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Explosions, Deflagrations & Detonations


In the broadest sense, an explosion is a rapid event during which gases created from the reacting materials cause an increase in the local pressure or volume. Due to the rapid nature of the event, the overpressure generated from an explosion can cause significant damage to surrounding life, structures, and property. If the explosion is sufficiently powerful, the shock wave or blast overpressure also creates fragments that cause secondary damage due to impact and penetration.

Explosions can be categorized into two general categories, as follows. 

In a deflagration, the combustion or reaction wave propagates at a velocity less than the speed of sound. Although all combustion (fires) can be defined as a deflagration, the ignition of a fuel-oxidizer mixture or a suspended cloud of combustible dust in a confined environment typically causes a significant and rapid increase in pressure that can cause catastrophic damage. These explosions are typically associated with natural gas or propane releases (gas explosion), gasoline and hydrocarbon vapors (vapor explosion), finely divided fuels (dust explosion), and certain reactive chemicals . These events can occur immediately before, or immediately after a fire and can propagate throughout a facility. Therefore, an engineering investigation of the event typically includes the preceding and subsequent events.

In a detonation, the combustion or reaction wave propagates at a velocity faster than the speed of sound. Due to the very fast reaction, these explosions create a high-pressure shock wave that causes significant damage at large distances from the seat of the blast. Detonations which can create significant brissance, or fragmentation, of containment vessels, causing impact and penetration damage are typically fueled by solid or liquid fuels but can also occur in pressurized or oxygen-rich-gas environments. They usually are associated with blasting agents or munitions (high explosives). Certain chemicals can also be boosted into detonation with a proximate high-explosive charge. A fire within a chemical warehouse or storage area may also cause a deflagration-to-detonation transition (DDT).

Over the last 40 years, Exponent’s engineers and scientists have investigated several thousand separate explosions that have involved a wide range of solid, liquid, and gaseous fuels. Our clients have included companies in the petroleum and chemical fields, defense and aerospace industry, oil and gas utility companies, insurers, law firms, and various federal and state agencies.

Our investigations have included theoretical analysis, experimental studies, and numerical modeling to determine:

Why Did the Explosion Occur?

  • What were the specific fuel and oxidizer?
  • What process upset or mechanical failure allowed the fuel and oxidizer to accumulate in a quantity sufficient to cause the explosion? 

How Did the Explosion Occur?

  • What was the ignition source?
  • How did the explosion propagate? What was the pressure profile that developed in the explosion, and how did it vent?

What Was the Strength of the Explosion?

  • What pressure profile developed in the explosion, and how did it vent? Was anyone injured, and were any structures damaged?
  • What was TNT equivalence of the blast, as determined from damage indicators around the seat of the explosion? 

 How Can a Similar Explosion be Avoided in the Future?

  • What process improvements are necessary?
  • Were any applicable state or federal codes or industry-accepted standards violated?

Exponent also assists clients in developing strategies to prevent or mitigate an explosion by conducting process hazard analyses of new or existing facilities, implementing methods to reduce explosion hazards, and designing testing programs to evaluate the hazards of various fuel and oxidizer mixtures.

Our staff actively serves on Industry Standards and rule-making committees, including the National Fire Protection Association (NFPA), Technical Committee on Explosives (responsible for NFPA 495, Explosives Materials Code), the Committee on Combustible Metals and Metal Dusts (responsible for NFPA 484 Standard for Combustible Metals, Metal Powders, and Metal Dusts), and the Committee on Wood and Cellulosic Materials Processing (responsible for NFPA 664 Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities).