BOF hood
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*BOF hood [#v8aeaf8c]
**1.BOF hoods system [#sc409258]
>In the BASIC oxygen furnace (BOF) process, oxygen is blown under pressure into a furnace bath containing hot metal, steel scrap and fluxes to produce steel. BOF hood systems are designed to capture gases and particulates generated in the furnance, cool the effluents and convey them for cleaning prior to discharge into the atmosphere. A schematic of a typical closed-hood system is shown in [[Fig.1>#fig1]].
>A worldwide survey of BOF hood systems completed in 1981 revealed that 30 % of all hood failures result from thermal stress cracking. Another 30 % are caused by thinning of hood wall tubes because of erosion or corrosion mechanisms. The remaining hood failures are related to poor water quality, localized superheating, skull buildup on the furnace lip ring or other reasons.
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CENTER:&aname(fig1);&attachref(BOFhoo72.jpg);&br;
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**2.Failure mechanisms [#s2f54794]
>Thermal fatigue cracking occurs when the hood wall is repeatedly cycled in response to rapid changes in effluent gas temperature. Thermal expansion and contraction of the hood wall create fluctuating tensile and compressive stress in the hood.
>Typically, thermal fatigue produces circumferential cracks along the crown of a tube or hot side of a panel. Cracking may initiate on the gas-side, water-side or on both surfaces simultaneously.
**3.Hood repair [#i2e91678]
>Hood repairs are commonly made on an emergency basis, as scheduling allows, so that the BOF can return to normal operations as soon as possible. During repair operations, cracks are quickly patch welded. Panels of tubes are often replaced only after it has been determined that the area is not repairable because of the extent of cracking damage or wall thinning ([[Fig.2>#fig2]]).
>However, there was report that cracks originated at the toes of repair welds ([[Fig.3>#fig3]]). Therefore, when possible, it is recommended that a tube or panel be replaced rather than weld repaired.
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CENTER:&aname(fig2);&attachref(BOFhoo73.jpg);&br;
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CENTER:Fig.2 Welded repair and thermal fatigue cracking on gas-side surface.
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CENTER:&aname(fig3);&attachref(BOFhoo74.jpg);&br;
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CENTER:Fig.3 Welded repairs and fatigue cracking on gas-side surface.
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**4.Case [#qde28128]
-Ⅰ．Water system : Open-recirculation water system was used in this excess air hood.
-Ⅱ．Circumstances of Repair :
>It was reported that many of the lower hood panels were replaced every 18 months to minimize unscheduled repair. However, two lower panels were failing repeatedly in less than one year.
-Ⅲ．Crack location : Membrane carbon steel tube hood panels.
>These panels were located in a portion of the lower hood, away from the highest heat flux zone.
>Numerous circumferential cracks were present on the gas-side surface, with at least one through-wall crack ([[Fig.4>#fig4]]). Inspection of the internal surface revealed a heavy water-deposit and transverse cracking.
>The external cracks were straight and oxide-filled ([[Fig.5>#fig5]]) and the internal cracks initiated at the base of pit ([[Fig.6>#fig6]]).
-Ⅳ．Cause of fatigue crack : Abnormally large temperature fluctuations.
>During a blow when peak gas temperatures were achieved, reduced heat transfer across the tube wall created excessive metal temperatures. Between heats, this area would cool significantly because this panel did not receive residual radiant heat from the furnace.
>The heavy buildup of water-side deposit and corrosion product contributed to the premature failure.
-Ⅴ．Repair method applied : To help the life of these hood panels, Improved water quality and reduced internal corrosion would be recommended.
&br;
CENTER:&aname(fig4);&attachref(BOFhoo75.jpg);&br;
&br;
CENTER:Fig.4 Thermal fatigue cracking on gas-side surface.
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&br;
CENTER:&aname(fig5);&attachref(BOFhoo76.jpg);&br;
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CENTER:Fig.5 Thermal fatigue cracking on external surface.
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&br;
CENTER:&aname(fig6);&attachref(BOFhoo77.jpg);&br;
&br;
CENTER:Fig.6 Corrosion fatigue cracking on water-side surface.
&br;
**Reference [#lc48dd2a]
>Hargrave R. E. Common failure mechanisms in BOF hoods. '''Iron and Steel Engineer,''' Nov., 1996,22-28.
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終了行:
*BOF hood [#v8aeaf8c]
**1.BOF hoods system [#sc409258]
>In the BASIC oxygen furnace (BOF) process, oxygen is blown under pressure into a furnace bath containing hot metal, steel scrap and fluxes to produce steel. BOF hood systems are designed to capture gases and particulates generated in the furnance, cool the effluents and convey them for cleaning prior to discharge into the atmosphere. A schematic of a typical closed-hood system is shown in [[Fig.1>#fig1]].
>A worldwide survey of BOF hood systems completed in 1981 revealed that 30 % of all hood failures result from thermal stress cracking. Another 30 % are caused by thinning of hood wall tubes because of erosion or corrosion mechanisms. The remaining hood failures are related to poor water quality, localized superheating, skull buildup on the furnace lip ring or other reasons.
&br;
CENTER:&aname(fig1);&attachref(BOFhoo72.jpg);&br;
&br;
**2.Failure mechanisms [#s2f54794]
>Thermal fatigue cracking occurs when the hood wall is repeatedly cycled in response to rapid changes in effluent gas temperature. Thermal expansion and contraction of the hood wall create fluctuating tensile and compressive stress in the hood.
>Typically, thermal fatigue produces circumferential cracks along the crown of a tube or hot side of a panel. Cracking may initiate on the gas-side, water-side or on both surfaces simultaneously.
**3.Hood repair [#i2e91678]
>Hood repairs are commonly made on an emergency basis, as scheduling allows, so that the BOF can return to normal operations as soon as possible. During repair operations, cracks are quickly patch welded. Panels of tubes are often replaced only after it has been determined that the area is not repairable because of the extent of cracking damage or wall thinning ([[Fig.2>#fig2]]).
>However, there was report that cracks originated at the toes of repair welds ([[Fig.3>#fig3]]). Therefore, when possible, it is recommended that a tube or panel be replaced rather than weld repaired.
&br;
CENTER:&aname(fig2);&attachref(BOFhoo73.jpg);&br;
&br;
CENTER:Fig.2 Welded repair and thermal fatigue cracking on gas-side surface.
&br;
&br;
CENTER:&aname(fig3);&attachref(BOFhoo74.jpg);&br;
&br;
CENTER:Fig.3 Welded repairs and fatigue cracking on gas-side surface.
&br;
**4.Case [#qde28128]
-Ⅰ．Water system : Open-recirculation water system was used in this excess air hood.
-Ⅱ．Circumstances of Repair :
>It was reported that many of the lower hood panels were replaced every 18 months to minimize unscheduled repair. However, two lower panels were failing repeatedly in less than one year.
-Ⅲ．Crack location : Membrane carbon steel tube hood panels.
>These panels were located in a portion of the lower hood, away from the highest heat flux zone.
>Numerous circumferential cracks were present on the gas-side surface, with at least one through-wall crack ([[Fig.4>#fig4]]). Inspection of the internal surface revealed a heavy water-deposit and transverse cracking.
>The external cracks were straight and oxide-filled ([[Fig.5>#fig5]]) and the internal cracks initiated at the base of pit ([[Fig.6>#fig6]]).
-Ⅳ．Cause of fatigue crack : Abnormally large temperature fluctuations.
>During a blow when peak gas temperatures were achieved, reduced heat transfer across the tube wall created excessive metal temperatures. Between heats, this area would cool significantly because this panel did not receive residual radiant heat from the furnace.
>The heavy buildup of water-side deposit and corrosion product contributed to the premature failure.
-Ⅴ．Repair method applied : To help the life of these hood panels, Improved water quality and reduced internal corrosion would be recommended.
&br;
CENTER:&aname(fig4);&attachref(BOFhoo75.jpg);&br;
&br;
CENTER:Fig.4 Thermal fatigue cracking on gas-side surface.
&br;
&br;
CENTER:&aname(fig5);&attachref(BOFhoo76.jpg);&br;
&br;
CENTER:Fig.5 Thermal fatigue cracking on external surface.
&br;
&br;
CENTER:&aname(fig6);&attachref(BOFhoo77.jpg);&br;
&br;
CENTER:Fig.6 Corrosion fatigue cracking on water-side surface.
&br;
**Reference [#lc48dd2a]
>Hargrave R. E. Common failure mechanisms in BOF hoods. '''Iron and Steel Engineer,''' Nov., 1996,22-28.
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