Before going into the physics of flashpoint we must know that flashpoint is a very important point in determining stowage of flammable liquids on board vessels. Document of Compliance specifies the holds for carriage of flammable liquids on the basis of flashpoint.
IMDG Code defines flashpoint as the lowest temperature of a liquid at which its vapour forms an ignitable mixture with air.
Wikipedia: The flash point of a volatile material is the lowest temperature at which it can vaporize to form an ignitable mixture in air. Measuring a flash point requires an ignition source. At the flash point, the vapor may cease to burn when the source of ignition is removed.
Let us see two examples of flashpoint
Petrol: -43 Deg C
Diesel: > 62 Deg C
If we light a match and bring it on the surface of petrol (DO NOT TRY IT) the vapours will flash but if same match is brought above diesel it won’t flash unless if diesel is heated at or above its flash point.
This shows that as long as the temperature of a liquid is below its flashpoint it will not have flammable hazard.
To determine the flash point there are various equipments. The liquid to be tested will be kept in a receptacle well below the expected flashpoint and heated slowly while bringing an open flame above its surface the lowest temperature the flash takes place is flashpoint. There are main two differences in this test, open-cup and closed cup. Where ever in IMDG Code a flashpoint is mentioned for any substance the figure is closed-cup.
Consignments of dangerous goods which have flashpoint at or below 60 deg C need to contain the flash point in closed-cup on the declaration.