What Does ‘Explosion Proof’ Mean, and Who Should Care?
Some workplaces are more dangerous than others, not because of the practices, but rather the products. Where do explosions occur, and what practices exist to reduce such risk for equipment and workforce?
As a young, naive engineer (which was not very long ago, that is to say, I’m still naive but not as young), I encountered ‘explosion proof’ ratings on industrial equipment and mostly passed over the detail without much consideration. After all, no explosions are expected anywhere near my equipment, so why should it matter to me?
What Explosion Proof Actually Means
Let’s get any misconceptions out of the way first. One, being explosion-proof does not mean a piece of equipment can be subjected to explosive forces and survive. That includes blasting equipment used in mining, for example. Simply, explosion proof does not mean a piece of equipment can withstand the heat and concussion of a blast and still operate.
Practically applied, the concept of explosion-proofing a device means that it is designed to eliminate or mitigate the device’s potential (while operating) to cause or contribute to a catastrophic detonation, even though it's operating in a risky environment such as a refinery or gas-cracking plant.
Figure 1. Industrial explosions from petroleum products can lead to immediate catastrophic effects on equipment, operators, and the environment. Image used courtesy of Adobe Stock
Where is an Explosion Proof Rating Required?
Industrial manufacturing and processing equipment often involve the making and breaking of electrical contacts to activate and operate them successfully. Each time a mechanical switch or relay is energized, a tiny spark is generated. Although solid-state components mitigate the risk of sparks to some extent, many devices, including mains disconnects and motors for example, cannot be substituted with solid-state equivalents.
Whenever there are sparks, they interact with the air around the device. More importantly, sparks interact with whatever is floating in the air. The environment around a system is the determining factor for the rating of the components. Several primary industrial environments prompt engineers to specify equipment rated as explosion-proof.
Figure 2. Electrical contacts will always create sparks (especially for higher voltages), as evidenced by the carbon buildup inside this motor starter’s auxiliary contact block.
Flammable Gasses and Vapors
It should be no surprise to anyone that flammable vapor can be ignited with a spark in the air. Fossil fuels, volatile gasses, solvents, and many processing chemicals are all on the list. The warnings posted on gas pumps and propane tanks offer the same rationale for explosion-proofing devices used in these environments.
Dust and Small Particulates
Although most dust is not explosive, its presence can represent a real danger and increase the potential for a fire or explosion. Fire requires three things: air, fuel, and heat. Any source of ignition can ignite the dust, creating a flashover effect that spreads quickly throughout the entire cloud of material. Fine wood dust, flour and grains, and other normally non-toxic particulates are some of the dangers. For example, even though wheat is not considered flammable, dust can be. If the silo’s contents aren’t managed correctly, biological processes in the wheat can generate enough heat to ignite the dust and cause an explosion.
Flammable liquids themselves are likely not to ignite because they don’t allow the ingress of oxygen. Any submerged spark would be extinguished. The fumes from flammable liquids are a different story. Even small amounts of flammable liquids can generate a thin layer of gasses from evaporation. If those gasses ignite, the heat from the fire will cause the evaporation rate of the liquid to rise, leading to an explosion. Although most process and manufacturing engineers aren’t dealing with explosive gasses directly, best practice dictates always to work very carefully with flammable industrial liquids.
Figure 3. Placards and warning signs can indicate explosive products in facilities, trucks, and trains. Image used courtesy of Adobe Stock
What Makes a Device Explosion-Proof?
One way to prevent sparks from a device from igniting nearby combustibles is to enclose the device and seal it off from the environment. It is quite simple in reality (although often complicated in design) to fully enclose the device with sealed electrical connection points and fanless enclosures.
A major problem to be solved when designing explosion-proof devices is cooling. Motion and electrical/electronic equipment create heat. That heat can’t be vented into the atmosphere, so screened openings for fans are not permissible, which means alternatives must be considered. Heat sinks, fully sealed air passageways, and liquid cooling are alternatives that are used to maintain low operating temperatures to mitigate any chance of igniting any combustibles present.
Is a Chance of Explosion the Same as Flammable?
In the context of explosion proof, the air surrounding the device holds the risk of ignition. The device itself may be adjacent to or mounted to a flammable object (a wooden fixture or cloth curtain, for example) even while the environment is not rated as an explosion hazard.
Any operating device that creates heat or a spark is at risk of causing a fire. That risk must be managed through proper design and maintenance to ensure there is no significant contact between a flammable object and a heat source. Fire certainly can be just as catastrophic as an explosion.
What is Intrinsic Safety for Hazardous Areas?
In these hazardous areas prone to explosion or flame, a design approach to all equipment must follow a consistent design strategy to ensure safety. It is well known what energy levels create explosions or ignite fires. By designing safeguards (distance, shielding, external cooling, etc.), the area of contact between the device and the environment can be kept to a minimum to reduce energy levels and prevent catastrophe.
Figure 4. Baking is harmless, right? Those small flour particulates can ignite quickly, leading to disasters in large storage and distribution facilities. Image used courtesy of Adobe Stock
Intrinsic safety, then, is a design process that limits any energy source below a specific threshold. This includes both sparks and bulk heat energy sources.
This design criteria, in collaboration with preventing explosive material from any exposure to ignition sources in the first place, drastically reduce the hazard in those problematic environments.
Safety in the Workplace
Although safety is a high priority in any industrial/commercial environment, it always comes in second place to the outcome of the process. Safety is simply the practices and procedures that ensure the project's success without increasing the risk to operators and equipment. By using well-regulated strategies, including concepts such as intrinsically safe and explosion-proof equipment, engineers and operators can ensure safe operations while successfully maintaining productivity and business continuity.