Document QkzyrYxqMzyOMYRMq525YEe24

DETECTOR 5 Time 45 Milliseconds Pressure 0 90 P S I 6 Time 50 Milliseconds Pressure 1 30 P S I 3 DETECTOR OPERATES Time 35 Milliseconds Pressure 0 20 P S I 4 SUPPRESSION COMMENCES Time 40 Milliseconds Pressure 0 55 P SI 7 Time 55 Milliseconds Pressure 1 65 P SI 8 SUPPRESSION COMPLETE Time 60 Milliseconds Pressure 2 00 PS I Fic 43-37 --Schematic diagram of suppression of an explosion in a large cylindrical tank Courtesy National Fire Protection Association pressuie changes When rapid changes take place, such as caused bv product withdrawal, a means is piovided which opens a large supply line until the pressure equalizes This tvpe of system is extremely efficient and has the advantage of reducing product vapor losses bv maintaining a slight positive pressuie within the chamber Preventing explosions There aie a number of methods for pre venting the development of explosive mixtures They aie the fiist line of defense against ex plosions Equipment foi handling and storing of flammable gases should be constiucted, in spected, and maintained so that the dangei of leakage and explosive mixture foimation is ledueed to a minimum Equipment design should comply with existing codes and othei regulatoiy ordinances Equipment should be inspected at legular inteivals bv qualified individuals eithei from the regular plant staff oi from an outside souice Ventilation will pi event excessive accumula tions of gases and vapois undei certain con ditions The method of ventilation neces- sanly vanes with the nature of the gas or vapor to be lemoved and depends upon whether it is heavier or lighter than air at the point of removal Inasmuch as heating or cooling of the gas oi vapor may change its densitv, the ventilation or exhaust system, or both, should be designed for operating condi tions and should not be based on the pub lished densitv figuies (See NFPA Standard No 91, Blower and Exhaust Systems, Dust, Stock and Vapor Removal ) Natural draft ventilation mav be through openings near the floor or near the ceiling or both However, the best method is a posi tive local exhaust system, using explosionpioof electrical equipment and taking suction as close to the source of a vapor or gas as possible A motor other than explosion-proof driving an exhaust fan, may be used if it is piopeily installed outside the dun- work and outside the hazardous aiea In general, industiial gases, such as acety lene, carbon monoxide, hvdrogen, and natural gas, are lighter than air The vapors of flam mable liquids are generally heavier than air Examples are alcohol, naphtha, gasoline, ben zene, kerosene, amvl acetate, and carbon di- 1377