The intent of explosives safety site planning is to increase safety in all aspects of handling and storage of energetic items. But what does “safe” mean?
The idea of safety is fairly difficult to quantify. Safety professionals and approval authorities can’t actually say that a person is “safe”. Instead, our criteria and approvals document the amount of risk that a person or facility is subject to. We attempt to quantify the risk in order to determine the level of safety provided. Put another way, “safety” is a lack of “risk.”
Okay, so what is IBD?
The default standard for explosives safety is the distance at which the public, or anyone unrelated to the explosives operation, is allowed to be located. This is commonly referred to as Inhabited Building Distance (IBD). NATO criteria also have a Vulnerable Building Distance (VBD), but that’s a discussion for another day.
IBD is commonly used as the threshold distance at which you don’t have to consider explosives hazards anymore. But that’s not entirely correct, and that’s what this post will discuss.
Within the explosives safety or planning departments of any military installation you’ll find an installation explosives map (sometimes referred to as a D-8 map) that shows one or more red bubble clouds. These are the aggregation of multiple IBD arcs generated by various explosives operations or storage locations. This type of map serves as a very quick “go/no-go” reference for where to avoid future development. An example installation explosives map for a fictitious military installation is shown below.
This type of map and the concept of IBD can lead to misunderstandings. By drawing a line and saying “you have to stay outside of this line” the map implies that we are in danger if we are within the line, but safe if we are outside the line. Does the hazard just shut off when I’m outside the line? No, that’s not true. So how is IBD to be interpreted?
IBD (as well as other explosives safety arcs) is referred to in the criteria as an “exposure” level. Hazards generated by an explosion reduce with distance but do not abruptly stop at a set distance. To be completely “safe” (removing all risk) you would have to be much farther away than IBD. But we can’t use that much space for explosives operations. So we rely on established thresholds for allowable risk. We do not say that there is zero risk at IBD. Instead, there is a tolerable level of risk that we would be “exposed to” at IBD. We define our required level of safety by establishing our tolerable level of risk.
It must be noted that establishing the acceptable levels of risk to people is a difficult and thankless job. There are risks inherent to handling explosives materials, and some level of risk must be accepted. Drawing that line is not an exact science. Averages, estimations and models are used to classify almost all explosives risks. The intent of this article is not to criticize these criteria, but to define and explain them. The author hopes that defining the risks will bring awareness and appreciation to the work of explosives safety personnel.
Ok, so what are these tolerable levels of risk? Am I “safe” at IBD?
Hazards at IBD
Explosives effects generally include blast overpressure, fragmentation hazards, and thermal hazards. For this discussion we’ll focus on Hazard Division (HD) 1.1 and its hazards of overpressure and fragments. It’s important to note that IBD arcs are almost always controlled by only one of these factors at a time (overpressure, fragments or thermal). The individual mechanisms will be described below, and a summary of the combined effects is provided at the end to explain how they interact.
Overpressure
Overpressure is the blast wave generated as the solid explosives material nearly instantaneously transitions to a much larger volume of gas (a detonation). The expanding gas creates a shock wave that dissipates as it expands further and further from the detonation point. There are a few factors used to quantify the strength of a blast wave such as peak pressure, duration, and impulse. At the range of IBD, only the peak pressure is used to quantify the blast hazard.
For U.S. DoD criteria, the established allowable peak incident pressure at IBD is between 1.2 and 0.9 psi (depending on the quantity of explosives – see DESR 6055.09). This is equivalent to scaled ranges of K40 to K50 (To learn more about K-factors, read this post).
These low pressures may not seem like a lot, and they have a very low probability of hurting a person (e.g., eardrum rupture or lung damage), but they still create a significant force when applied to a large surface facing the blast wave. Windows and other lightweight construction components commonly break at IBD. The most likely hazard at IBD is being struck by broken glass or dislodged structural components.
Fragmentation
Fragments encompass any physical material or item thrown outward from the detonation. This includes “primary fragments” such as munitions casing or any other metal object in direct contact with the explosives material, as well as “secondary fragments” (also referred to as debris) which are generated by the breakup of any structure surrounding or enclosing the explosives. Secondary fragments can be metal, concrete, or soil.
The distribution of fragments depends on many factors, such as the loading density (lbs/ft3) of the donor structure, the amount of venting (areas where the blast overpressure can leak or escape), and the type or density of the material being thrown. Quantifying how many fragments are thrown, what direction they are thrown and how far they travel is difficult. However, criteria have been established to define tolerable levels of risk for fragment hazards.
U.S. DoD criteria (as well as NATO criteria) define the acceptable/tolerable risk for fragments at IBD as “the distance at which the density of hazardous fragments becomes 1 per 600 square feet (ft2)”. What on earth does that mean? There are a couple of concepts to dive into here.
First is the definition of a “hazardous fragment”. Most people don’t want to be struck by any fragment. What makes one hazardous or not hazardous? This is another example of an acceptable threshold of risk that had to be established. Fragments come in all sorts of shapes and sizes. More importantly, they also arrive with different velocities. A large fragment traveling relatively slowly can be hazardous (e.g., being struck by a large panel in a windstorm), and so can small fragments traveling very fast (e.g, a bullet). Hazardous fragments are classified using the kinetic energy of the object (KE = 0.5 x mass x veloicty2). A hazardous fragment is defined as one that can cause a lethal injury. The threshold for this classification is 58 ft-lbs of kinetic energy. This is equivalent to a 75 mph baseball. Only fragments with a kinetic energy of 58 ft-lbs or greater are considered when establishing IBD.
The second concept is the fragment density. Why is the value of 1 hazardous fragment per 600 ft2 used? This one originates from the concept that the profile of an average adult (height x width) is about 6 ft2. If the density of hazardous fragments is 1 per 600 ft2, there is roughly a 1% chance that a 6 ft2 person will be struck by a hazardous fragment. It’s not perfect, but it’s a model that lets us move forward with safety criteria.
So, what does this mean collectively? The acceptable/tolerable level of risk (and therefore the level of safety afforded) at IBD is a 1% chance of being struck by a lethal fragment. This leads to two important outcomes:
Hazardous/lethal fragments can and do in fact travel farther than IBD (but at a lower density), and
The probability of being struck by a fragment beyond IBD can be larger than 1% when you include those fragments that fall under the threshold of what is deemed lethal on average.
For explosives weights of 450 lbs or less of general HD 1.1 material, the Hazardous Fragment Distance (HFD) for IBD ranges between 236 ft and 1,250 ft. For any quantity of explosives greater than 450 lbs, the HFD plateaus at 1,250 ft. This is partially due to the ballistic limitations of most fragments. This is a recognized simplification of a complex issue but serves as the U.S. DoD criteria for planning and approval purposes.
Combined Effects
IBD for HD 1.1 materials is the combination of required standoff for overpressure and fragments. It is measured as the greater of the two individual requirements. IBD is dependent upon the amount of explosives being considered. The graph below illustrates which hazard controls the combined IBD. For HD 1.1 explosives weights less than approximately 30,000 lbs IBD is dominated by the fragment hazard. This means that for explosives weights less than 30,000 lbs, at IBD the overpressure will be less than 1.2 psi (most often significantly less) and the fragmentation hazards are the concern. On the other hand, for scenarios with explosives weights greater than 30,000 lbs, at IBD the primary hazard is overpressure. This isn’t to say that there are not fragments at this point, and even potentially hazardous fragments, but the IBD is controlled by the overpressure hazards.
But What Are the Chances?
If hazards still exist to some degree at IBD, why is the public allowed at this distance? The answer is in the probability of an explosion. The criteria presented in this article are for accidental or unintended initiations or detonations. The probability of an event is very low. This is a required balance of accepting risk. How bad would the consequences be AND how likely is the event to occur? Some risk is accepted to the public due to the low probability. For other explosives operations, such as test ranges or demolition ranges, the required IBD is calculated in a completely different, more conservative, manner (but that’s another article on another day). This is because the probability is much higher – the incident is planned to happen. In these cases, the risks accepted for standard IBD would be unacceptable.
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You can find more articles about explosives safety on our website (Explosives Safety Articles). If you are looking for assistance with explosives safety site planning, please visit our website to learn more about how The Schreifer Group can help. Our growing team of five explosives safety SMEs are ready to support you.
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