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Safe Operation of Endothermic Atmospheres

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Safe Operation of Endothermic Atmospheres D. SCOTT MACKENZIE, PH.D., FASM HOT SENIOR RESEARCH SCIENTIST–METALLURGY SEAT HOUGHTON INTERNATIONAL INC. Atmosphere Safety Hot Seat: Atmosphere Safety D. Scott MacKenzie, Ph.D., FASM This month the author describes typical scenarios where endothermic atmospheres can result in Hot Seat: Atmosphere Safety Table 1. Typical compositions of endothermic atmosphere produced by natural gas, propane or fires and serious explosions,Safe and operation how they can beof avoided. endothermic atmospheres This month the author describes typical scenarios where endothermicnitrogen atmospheres-methanol. can result in By D. Scott MacKenzie, Ph.D., FASM Natural Nitrogen- fires and serious explosions,Typical and scenarios how they where can be endothermic avoided. Gas Species Formula Propane Senior research scientist - metallurgy, Houghton International Inc. Gas Methanol By D. Scott MacKenzie, Ph.D., FASM atmospheres can result in fires and serious Senior research scientist - metallurgy, Houghton International Inc. Carbon Monoxide CO 20% 23% 20% In the last column, weexplosions, discussed the and importance how they of canmaking be avoided.sure that the endothermic atmosphere Hydrogen H2 40% 31% 40% was the right composition to ensure proper heat treatment. When neutral hardening, if the In the last column, we discussedn the last column, the importance we discussed of the making importance sure of makingthat the sure endothermic that Carbon atmosphere Dioxide CO2 0.30% 0.30% 0.30% atmosphere is incorrect,the decar endothermicburizatio atmospheren or carburization was the right can composition result. If to carburizing, ensure an incorrect was the right compositionI to ensure proper heat treatment. When neutral hardening,Water if Vapor the H2O < 0.1% < 0.1% < 0.1% atmosphere could resultproper in soot, heat treatment.or incorrect When carbon neutral profiles. hardening, if the atmosphere atmosphere is incorrect, decarburization or carburization can result. If carburizing,Methane an incorrect CH4 < 0.1% - - is incorrect, decarburization or carburization can result. If carburiz- atmosphere could result in soot, or incorrect carbon profiles. Propane C3H8 - < 0.1% - In this column, we areing, going an incorrect to discuss atmosphere atmosphere could result safety. in soot If you or incorrect recall fromcarbon last month, the profiles. Nitrogen N2 40% 46% 40% composition of a typical endothermic atmosphere produced from natural gas, nitrogen-methanol In this column, we are goingIn this to column, discuss we atmosphere are going to discuss safety. atmosphere If you recall safety. from If you lastTable month 1: Typical, the compositions of endothermic atmosphere produced by natural or propane is found in Table 1. One thing to examine is the amount of hydrogen present. It is 30- composition of a typicalrecall endo fromthermic last month, atmosphere the composition produced of a typicalfrom naturalendothermic gas, nitrogengas, propane-methanol or nitrogen-methanol. 40 percent of the total atmospheregas present. produced Needless from tonatural say, gas,hydrogen nitrogen-methanol, is an extremely or pro- explosive gas. or propane is found in Table 1. One thing to examine is the amount of hydrogen present. It is 30- Methane and propane paneare also is found explosive in Table 1.under the proper conditions. The reactions are 40 percent of the total gas present. Needless to say, hydrogen is an extremely explosive gas. predominately: One thing to examine is the amount of hydrogen present. It is 30 Methane and propane percentare also to 40explosive percent of underthe total the gas properpresent. Needlessconditions. to say, Thehydrogen reactions are predominately: is an extremely explosive gas. Methane and propane are also explosive under the proper conditions. The reactions are predominately: $ ' ' Further reactions establish the proper2 equilibrium: + ↔ 2 + 4 Further reactions establish the proper equilibrium: $ ' ' Further reactions establish the proper2 equilibrium: + ↔ 2 + 4 Figure 1. Fire Triangle. If any of the legs are absent, a fire or explosion cannot occur. These reactions are strongly endothermic, meaning that it takes ' ' ' These reactions are stronglyenergy toendothermic, cause the reaction+ meaning to⇔ go forward.that +it takes This alsoenergy means to thatcau se the reaction to go forward. This also meansthe reactions that the are reactions strongly exothermicare' strongly to' reverseexothermi' direction.c to Itreverse is this direction. It is this These reactions are stronglystrong exothermic endothermic, reaction + meaning and ⇔ the resultantthat +it takes release energy of energy to thatcau se the reaction to go strong exothermic reaction, and the resultant release of energy, that we must avoid.Figure Carbon 2: Typical integral quench furnace found in a heat treating shop. (Courtesy forward. This also meanswe must that avoid. the reactions Carbon monoxide are strongly (CO) is increasingly exothermi stablec to reverse at high direction. It is this monoxide (CO) is increasingly stable at high temperatures, but it becomes unstableSurface at Combustion, lower Maumee, Ohio.) strong exothermic reaction,temperatures, and the but resultant it becomes release unstable of at energylower temperatures., that we must avoid. Carbon temperatures. monoxide (CO) is increasinglyAtmospheres stable containing at high a temperatures,total of more than but 5 percent it becomes of the com unstable- zone atfrom lower the quench vestibule. An outer door with flame curtain bustible gases (H , CO, and CH , C H , CH OH) constitute a potential and pilot separates the outside from the quench vestibule (Figure 3). temperatures. 2 4 3 8 3 Atmospheres containingfire aand total explosion of more hazard. than It is 5 generally percent considered of the combustible that endothermicFigure gases2. Typical (HPrior2 ,integral CO, to loading and quench the furnace furnace, found the furnacein a heat is treatingfilled with shop. endothermic Photograph courtesy atmospheres are considered explosive at temperatures belowSurface 750°C Combustion, gas. A positive Maumee, partial OH. pressure in the furnace and quench vestibule CH4, C3H8, CH3OH) constitute a potential fire and explosion hazard. It is generally considered Atmospheres containing(1,382°F). a total This of meansmore thanthat any 5 percent atmosphere of the combustible gases (H2, CO, and prevents the infiltration of oxygen. As loads that endothermic atmospheres are considered explosive at temperatures below 750°C (1382°F). CH4, C3H8, CH3OH) constitutebeing admitted a potent to furnacesial fire below and 750°C explosion may hazard. It is generally considered are introduced into the furnace, the outer This means that any atmosphere be admitted to furnaces below 750°C may result in a fire or that endothermic atmospheresresult in a fireare orconsidered explosion. explosive at temperatures below 750°C (1382°F). door opens. As the door is opened, a limit explosion. The fire triangle (Figure 1) is an excel- switch senses that the door is opening and This means that any atmosphere be admitted to furnaces below 750°C may result in a fire or lent way of understanding the cause and initiates the flame curtain. The flame curtain explosion. The fire triangle (Figureprevention ) is an excellentof fires and way explosions. of understanding If any the cause and prevention of fires is sized to extend well past the door opening. of the three legs — fuel, oxygen or ignition Parts are loaded through this flame curtain, and explosions. If any of the three legs – fuel, oxygen or ignition source – are removed, then a The fire triangle (Figuresource ) is — an are excellent removed, thenway aof fire understanding or explo- the cause and prevention of fires and into the quench chamber. The outer door fire or explosion cannot occur. and explosions. If anysion of thecannot three occur. legs – fuel, oxygen or ignition source – are removed, then a is closed. Parts are maintained in the quench There are many ways this can happen. The chamber until the chamber is again pressur- fire or explosion cannot occur. There are many ways mostthis commoncan happen circumstance. The most is the common integral circumstance is the integral quench or ized with endothermic gas. Usually, a high sealed quench furnacequench (Figure or sealed1: Fire quench triangle furnace – if any(Figure of the2). legs are absent, a fire or explosion cannot flow of endothermic gas is initiated to rap- In the integral quench furnace, there are idly fill the vestibule and exclude any residual occur.There are many ways this can happen. The most common circumstance is the integral quench or sealed quench furnacethree (Figure of four 1: chambers: Fire triangle the hot– i fzone, any whichof the legs are absent, a fire or explosion cannot oxygen that was not burned as the result of occur. is used for austenitizing parts; the quench the flame curtain. Figure ). In the integralvestibule; quench quenchfurnace, tank; there and are possibly three ofan four chambers: the hot zone which is While the chamber is rapidly filling the atmosphere cooling zone above the quench Figure 1: Fire triangle — if any of the legs are vestibule with endothermic gas, some resid- used for austenitizing parts; the quench vestibule; quench tank; and possibly an atmosphere Figure ). In the integralvestibule. quench An furnace, inner door there separates are three the ofhot four absent, chambers: a fire or explosion the hot cannot zone occur. which is ual oxygen may be present. The chamber is used for austenitizing
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