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 , 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 - 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. CO 20% 23% 20% In the last column, weexplosions, discussed the and importance how they of canmaking be avoided.sure that the endothermic atmosphere 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 , 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, 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 proper2𝐶𝐶 equilibrium:𝐻𝐻 + 𝑂𝑂 ↔ 2𝐶𝐶𝐶𝐶 + 4𝐻𝐻 Further reactions establish the proper equilibrium: $ ' ' Further reactions establish the proper2𝐶𝐶 equilibrium:𝐻𝐻 + 𝑂𝑂 ↔ 2𝐶𝐶𝐶𝐶 + 4𝐻𝐻

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 the𝐶𝐶𝐶𝐶 reaction+ meaning𝐻𝐻 𝑂𝑂 to⇔ go 𝐶𝐶forward.that𝑂𝑂 +it takes𝐻𝐻 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,𝐶𝐶 reaction𝐶𝐶 + meaning𝐻𝐻 and𝑂𝑂 ⇔ the 𝐶𝐶resultantthat𝑂𝑂 +it takes𝐻𝐻 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 , 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 thatthan 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 of an 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 28parts; gearsolutions.comthe quench vestibule; quench tank; and possibly an atmosphere severe explosion can result, with hot oil ignited by the hot parts, with the violent explosion of the endothermic gas. As a young engineer, my employer had a small integral quench furnace with a work zone dimension of 18” x 24” x 30”. We were required to check thermocouples and other devices weekly for proper operation. So as not to interrupt production, an electrician briefly powered off the heating elements in the furnace while endothermic gas was flowing. All other power was left on to the furnace. The electrician made his checks of the furnace, and was filling out the necessary paperwork required for continued operation. It took him a bit longer than expected, and the temperature in the furnace con- tinued to drift down to approximately 675°C (1,250°F). He completed Figure 3: Schematic of a typical integral quench furnace, showing the hot zone; quench vestibule; and quench tank. Inner and outer doors are shown. (Schematic his paperwork and started heat to the furnace. Immediately, once courtesy: Surface Combustion, Maumee, Ohio.) the heating elements energized, there was a loud explosion, and the outer door of the furnace was thrown approximately 30 feet. The vestibule was separated from the hot zone by approximately 6 inches. Luckily, no one was injured, but the furnace was a total loss. Immediately after the accident investigation, we implemented a nitrogen purge. This is by far the most effective method to prevent- ing fires and explosions resulting from endothermic atmospheres. In a nitrogen purge system, gaseous nitrogen is used to purge the vestibule (and the furnace) of oxygen. Depending on how leaky the furnace is, at least five to eight volume changes are necessary to purge the atmosphere and oxygen from the furnace. A nitrogen purge is initiated: Whenever the outer door is opened for part removal or part insertion into the vestibule. When the inner door is opened for quenching. Figure 4: Typical nitrogen purge system. Valves, flowmeters During a power loss (intermittent or long term). and other ancillary equipment not shown for clarity. In a low generator atmosphere pressure. In a high generator pressure. cold (below 750°C). Only an ignition source is needed to cause the Due to a flame curtain failure. furnace atmosphere to explode. It is this short period of time, when Due to pilot failure. the furnace atmosphere is unstable, that an explosion can occur. When it is initiated by the operator (hung load, load jams or The resultant explosion has enough energy to throw the outer door other event). many feet from the furnace. Injury to equipment and personnel Once a nitrogen purge is initiated, it was to remain flowing until can occur. This is why the “old hands” never stand in front of an the operator manually initiated atmosphere flow. There was a brief integral quench furnace after it is first loaded. Only after a positive overlap of the atmosphere flows to prevent a partial pressure from partial pressure of gas is present in the vestibule (typically at least forming in the furnace. A typical nitrogen purge system is shown 3-5 volume changes) is the inner door opened and the workload in Figure 4. pushed into the hot zone. While some shops rely on maintaining a high flow of endother- The second way a fire or explosion can occur, is during quenching, mic gas into the quench vestibule to maintain safe operations, a or rather transfer of the load from the hot zone to the quench vesti- nitrogen purge is a safer alternative. bule. In this scenario, after the load is austenitized, the inner door is In conclusion, this article has described typical scenarios where opened. Often, a high flow is initiated prior to the inner door opening. endothermic atmospheres can result in fires and serious explosions. The inner door is opened, and the radiation for the hot zone causes It is never recommended that furnaces be operated at temperatures the gases in the vestibule to expand. The load is transferred into the below 675°C (1,250°F) while filled with endothermic atmospheres. vestibule, and the inner door is closed. As the door is closing, the radia- The use of nitrogen purge, and maintaining a positive pressure inside tion from the hot zone is reduced. This causes the furnace atmosphere the quench vestibule at all times is recommended for safe operation gases to contract, and an atmosphere collapse occurs. This contraction, of endothermic atmospheres and integral quench furnaces. provided that there is inadequate atmosphere flow, causes a partial vacuum inside the vestibule. If the vestibule is not properly sealed, REFERENCES then oxygen can infiltrate the vestibule through leaky door seals or [ 1 ] ISO 13577-3 “Industrial furnaces and associated processing equipment the exhaust gas f lue. At this point in time, we have all three legs of the – Safety,” International Organization for Standardization, Vernier, Geneva, fire triangle present: fuel from the endothermic gases; oxygen that has Switzerland. infiltrated into the vestibule; and an ignition source – the hot parts. A [ 2 ] NFPA 86C, “Standard for Ovens and Furnaces,” NFPA, Quincy, MA

ABOUT THE AUTHOR D. Scott MacKenzie, Ph.D., FASM, is senior research scientist-metallurgy at Houghton International Inc. For more information, go to www.houghtonintl.com.

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