Precast-Shape Design and Manufacturing
In order to realize the true benefits to be gained from the use of precast shapes, a thorough knowledge of how the shape system will be used and installed in the field is an absolute requirement during the design phase. The successful design and manufacture of a high-performance refractory-shape system requires a unique understanding of refractory materials, manufacturing, anchoring systems and construction practice. Dimensional tolerances, construction sequencing, lifting and handling capabilities at the site, anchoring facilities and the actual service demands within the refractory-lining environment are all factors that must be well known before the shape is designed.Precast-shape manufacturing inherently requires the use of a mold or pattern to form the shape. There are several methods for mold-making that are routinely employed, and the type of mold construction and materials used depend on the size, complexity, dimensional tolerances required in the shape and sometimes the quantity of shapes required. For simplistic shapes with loose dimensional tolerances (+/-1/16 inch), plywood or metal-fabricated forms can be used. Toward the other extreme, some shapes may require very tight tolerances, which necessitate the use of a more sophisticated mold made from wood, plastic or metal. These molds may be of the type made by a foundry pattern maker or machine shop.
Another factor in the design of a precast shape involves the schedule and sequencing of the actual field installation. The shape design must take into account job accessibility, the lining components already in place when the shapes are to be installed and how the shape can be handled physically on the job site. Weight and lifting limitations must be considered and planned for, as well as the type of access available into the furnace or vessel. If necessary, lifting lugs or other fixtures can sometimes be incorporated into the shape design.
The design of the anchoring system to be used in the shape is of tremendous importance. In addition to the normal considerations of alloy type and anchor size, the precast-shape design must also consider all alternatives for attaching the shape to the structure. Numerous methods can be used, including threaded-stud attachments through the wall, welded fixtures or bolted assemblies.
Perhaps most importantly, the proper refractory material must be selected to suit the demands of the application. Factors such as the desired temperature profile through the lining, expected mechanical stresses, potential chemical attack on the lining, erosion mechanisms and expansion allowance must all be understood prior to selecting a material to be used in the precast shape.
A well-equipped precast manufacturing facility should include high-energy, large-capacity mixers, automated mixing stations with conveyors for material delivery, vibration tables, digitally controlled water addition, mixing-time controllers and adequate lifting capabilities for large shapes. Firing of shapes is accomplished with a digitally controlled furnace with burners capable of firing to 1300°F minimum. In-house mold/pattern fabrication capabilities and CAD-generated drawings for design assistance should also be expected.
Fig. 1. Precast shape being installed
Benefits from Material-Property Enhancement
Regardless of how complex or sophisticated the refractory castable that is selected for an application, the physical properties of the material can be drastically reduced if care is not taken during the mixing, pouring and curing processes. Particularly with the use of more complex refractory castables to solve specific wear issues, installation variables become even more critical to the performance of a lining. Unfortunately, lining quality is often compromised by field conditions during material placement. Project schedules, crew skill levels, equipment availability, job-cost pressures or other demands can sometimes have an impact on proper refractory installation. Improper water addition, mix-time variations, over- or under-vibration and improper curing can drastically affect material quality. With precast shapes – cast in a controlled shop environment – the physical properties of a castable can be more fully optimized.Initial drying and firing of a refractory castable is a critical installation variable that can influence lining performance. Precast shapes are typically fired in a digitally controlled furnace prior to shipment, ensuring that the refractory manufacturer’s recommended bake-out schedule is closely followed. Since the shapes are fired slowly from all sides, the moisture is removed through the entire thickness of the shape in a controlled manner. The temperature to which the shape is fired can optimize the physical properties of the material through the entire thickness of the shape – not just the hot-face surface. This results in a truly homogeneous lining. Micro-cracks within the shape, which are often introduced during field bake-out but may go unnoticed, may also be reduced since the initial firing is more controlled.
In service, linings comprised of precast shapes often see less stresses and cracking due to the independent, “floating” nature of the lining. The performance of the lining can also be more predictable, resulting in better opportunities to plan for maintenance and repairs (Fig. 1).
Fig. 2. Precast for roof application
Benefits from Installation Logistics
Other major benefits to be gained from the use of precast refractory shapes are related to simplified installation and repair logistics, which can lead directly to reduced costs and shorter downtimes. With the use of precast shapes, forming labor, materials, equipment costs, actual placement time and expense, and associated costs during form removal, curing and cleanup are all eliminated. These costs are shifted back to the manufacturer of the shape, who can absorb them much more efficiently when spread over his entire production capacity.Refractory installation contractors have begun to consider precast refractory shapes much like they do any other premanufactured item such as block insulation, ceramic-fiber blanket, anchors, etc. These items can be bought and then re-sold as a component of their installation projects.
Whenever any portion of refractory repair work can be completed prior to crews being on-site, costs are automatically reduced. Installation contractors have also found that the use of precast shapes can often give them a substantial advantage in competitive bid situations. With the use of precast shapes, crew sizing can be minimized. Speed of installation is another obvious benefit to both the installer and the owner, resulting in reduced costs due to shorter job duration (Fig. 2). Material usage is also reduced when compared to other installation methods such as guniting, where as much as 45% of extra material is required to compensate for rebound and other job losses. Environmental hazards such as dusting and tripping hazards associated with equipment and hoses are also reduced substantially, if not eliminated.
Future repairs also become much more economical and quicker to accomplish. Repair areas can be isolated to just the immediate wear area within the boundaries of a shape. Anchor attachment points can typically be reused. Replacement shapes – purchased early and kept as spare parts on-site – can be easily installed in a fraction of the time required for conventional repair methods.
The initial bake-out of a new refractory lining on-site can be a very expensive and time-consuming component of a refractory repair project. The use of precast and prefired refractory shapes can sometimes reduce or even eliminate the need for an extensive initial bake-out. If an entire repair is made with a prefired system, normal furnace start-up schedules can be used without the fear of steam spalls or other damage during the initial heating. Bake-out of multi-component linings, which may include a combination of precast shapes and other materials placed in the field, can often be reduced by the prefiring of the castable shapes, particularly if that material would have been the critical item determining the bake-out schedule. This can have a positive impact on not only job costs but in reducing downtime as well.
Precast refractory shapes will continue to be a growing specialty in the refractory industry in coming years. With the improved quality that can be achieved through controlled manufacturing processes, their expanding use will play a major role in improving refractory-lining performance and reducing maintenance costs across all industries.IH
For more information:TFL, Incorporated is a supplier of refractory materials and related services located in Houston, Texas, and is a leading manufacturer of custom-designed precast shapes. web: www.tflhouston.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: precast refractory shape, refractory castable, burner block
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