When a tank is exposed to sustained heat such as that from a fire, the liquid within the tank is forced to boil or vaporize resulting in an increase in pressure within the tank. To prevent this over stressing the tank is enabled with a relief device which activates when such a fire occurs thus venting any excess pressure and as the pressures decreases in the tank the valve begins to close.
However with continued heating occurs, pressure again rebuilds causing the relief valve to reopen. Where flames impinge on the tank below the liquid level, the liquid absorbs the heat allowing the tank metal to remain at a safe temperature.
But with continued relief valve operation the liquid levels drop thus exposing a greater area of the tank to the effects of the heating. With the flames impinging on the vapor space of the tank, the temperature of the tanks metal (un-cooled by liquid) rises and at some point (due to the vapor pressure within) the tank metal begins to weaken causing stretching and eventually tears within the tank.
As the tank comes apart large quantities of both liquid and vapor are released in a powerful explosion and the heat radiated is sufficient enough to ignited combustibles which can cause burns to objects that are up to 1 kilometer away from the explosion.
Tank sections that contain rapidly ignited fuel can become missiles traveling great distance and again can cause at the very least secondary burns to a humans body along with substantial damage to material goods or property´s.
It all happens very quickly with little to know warning and is known as a Boiling Liquid Expanding Vapor explosion or the BLEVE Effect!
Prevent deflagration explosions due to combustible dust particles, gases or vapors with NFPA 69. Combustible dust, gases and vapors produced in industrial settings can pose a significant safety hazard.NFPA 69: Standard on Explosion Prevention Systems offers definitive guidance on explosion protection and prevention systems.
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