Fire

1. Fire Fire (or combustion) is a chemical reaction in which a substance combines with an oxidant and releases energy. Part of the energy released is used to sustain the reaction, or Fire can be considered as the rapid exothermic oxidation of an ignited fuel.

2. The Fire Triangle (Fig. 3-1) • Fuels Liquids: gasoline, acetone,ether, pentane. Solids: plastics, wood dust, fiber, metal particles. Gases: acetylene, propane, CO, hydrogen. • Oxidizers Gases: oxygen, fluorine, chlorine. Liquids: hydrogen peroxide,nitric acid, perchloric acid. Solids: ammonium nitrite, metal peroxides. • Ignition Sources sparks, flames, static electricity, heat.

3. Flammability Limits • Vapor-air mixtures will only ignite and burn over a well-specified range of compositions. • The mixture will not burn when the composition is lower than the lower flammable limit (LFL) or higher than the upper flammable limit (UFL). The commonly used units are volume percent fuel in mixture. (Table 3-1) • The explosive limits (LEL and UEL) are used interchangeably with LFL and UFL. (Figure Next)

4. LFLs and UFLs of Vapor Mixtures The Le Chatelier equations:

5. Temperature Dependence The flammability range in general increases with temperature. Where is the positive net heat of combustion (kcal/mole); T is the temperature ( ).

6. Pressure Dependence Pressure has very little affect on the LFL except at very low pressures, where flames do not propagate (see Fig. 3-2). The UFL increases significantly as the pressure is increased (see Fig. 3-3). Where P is the pressure in mega pascals absolute; UFL is the upper flammable limit in volume % at 1 atm.

7. Estimating Flammability Limits where Cst is volume% fuel ( ) in mixture.

8. Flash Point The flash point (FP) of a liquid is the lowest temperature at which it gives off enough vapor to form an ignitable mixture with air. At flash point the vapor will burn, but only briefly; inadequate vapor is produced to maintain combustion. (Figure 3-5, Table 3-1) The flash point generally increases with increasing pressure.

9. Fire Point The fire point is the lowest temperature at which a vapor above a liquid will continue to burn once ignited; the fire point temperature is higher than the flash point.

10. Ignition • Ignition: Ignition of a flammable mixture may be caused by a flammable mixture coming in contact with a source of ignition with sufficient energy (see Table 3-3) or the gas reaching a temperature high enough to autoignite. • Autoignition Temperature (AIT): A fixed temperature above which a flammable mixture is capable of extracting enough energy from the environment to self-ignite.

11. 點火源 直接燃燒的火焰 (44%) 物理點火源 (12%) 化學點火源 (15%) 電點火源 (25%) (見 表3-4)

12. 直接燃燒的火焰 • 鍋爐、加熱爐、焚化爐等關閉式火焰 • 火炬、燃燒塔等開放式火焰 • 香煙 • 設備或煙囪排放的高溫油煙 • 無焰燃燒的悶煙 • 高溫設備高溫外表 • 車輛或設備的內燃機 • 縱火 • 焊接、切割用火焰

13. 物理點火源 • 壓縮生熱 • 機械磨擦撞擊產生的火花 • 吸附

14. 電點火源 • 靜電 • 電機設備 • 雷電 • 雜散電流(stray current)

15. 靜電 靜電是由兩種不同物質在接觸後產生的表面效應。化學程序中的不同物質表面在相互接觸、移動再分離後也會產生靜電，例如表3-5、圖3-6。

16. 液體流動引起的靜電 非極性(nonpolar)液體在管線中流動會產生靜電而帶電荷，當電阻係數(resistivity)超過 ，即具危險性 (表3-6)。 防範措施： • 強極性添加劑 • 其他見76頁

17. 粉體輸送引起的靜電 粉體的傳送、研磨、混合、篩選、充填桶槽或在空氣中飛散時，由於與其他物質不斷相互接觸再分離，每一粉粒皆會帶電，且會將所接觸物體充電。防範措施見78頁。

18. 人體接地 工作人員宜穿著 • 具導電性鞋底的鞋靴，或 • 連接膝部與鞋底的導電帶。

19. 電源開闢 電路斷電器 馬達啟動器 按鈕站 電源插座 照明設備 馬達 可能造成火花的電機設備

20. 雜散電流之原因 • 無線電波引起的天線效應 • 高壓電線附近之電場 • 陰極防腐系統 • 化學電流