What are flammable or explosive limits?

Flammable or explosive materials are incredibly dangerous to workers. On top of that, these materials can exist in almost any environment whether that be in the construction industry, manufacturing, or in a commercial kitchen.

There are some jobs that pose a higher risk of encountering these hazardous substances than others. Those whose job it is to enter confined spaces for equipment installations, repairs, and even for just general maintenance must be incredibly careful as vapors or other substances suspended in air can quickly fill a space. Dangerous conditions like these can even persist into outdoor environments depending on the hazardous substances present.

Buildup of toxic vapors not only do harm to employees if they are not wearing the right protection equipment, but these vapors also tend to be flammable and even explosive in low concentrations.

The concentration of a flammable vapor or solid particles in air are often expressed in percent of fuel by volume. This measurement is used to denote the concentration of both the lower flammable limit and the upper flammable limit as it reaches its flammable or explosive qualities in the presence of an ignition source.

Lower Explosive Limits vs. Upper Explosive Limits

The terms upper and lower flammable limit as well as upper and lower explosive limit are often used interchangeably. That said, all workers have the potential to come across situations where flammable or explosive limits must be acknowledged.

These limits specifically apply to the concentration of chemical vapors in air. Anything between the lower explosive limit and the upper explosive limit, also referred to as LEL and UEL, can ignite or explode in the presence of an ignition source such as a spark or open flame.

Let’s go over what exactly lower and upper explosive limits are and why they’re so dangerous. Each are defined as the following:

  • Reaching the upper explosive limit means the air and substance mixture is too rich in fuel to burn. This also means there’s not enough oxygen present. The upper explosive limit is sometimes referred to as the UEL.
  • If the concentration of fuel is below the lower explosive limit, the mixture of substance and air lacks enough fuel to burn. The lower explosive limit is sometimes referred to as LEL.

There are all kinds of different flammable substances that have the potential to ignite once the concentration in air meets a certain threshold. One of the most common groups are VOCs, also known as volatile organic compounds. The following are a few common examples:

  • Solvents
  • Paint thinner
  • Fuels such as gasoline, diesel, jet fuel, and kerosene
  • Hexane
  • Toluene
  • Benzene

LEL and UEL can also apply to powdered solids in the form of dusts. Examples of these include the following:

  • Aluminum or magnesium dust
  • Wood dust
  • Carbon dusts
  • Organic dusts such as flour, sugar, and paper
  • Dusts from textiles

Employers must provide both engineering controls as well as proper ventilation and spill cleanup protocol to control and minimize any potential for an explosive reaction.

Safety Controls to Minimize Explosion Risk

Minimizing injuries and illnesses in the presence of hazardous substances almost always begins with the hierarchy of hazard controls. This topic has been touched on in previous articles, but anytime is a good time to refresh your memory.

  1. Elimination – The best way to prevent injuries from hazardous substances is to eliminate it entirely. This can be very difficult or outright impossible if the substance is an integral part of the business’ process. Regardless, the hazard control method always starts here, it’s never a bad idea to consider this as an option.
  2. Substitution – As the next best option, substitution should be considered in case there is an option to use a less lethal substance in the workplace.
  3. Engineering Controls – This is often where the real work begins in establishing a safer workplace while around flammable liquids and dusts. An excellent option for maintaining a space that contains hazardous substances is the installation of a better ventilation system or HVAC system. This way, the particulates in the air can’t accumulate quickly to dangerous levels within the flammable range of a substance. Better storage methods can also apply to these dangerous conditions. Even removing ignition sources is an excellent option.
  4. Administrative Controls – Improving training methods should always be on the docket for facility improvements as well as improving process steps, emergency response procedures, and even standardized housekeeping rules within the facility. Improvements like these can go a long way in preventing catastrophic accidents.
  5. Personal Protective Equipment – PPE is the last protective/mitigation method to use. In theory all the other admin, engineering, substitution, and elimination efforts leave the worker with minor hazards to contend with, which even still may require hefty safety equipment.

Remember, there are three ways to effectively control flammable and combustible materials in the workplace to prevent injuries and property damage in the workplace. These include:

  1. Providing the necessary amount of ventilation
  2. Removing all ignition sources from the area
  3. Storing the substances correctly, this also applies to cleaning up dust in the workplace

Paired with the appropriate mitigation methods, the risk of a substance reaching its explosive or flammable threshold decreases significantly.

Common Jobs Where Gas Detection is Necessary

When we think of explosions and vapors causing issues in the workplace, the immediate subject that comes to mind are confined spaces. There are all sorts of spaces that are classified as confined even though they may not seem to be so. However, many vapors are heavier than air, causing them to pool in poorly ventilated areas. Because of this, confined spaces aren’t necessarily the only places to worry about reaching the LEL or UEL.

Sewers, holding tanks, and pits are excellent examples of confined spaces. But have you thought about the dangers associated with semiconductor facilities or wastewater treatment plants?

The following list includes common jobs and always need gas detection systems and gas monitoring efforts:

  • The Mining Industry – While considered a confined space, these environments are risky for workers if the right ventilation efforts are not pursued. However, these mines often naturally exude flammable vapors such as methane and H2S or flammable particulates such as coal dust rather than the substances leaking from a storage container. Overall, it’s not a question of if employees encounter these dangerous conditions, but when and what protective measures are being taken.
  • Chemical Manufacturing Plants – Aside from manufactured chemicals potentially being toxic, a leak caused by improper storage or transportation can displace oxygen in the workplace, driving up the air concentration and exposing workers to explosion hazards. Again, ventilation is always necessary.
  • The Oil and Gas Industry – Everyone knows fuel is extremely flammable in its liquid form. However, these vapors, especially that of gasoline, have the tendency to pool in low lying areas away from an initial spill even in outdoor situations. When lit, the explosion travels from its initial point to wherever those vapors drifted to.

Take the time to install gas detection equipment in your facility. This way you and your employees know when extra ventilation is needed in case a certain substance has reached its explosive threshold.

Explosivity, Not Toxicity

The fire triangle, LEL, UEL, and exposure limits are often entwined into one large problem that employers must face on a regular basis. Isolating each hazard and developing an action plan is often the best plan.

To start we must go over the difference between explosiveness and toxicity as they often are mistaken for each other in terms of LEL and UEL when in fact, while they often occur in the same space, cause entirely different problems to the worker.

Toxicity is a chemical’s effect on the body when exposed. This can be skin, eyes, or lung irritation, body weakness, gastrointestinal distress, organ failure, the list could go on. Depending on the substance, this can either be a small amount of chemical exposure or even in larger quantities. There are also exposure limits that OSHA has set for the duration of exposure to these substances.

Explosivity is the chemical reacting to its environment. As mentioned above, a fire triangle describes the three essentials for ignition. Fuel, oxygen, and heat or an ignition source. In the case of LEL or UEL levels, the amount of fuel and oxygen ratio must be just right to begin the reaction. If too little fuel or if the air is too saturated, there won’t be an explosion.


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