce an extremely small amount of a flammable gas, vapor, or mist into the atmosphere of a confined space, it might not ignite when exposed to an ignition source (such as a spark or flame) because there is not enough of the material present to form a flammable mixture in the air. How much flammable gas, vapor, or mist is needed in the atmosphere for a fire or explosion to occur? That depends on the gas, vapor, or mist present inside either space, as they all have a different LFL.

What is LEL?

Be aware the term “LFL” is also referred to as “LEL”, or Lower Explosive Limit. The terms LFL and LEL are basically interchangeable. While OSHA uses LFL in their confined space standards, LEL is commonly used by many gas detection equipment manufacturers, as well as in many written confined space entry programs. Also, I may on occasion use the term LEL throughout future blog posts.

LFLs of Common Flammable Gasses

The LELs of a few common combustible materials are listed in the chart below. An LEL is expressed as a percentage of the atmosphere within the confined space that is comprised of a particular combustible gas, vapor, or mist. For example, the LEL of Methane gas, with the chemical identifier of CH4, is 5% of the atmosphere at any given point. As you can see, the LELs of various gases differ quite a bit.

Upper and lower flammable limits of common gasses

What does UFL mean?

Note that each of the combustible materials listed in the chart also have their own UFL, or Upper Flammable Limit (expressed in the chart as an UEL). If a flammable gas, vapor, or mist is present in the atmosphere in quantities greater than its UFL, it will not ignite and burn. I bring up this term because while OSHA does not regulate UFLs, they do come into play when we discuss gas detection equipment in later blog posts.

Flammability Characteristics of Methane Gas

As mentioned earlier, the LFL of Methane gas is 5%. So, if Methane gas is present in a confined space, but it does not make up at least 5% of the atmosphere at any point within the space, there will be no flash fire or explosion if there is a spark or other source of ignition. And the upper flammable limit, or UFL, of Methane gas is 15%; So, a confined space containing higher levels of Methane would not burn or explode either. However, if the amount of Methane gas increases or decreases to fall somewhere between its LFL and UFL at any point inside a confined space and there is any source of ignition in that same area, a fire or explosion will occur.

Since Methane gas has an LFL of 5% of the atmosphere, a hazardous atmosphere as defined by OSHA would be present inside a confined space if the amount of Methane gas in the atmosphere exceeds 10% of that level. The presence of that level of Methane gas, which equates to just 0.5% of the total atmosphere, means the space would have to be classified as a permit-required confined space.

Other Hazards Associated with Methane Gas

Before moving on, let’s take a moment to discuss Methane gas, or CH4, a little more, as this is a hazardous gas commonly found in a variety of confined spaces. Methane is a colorless, tasteless gas that has no odor, so its presence cannot be detected by your senses alone. But too much Methane gas inside of a confined space creates not only a potentially flammable atmosphere, as previously discussed, it can also displace oxygen in the space, creating a deficiency of oxygen for an entrant to breathe. Fortunately, while Methane is a highly flammable gas that can also displace oxygen, it is one of the few flammable gases that is not toxic.

Where is Methane Gas Commonly Found?

Methane is created when organic matter decomposes. Therefore, it is commonly found in potentially harmful quantities within confined spaces such as sewers and related facilities, manure pits, and in silos that have held grain or similar materials that has rotted. Methane is also commonly found in confined spaces such as underground utility vaults, especially those located near landfills (the rotting trash creates Methane). Therefore, these spaces are typically classified as permit-required confined spaces.

More Examples of Confined Spaces With Potentially Flammable Atmospheres

Other examples of confined spaces with a reasonable potential to contain an atmosphere containing a flammable gas, vapor, or mist above 10% of its LFL include pipelines, tanks and other vessels that have contained fuels or other flammable or combustible materials, even when they have been emptied of their contents. Also, confined spaces where work processes such as cutting with an oxy-acetylene torch or painting with flammable paints and solvents is taking place inside could potentially contain a hazardous atmosphere. And confined spaces such as vaults or tunnels in which there are leaking valves or piping used to transport flammable gas could also contain a hazardous atmosphere. As such, each of these spaces would also likely be classified as a permit-required confined space.

So, in review, any confined space with an actual flammable gas, vapor, or mist present at levels greater than 10 percent of its lower flammable limit, or LFL, is considered to have a hazardous atmosphere. And that means the space must be classified as a permit-required confined space. The same is true for any confined space in which that level of a flammable atmosphere could reasonably be expected to develop during entry operations. You may also hear the term LEL, which stands for lower explosive level, being used to describe a hazardous flammable atmosphere inside a confined space, as that term is synonymous with LFL.

In our next blog post (#10), we will examine the second criteria of a hazardous atmosphere that could be present inside of a confined space (combustible dust). In the meantime, please provide your feedback and questions to this blog post in the comments section below. And as always, I urge you to share a link to this confined space training blog post with anyone in your network who could benefit from this information. Thanks – Curtis

Definition of a Hazardous Atmosphere – Airborne Combustible Dust =/> LFL