Hot surface ignition

As a guide, because of convection that occurs from hot surfaces, ignition by a hot surface in open air should not be assumed unless the surface temperature is at least 200°C above the published minimum autoignition temperature (American Petroleum Institute, Ignition Risk of Hot Sui faces, API PSD 2216, Washington, 1980). 

Note Where normal process and compressor operating temperatures are low and there are no nearby hot surface ignition sources, the probability of fire may be low. In these cases, specific compressor water spray protection may not be needed. 

Calculation of hot surface ignition temperature is complex and depends on many variables. From a safety perspective, it is often safest to assume that a surface with a normal temperature slightly below the autoignition temperature is a potential ignition source. 

Mixtures of natural gas and air are similarly unpredictable, being quite stable and unreactive until a spark or a hot surface ignites reaction, and then thermal and chain branching autocatalysis takes off and so does the building. 

Haz-1 Hot surface ignites fuei ieak and fire suppression faiis,... 

Consequendy, convective heat transfer determines the intensity of warming up and ignition. In addition, convective heat transfer also plays an important part in the overall dame-to-surface transmission. The reaction of gases is greatiy accelerated by contact with hot surfaces and, whereas the reaction away from the walls may proceed slowly, reaction at the surface proceeds much more rapidly. 

Hot surfaces and electric sparks are potential ignition sources for carbon disulfide. The ignition temperature depends on specific conditions, and values from 90 to 120°C in air have been reported (2,22). Data on carbon disulfide oxidation and combustion have been summarized (18). Oxidation products ate generally sulfur dioxide [7446-09-5] and carbon dioxide [124-58-9J ... 

Ignition can also be produced by a heated surface. During the process of heat transfer from a hot surface to a flammable mixture, reactions are initiated as the temperature rises and the combination of additional heat transfer from the surface and heat release by chemical reactions can lead to ignition of the mixture. 

Insulation provides other functions in addition to energy conservation. A key role for insulation is safety. It protects personnel from bums and minimizes hot surfaces that could ignite inflammables. It also protects equipment, piping, and contents in event of fire. Thus materials such as mineral wool are sometimes used despite relatively poor thermal quaUties. 

Mechanically generated hot surfaces represent an ignition hazard if, irrespective of the MIT and the MIE, the. surface temperature is 1100°C or higher and the hot. surface area by itself is large enough (see Fig. 26-35). Higher surface temperatures and larger surfaces have a better incendivity lower temperatures and smaller surfaces have a poorer incendivity. 

Equipment Failures Safety system Ignition Sources Furnaces, Flares, Incinerators, Vehicles, Electrical switches. Static electricity, Hot surfaces. Cigarettes Human Failures Omission, Commission, Fault diagnosis. Decisions Domino Effects Other containment failures. Other material release External Conditions Meteorology, Visibility... 

Thermal ignition sources A source that will cause the ignition of a flammable gas, vapor, or dust, such as an electric spark, flame, or hot surface. 

Ignition in 23% of the cases where this occurred, the source was unknown in about a third of the incidents in which the source was known, it was auto-ignition. Other common sources were flames, hot surfaces, sparks, lighming, static electricity and electrical equipment. In many cases conditions changed after a permit-to-work had been issued (see Section 1.3.2). 

Investigations revealed that the initial fire was due to a small, continuous release from the transfer lines. The leakage was ignited by hot surfaces of the truck s engine. The fireball was found to have a maximum diameter of approximately 40 m (130 ft). It rose to 25 m (80 ft) above ground level. 

The time delay decreases as the ignition temperature increases. Tliis means that when a flammable fluid in a flow system comes in contact with a hot surface for a time (shorter than tlie time delay), ignition mtiy not occur. 

Ignition can take place for any flammable mixture within the concentration ranges for the respective LEL and UEL. The conditions for ignition may vary with the specific mixture, the type of oxidant (usually air or pure oxygen), the temperature, and pressure of the system. Ignition may result from electrical spark, static spark, contact with hot surfaces (autoignition) (see Figures 7-48,... 

These explosions in air are usually the result of the release of flammable gas and/or mists by leaks, rupture of equipment, or rupture of safety relierdng devices and release to the atmosphere, which become ignited by spark, static electricity, hot surfaces, and many other... 

Bruderer, R. E., Ignition Properdes of Mechanical Sparks and Hot Surfaces in Dust-Air Mixtures, Ibid., p. 152. 

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