Ignition hazards

Thiophene and 3-methylthiophene are Hsted on the TSCA chemical substances inventory. Thiophene is regulated as a hazardous material under OSHA and also regulated under the Clean Air Act, Section 110, 40 CFR 60.489, but there are no exposure limits or controls set for 3-methylthiophene. Both materials are regulated under sections 311/312 of the Superfund Amendments and Reauthorization Act, 1986 (SARA), as materials with an acute health and fire hazard, and under the Resource Conservation and Recovery Act, as ignitable hazardous wastes (DOOl). 

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. 

Static charge generation causes an ignition hazard only if the accumulated charges create an electric field sufficient to produce an electrical discharge in a flammable atmosphere. In most processes, this means that the electric field intensity at some location must reach the breakdown strength of air (nominally 3 X lO " V/m). The objective of static-control measures is to ensure that electric field intensities cannot reach this value. 

API Publ 2027, Ignition Hazards Involved in Abrasive Blasting of Atmospheric Storage Tanks in Hydrocarbon Service, 2nd ed., July 1988. 

Britton, E. G., 1999. Avoiding static ignition hazards in chemical operations, American Institute of Chemical Engineers, Center for Chemical Process Safety. New York. 

Some typical values for breakdown strength are tabulated in Appendix B. For example, a resistivity of < lO -m might be required to avoid pinholes in a lining whose breakdown strength = 20 kV/mm, assuming /c = 10 /rA and A = 10 m. Note that this evaluation does not address ignition hazards. 

Since static discharges frequently occur at the liquid interface as liquid drains from the wetted wall, a vapor ignition hazard may also exist [ 157,159]. These discharges may be either brush types or PBDs (2-6.2 and 2-6.5). In 1997 it was concluded that Manufacturers have so far not paid attention to requests by the operators of enamel apparatus for a coating which is completely or partially capable of dissipation [159]. However, this situation is believed to be changing and it might now be possible to specify special static dissipative coatings for the vessel and stirrer. 

While some video display screens such as liquid crystal, gas plasma or vacuum fluorescent displays do not present the same charged screen hazards as CRTs, this does not imply that they are safe for use in hazardous locations. This requires special design and certification for use with a given flammable atmosphere. Non-certified equipment used in locations classified as hazardous under Article 500 of NFPA 70 National Electrical Code require a purged or pressurized enclosure to control ignition hazards as described in NFPA 496 Standard for Purged and Pressurized Enclosures for Electrical Equipment. The screen in this case is located behind a window in the enclosure. 

Button, L.G. (2000) Avoiding Static Ignition Hazards in Chemical Processes, American Institute of Chemical Engineers. Browning, E. (1965J Toxicity and Metabolism of Industrial Solvents, Elsevier, Amsterdam. 

The mere fact that voltage, current, or even both, are at low levels does not guarantee a circuit to be intrinsically safe, even though intrinsically safe circuits do utilize relatively low voltage and current levels. Intrinsically safe systems employ electrical barriers to assure that the system remains intrinsically safe. The barriers limit the voltage and current combinations so as not to present an ignition hazard should a malfunction develop. Typically, devices upstream of barriers are not intrinsically safe and are installed in control rooms or other unclassified locations. All devices and wiring on the downstream side of the barriers are intrinsically safe and can be installed in classified areas. 

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