A simple mechanical explosion of the container.

Again NFPA 921 has long ago addressed this issue:

21.2.1 Mechanical Explosions. Mechanical explosions are explosions in which a high-pressure gas produces a purely physical reaction. These reactions do not involve changes in the basic chemical nature of the substances in the container. A purely mechanical explosion is the rupture of a gas storage cylinder or tank under high pressure resulting in the release of the stored high-pressure gas, such as compressed air, carbon dioxide, or oxygen.

21.2.3.1 In chemical explosions, the generation of high-pressure gas is the result of exothermic reactions wherein the fundamental chemical nature of the fuel is changed. Chemical reactions of the type involved in an explosion usually propagate in a reaction front away from the point of initiation.

21.2.3.2 Chemical explosions can involve solid combustibles or explosive mixtures of fuel and oxidizer, but more common to the fire investigator will be the propagating reactions involving gases, vapors, or dusts mixed with air. Such combustion reactions are called propagation reactions because they occur progressively through the reactant (fuel), with a definable flame front separating the reacted and unreacted fuel.

21.2.4 Combustion Explosions. The most common of the chemical explosions are those caused by the burning of combustible hydrocarbon fuels. These are combustion explosions and are characterized by the presence of a fuel with air as an oxidizer. A combustion explosion may also involve dusts. In combustion explosions, the elevated pressures are created by the rapid burning of the fuel and rapid production of large volumes of combustion by-products and heated gases. Because these events are likely to be encountered by the fire investigator, combustion explosions are considered here as a separate explosion type.

Pat Kennedy, CFEI, CFPS, MIFireE
Fire and Explosion Analyst
Sarasota, Florida
[www.kennedy-fire.com]