What autoignition describes
Autoignition temperature characterises the onset of combustion without applying an external spark or flame, under test conditions. It is useful for assessing hot sources in the presence of combustible substances. It does not describe a universal safe temperature for every item of equipment, mixture or working situation.
Related concept: Explosive atmospheres.
Difference from flash point
A liquid may generate flammable vapours at a temperature far below its autoignition temperature. Those vapours could burn if they encounter a spark. Controlling hot surfaces therefore does not allow static electricity, flames or electrical contacts to be ignored.
Conversely, a sufficiently hot source can initiate combustion without a visible spark. The absence of an open flame does not demonstrate that a maintenance operation is safe.
Preventive applications
The value is considered when selecting equipment for potentially explosive atmospheres and assessing surface temperatures. Selection also depends on the substance, equipment group and conditions of use. Marking and instructions must be interpreted together by competent personnel.
For dusts, the behaviour of a cloud must be distinguished from that of a deposited layer. Accumulation on hot equipment may change heat dissipation and create conditions different from those considered for clean equipment.
Information to check
Product identity, test method and the conditions to which the value refers must be confirmed. Mixtures and process changes require their own review. Residues in a line or vessel may differ from the product normally passing through it.
The assessment also examines hot spots caused by friction, bearings, motors, heaters and cutting work. Foreseeable abnormal operation should be considered, because a fault may raise the temperature even when normal operation is acceptable.
Hypothetical example
A motor in a dust-conveying system is replaced with one of similar power. Before accepting it, the team reviews suitability for the location and surface temperature, not just dimensions and energy consumption. It also checks cleaning and bearing monitoring. Mechanical equivalence does not demonstrate suitability for the ignition risk.
Limits of the value
An improvised margin must not be established by subtracting a few degrees from the published temperature. Applicable technical criteria depend on the product and system. Autoignition, spontaneous combustion through self-oxidation and thermal decomposition must not be confused either: they can be related but require different mechanisms to be examined. The safety data sheet and technical documentation are starting points for that assessment.
