How are flammable and combustible materials an explosion hazard?

Flammable and combustible materials achieve explosive tendencies depending on two factors: the concentration of vapors or particles suspended in air as well as the presence of an ignition source, whether that be heat or a spark.

However, the terms flammable and combustible are often used interchangeably when, in fact, there are some significant differences between the two as well as the substances they describe.

Flammable materials ignite at lower temperatures than their combustible counterparts. These temperatures are often referred to as flashpoints. Unfortunately, the literature behind the difference in temperature that defines flammable vs. combustible is a bit confusing. The reason is because both the NFPA and OSHA have their own definitions for each temperature threshold.

The NFPA defines a flammable liquid as one that has a flash point below 100°F (37.8°C) proven by the closed-cup test method. A combustible liquid is defined as having a flash point that is at 100°F (37.8°C) or higher. These are established as flammable liquid classes.

On the other hand, OSHA defines a flammable liquid as one that has a flashpoint at or below 199.4°F (93°C). Within that range, flammable substances are placed into categories ranging from 1 to 4. You’ll find that the word combustible has been taken away from the language within the standard entirely and replaced with those flammable categories.

The deviation from NFPA’s original definition of flammable and combustible liquids, of course, has not changed the explosive tendencies of these substances. Rather, it offers a different perspective on how to approach this fire safety hazard. That said, the NFPA standard is allowed to be used according to OSHA if the facility can prove that this system is suited to their unique safety protocol as well as offers just as much protection as the revised OSHA standard.

The Different Types of Flammable and Combustible Materials

There are two primary groups of explosive materials that workers can encounter during their shift: flammable liquids and combustible dust. Each of these explosive hazards have their own unique properties, which include flammable vapors and solid particulates.

The following are a few examples of flammable chemicals:

Heptane – This chemical can be found in laboratories as a solvent, in cement, as a component in gasoline, etc.
Isopropyl Alcohol – This chemical is often used as a mode of sanitization. It can also be found in industrial environments as an inexpensive solvent.
Turpentine – Also a solvent, you’ll find turpentine being used as a paint thinner, a component of varnishes, etc.
Diesel Fuel – The transportation industry is one of the prime consumers of diesel fuel. But, this flammable liquid is also used in agricultural machines, boats, etc.
Formaldehyde – Aside from encountering this chemical in the high school classroom for dissection day, formaldehyde is commonly used in building materials like glue, particleboard, and insulation materials.
Vegetable Oils – Made from canola, soybean, sunflower, and other types of food products, vegetable oil is found in the home kitchen as well as industrial kitchens. Oil fires can be quite hazardous and difficult to extinguish without the right fire extinguisher.

As for combustible dust, here are a few common examples that can be found in many industrial environments:

Most solid organic materials from agricultural environments
Carbonaceous materials such as coal, charcoal, and cellulose can be found in mines and woodworking shops
Textile fibers such as cotton are present in clothing manufacturing facilities
Metals such as aluminum, zinc, and magnesium in places like the automotive industry and elsewhere.

If you don’t think you have any flammable or combustible materials in your facility, check again. Without the right measures, these hazardous materials can wreak havoc on both the employees’ health and safety and the building itself in the event of an emergency.

Combustible/Flammable Liquid Explosions

Flammable liquids aren’t flammable. It’s the vapors they emit that are the real danger.

Chemical vapors are almost always heavier than air. This results in pooling in low-lying areas with poor ventilation. Under the right concentration levels, also known as the upper and lower flammable limits, along with an ignition source such as heat or a flame, the reaction can be devastating.

There are all kinds of different flammable liquids held in laboratories, manufacturing facilities, and even in wastewater treatment plants. All of which are either a byproduct of a process or are manufactured for use elsewhere.

Make sure you and your employees are familiar with all the storage requirements, transportation regulations, and basic safety measures for emergency prevention. Of course, having an emergency plan for spills is also critical for workplace safety.

Combustible Dust Explosions

Similar to how a liquid substance isn’t necessarily flammable but rather the vapors that it releases or heat that it’s exposed to are the source of the fire, dust works along the same lines. A pile of some type of dust isn’t going to alight and result in an explosion. But dust that has been suspended in the air as a cloudy mass poses a huge risk to those in the area.

OSHA defines combustible dust as "a solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition, which presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations".

If you need a more comprehensive list of combustible dusts, take a look at this page from OSHA. Their extensive list of combustible dusts can help you identify any you may have in the workplace. The presence of one of these dusts will require immediate mitigation or elimination efforts.

How do these dangerous events happen, you ask? Well, there are three primary components with an additional two specifically for combustible dust explosions. They are as follows:

Fuel
Oxygen
An ignition source

If you notice, the first three are the essential elements to the fire triangle. This also applies to flammable liquids. While these three in the right quantities will always produce a fire, there are an additional two elements that must be present for a dust explosion. They are as follows:

The right concentration of dust particles suspended in the air
The dust cloud is confined to a single area

What makes these instances so dangerous is that a dust explosion can push even more dust into the air, leading to a cascading effect of explosion after explosion. The buildup of gases and heat has the potential to happen so fast that extreme air pressures build up, blowing out walls and leveling buildings in the process.

Safety Tips for Working Around an Explosion Hazard

Some of the best rules to follow when it comes to preventing these scary situations is to look to the Hierarchy of Hazard Controls. This 5-step process looks to:

Eliminate any unnecessary chemicals or buildup of dust
Substitute hazardous materials for those that are less hazardous or a less dangerous process
Engineer protection measures in the building and around these dangerous substances
Use administrative controls to change the process to one that is safer
Acquire code compliant PPE. Note that this is the last component to the Hierarchy of Hazard Controls because the remaining hazards should be in control.

Within this control method for flammable and combustible materials, it’s best to start out with the following three protection measures:

Provide plenty of ventilation
Train and provide refresher courses on protocol for employees regularly
Use visual communication as well as safety data sheets
Store chemicals according to best practice methods.

With this information in hand, get started on reading the applicable standards revolving around combustible and flammable materials. That includes NFPA 30 and OSHA’s regulation 1910.106. You can also find OSHA regulations pertaining to combustible dusts here.