Ignition sources purging

Of the instmments described only some are suitable for use outside the laboratory. Where any instrument is carried into a flammable environment it should either be certified as intrinsically safe for exposure to the flammable atmosphere or isolated from the atmosphere such as by keeping it within a purged enclosure. Any probe connected to the instmment must be separately considered as a possible ignition source. Electrometers are described in [ 1531. A more general review of electrostatic instruments is given in [ 136]. 

In the combustion chamber, it is essential to purge down to below the LEL before initiating the ignition source. The usual criterion is 25 per cent LEL. 

Various protection techniques can be used to prevent electrical equipment from becoming an ignition source. Explosion-proof enclosures, pressurized and purged equipment, and intrinsically safe circuits can be used in Division 1 and Division 2 locations. Nonsparking equipment can be used in Division 2 locations. Details of the types of equipment permitted in each classified location are given in NFPA 70. 

After its investigation into the June 9, 2009, catastrophic natural gas explosion at the ConAgra Slim Jim manufacturing facility in North Carolina, the CSB issued formal safety recommendations to the National Fire Protection Association and the International Code Council to prohibit indoor purging and require companies and installers to purge flammable fuel gases to safe locations outdoors, away from workers and ignition sources. 

Of the three general ways by which a fire or explosion can be prevented when flammable materials are present in air (Oxygen), one involves management of ignition sources. One can (A) contain the explosion or fire via enclosures [so-called explosion proof] (B) pressurize or purge the affected volume with an inert gas such as Nitrogen or occasionally carbon dioxide, and manage the concentration of flammables vs. 25% of the lower explosive limit [LEL] or (C) use intrinsically safe facilities (Endnote MM). 

Ignition sources must be minimized during the purging process. For example, all hot work in the area should be stopped. 

The hazards associated with liquid hydrogen are fire, explosion, and exposure to extreme cold temperatures. The elimination of sources of ignition minimize fire hazards. Careful purge operations performed by drivers using helium eliminate the formation of explosive mixtures during the start-up and shutdown of transfer operations. 

NFPA 51,4-4.2 NFPA 51,4-5.2 29CFR1910.253(d)(5)(ii) 4.5.60 When compressed gas lines are being purged of air, oxygen or combustible gas, then sources of ignition shjdl not be allowed near uncapped openings. 

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