Step 5: Review the installation procedures
The focus of the final step of the process is how the refractory was installed. The following paragraphs detail the most common reasons for refractory material lacking proper strength, making it more likely to fail.
Refractory materials have a fairly short shelf life between their manufacture and installation. One year is the recommended limit for cement-bonded materials used for conventional seals inside boilers. Three months or less is the limit for materials used in high-temperature and high-abrasion areas, such as those inside fluidized-bed or cyclone-fired boilers or inside wet bottom-ash hoppers. Refractory material should always be stored in a dry, well-ventilated space.
Many common industrial compounds can easily contaminate a refractory mix and seriously weaken it. Certain salts reacting with the refractory cement will make the material almost useless. For this reason, most refractory manufacturers recommend using potable water for mixing.
Using the right type of mixer, following proper mixing procedures, and knowing the material's "pot life" (how long it remains viable after mixing) are essential. Using the wrong mixer or pneumatic gun, or even interrupting the operation of the gun for longer than the pot life could compromise the strength of refractory material. In the latter case, material left in the mixer and hoses should be discarded and not re-used. Furthermore, many pneumatically applied refractory materials require pre-wetting prior to their mixing and installation. If the installation contractor used a continuous-feed mixer (in which dry material is added to a hopper, followed by water from a nozzle), pre-wetting would have been impossible.
Hot or cold weather on mixing day also could adversely affect refractory strength. Most refractory manufacturers recommend that the final mix temperature be within a specific range, typically 40F to 90F. Obviously, it likewise is very important to protect the installed refractory from freezing for a minimum of 48 hours or until it is thoroughly dry.
Taking the cure
Only after the refractory material has been cured and then dried will it be at its proper strength. Almost all refractory materials (except those that are phosphate-bonded) require both steps. Failure to properly cure cement-bonded refractory material is the primary cause of its failure and lack of longevity. During this critical period, it is recommended that the surface of the refractory be kept moist (either by a curing compound, wet canvas bags, or water sprays) and the surrounding atmosphere kept humid for at least 24 hours.
After the refractory is cured, it is dried out or baked out to remove all water left in the installed material, allowing it to reach its maximum strength. Unlike curing, the performance of this step is not time-sensitive—unless the refractory material is phosphate-bonded. Such materials must be cured and dried at the same time—and within two to three weeks of installation—because the phosphate will immediately begin to absorb moisture from the atmosphere. Eventually, over a period of two or three weeks the material will begin to slump and fall off.
—Gary J. Bases, author of The Bril Book (a complete guide to brick, refractory, insulation, and lagging systems), is president of BRIL Inc. (Copley, Ohio). He can be reached at 330-665-2931 or brilinc@raex.com.
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