“What is wrong with my furnace refractory life? I’m not operating as much. Shouldn’t the refractory last as long, or longer, than when we we’re running full out?”
The answer is usually no. Thermal cycling is refractories enemy. There are refractory materials that are naturally thermal cycle resistant like fused silica linings. However, maintenance and rebuilds utilize traditional castables which are not as thermal cycle resistant. In general, the following basics can begin the process of improving refractory life until production levels out. Consistent production levels help in reducing the cycling of your refractory lined equipment. If you cannot keep it hot, control the cool down and go slow. Remember, cycling damage can occur on both rapid heat-up and rapid cool down scenarios.
Keep It Hot
Anything you can do to keep a temporarily idled refractory lined unit hot will be your best approach. Certainly not at the normal full preheater operating temperature, but rather a slightly lower (50% -70% of your normal preheat temperature) temperature based upon your material supplier’s recommendation. Keeping the lining hot means the refractory lining will remain expanded (tight). The reduced thermal cycling may prove to be well worth the extra fuel cost. Give it a try!
If your facility is not running a full 24/7 schedule, you may not be able to monitor the preheating equipment. Much of the current equipment is outdated lacking proper safety controls which may lead to “babysitting” a ladle/furnace over a weekend.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 with a 2” to 3” thick stack bonded ceramic fiber lining. Alternatively, covers can be fabricated from a high temperature cloth encapsulating ceramic fiber which is very flexible and forms around contours. Both are mechanically sound and provide for much better heat retention than your refractory requires. Enough residual heat may be maintained to give you a head start on the Monday morning start-up.
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. Take the heat up slow! Exacerbating the issue above, the preheaters and burners can rapidly heat the equipment, easily melting any residual metal before the lining has “soaked” with temperature. This means the refractory may not have had time to deal before any molten metal gets in the joints. This unfortunate phenomenon provides potential joint penetration, breakouts and certainly reduced refractory life. Do not rush the startup.
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. Maybe you need custom fabricated cooling covers. Our combustion team is able to upgrade your preheaters or service outdated preheater systems and controls. Engineers are able to run thermal calculations so you know what to expect at that cold face. Call us for technical assistance to get you through any issues you may have.