Class 3 Flammable liquids

Some flammable liquids derive from petroleum, e.g. petrol, kerosene. Others are manufactured through natural or industrial processes, e.g. alcohols.

Vapours are created when some molecules in the liquid have sufficient energy and are moving with sufficient velocity to break clear of the surface into the air space above.  The warmer the liquid, the more molecules reach this energy and velocity level, and the faster the vapours is formed.

The vapours are invisible, and always much heavier than air.  They will flow downhill and collect at the lowest point.  The vapours mix readily with air, when the mixture is within the explosive limits for the particular material, it will burn or explode when ignited.

The flashpoint is the temperature above which the liquid releases just enough vapour to create an ignitable mixture with air, i.e. at the lower explosive limited.  Below the flashpoint, insufficient vapour forms to create an ignitable mixture.  The lower the flash point, the more readily the vapour forms at normal temperatures and the greater the risk.

The FP of petrol is -40º C, so it burns readily at normal temperatures.  The FP of diesel is +65ºC, so it has to be heated before it will burn.  The UN upper limit for Class 3 is normally FP 60ºC, above which the material is not regarded as dangerous for transport. However, diesel came within the full scope of the Regulations recently.  Beyond that, a flammable liquid is included in Class 3 if it has a FP above 60ºC and is carried at a temperature above its FP.  If it is carried at a temperature above 100º C and below its FP it is included in Class 9.

The auto-ignition temperature is the temperature at which the vapour will ignite in air without an ignition source.  The AIT is much higher than the FP, e.g. for petrol it is 300ºC, the effect is used in diesel engines, where no spark plug is required.

Flammable liquids are placed in Packing Groups according to boiling point and flash point.

 Packing Group  Initial Boiling Point  Flashpoint (closed cup)
 Packing Group I  Boiling point below 35º C  
 Packing Group II  Boiling point above 35º C  Flashpoint below 23º C
 Packing Group III  Boiling point above 35º C  Flashpoint >23º C and <60º C


Flammable liquids are mostly used as fuels in internal combustion engines for motor vehicles and aircraft, and as such represent by far the largest tonnage of dangerous goods moved by surface transport.  They are also used in much smaller quantities as chemical intermediates, or as the medium for paints, varnishes, inks, adhesives, etc.

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