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“KEEP IT HOT” IMPROVING REFRACTORY LIFE DURING CYCLICAL BUSINESS CONDITIONS

The first comment to the above statement is usually: “I’m not operating as much, so shouldn’t the refractory last as long, or longer, than when we we’re running full out?”  The answer is usually no, with the culprit being refractories enemy, thermal cycling. For the sake of today’s discussion, I am not focusing on naturally thermal cycle resistant materials, like fused silica linings, rather on traditional castable utilized more as maintenance and rebuild items. In general, the following basics can begin the process of improving refractory life until production levels “Keep it hot” (increased production helps in reducing the cycling of your refractory lined equipment). If you cannot keep it hot, cool it down slow and controlled. Remember, cycling damage can occur on both rapid heat-up and rapid cool down scenarios. 

  • Keeping it hot! Anything you can do to keep a temporarily idled refractory lined unit hot (temporary means a few days or over a weekend), will be your best approach. Certainly not at the normal full preheater operating temperature, but rather a slightly lower (say 50% -70% of your normal preheat temperature) temperature based upon your material supplier’s recommendation. Keeping the lining hot, expanded (tight), with reduced cycling, may prove to be well worth the extra fuel cost and attention. Give it a try. 
  • Cannot keep it hot? If your facility is not running a full 24/7 schedule, you may not be able to monitor the preheating equipment, noting that much of the current equipment is outdated (lack safety controls), or quite simply, you may not be able to   justify “babysitting” a ladle/furnace over a weekend. In this case, the slowest possible controlled cool down is recommended. This means more than a 1” piece of ceramic fiber draped over a well or an open ladle. Consider the use of fabricated lids and sewn covers for these units. These covers are easily fabricated from hex mesh (for weight reduction) with a 2” to 3” thick stack bonded ceramic fiber lining or fabricated from a high temperature cloth encapsulating ceramic fiber ( very flexible and forms to many contours). Both are mechanically sound and provide for much better heat retention than your refractory requires. You may even find that enough residual heat is maintained to give you a head start on the Monday morning start-up.
  • Now for the heat-up (This discussion is being applied to refractory lined equipment that has been in service and is subject to thermal cycling): First, and most important, cold clean these units as thoroughly as possible. Residual slag, large bottom heals, and metal fins are all potential troublemakers when left in the unit during heat up, leading to these next words of advice, TAKE IT SLOW!! Exacerbating the  issue above, the  preheaters and burners   can rapidly heat up most any piece of  refractory lined equipment (especially smaller units), easily melting any residual metal before the lining has “soaked” with temperature and long before the refractory  has had a chance to expand and seal any of the  open joints from molten metal. This unfortunate phenomenon provides potential joint penetration, breakouts and certainly reduced refractory life. Do not rush it if you do not have to.
  • Soak, Soak, Soak!: Bring the equipment up slowly in temperature (say 100 /hr.), with one or two holds in the schedule. This allows heat to “soak” into the lining. To “soak”, use 1 hour per inch of thickness as the rate that temperature can soak during holds, heading towards the cold face. Once you reach the temperature (say 1500° f), use an (inexpensive) optical pyrometer to establish your baseline shell temperature for future cyclical conditions and proper preheating of your equipment. This baseline will also help as the lining thins and emits more heat loss, providing heads-up to a future potential rebuild or a large metal fin ready to cause trouble. And remember, no burner flame impingement. 
  • How can Onex help? We can provide proper cold clean and patching services, fabricated cooling covers, preheater upgrades,  service to outdated preheater systems and controls, thermal calculations (so you know what to expect at that cold face) and technical assistance to get you through any issues you may have. We are here to help!
  • How can Onex help? We can provide proper cold clean and patching services, fabricated cooling covers, preheater upgrades, service to outdated preheater systems and controls, thermal calculations (so you know what to expect at that cold face) and technical assistance to get you through any issues you may have. We are here to help!

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