When it comes to high-temperature industrial equipment, the longevity and performance of furnace linings largely depend on the proper installation of refractory castables. One of the most overlooked but critical elements during installation is the expansion joint. If not properly designed and constructed, thermal expansion during operation can lead to cracking, bulging, or even collapse of the furnace lining.
Why Are Expansion Joints Necessary in Refractory Castables?
Unlike refractory bricks, where the mortar-filled joints can absorb some thermal movement, castable refractory linings behave as large monolithic structures. When subjected to high temperatures, these linings undergo considerable linear expansion. Without adequate expansion space, the castable will exert force against steel shells, support anchors, or adjacent cast sections-causing mechanical failure.
Expansion joints in monolithic refractory linings relieve internal stress by allowing the lining to expand freely during heating cycles. This preserves the mechanical integrity and prolongs the service life of furnaces, kilns, incinerators, and boilers.
1. Setting expansion joints in refractory castables
For furnace walls built with refractory bricks, the refractory mud in a large number of brick joints can be compressed, which facilitates the expansion of the masonry after heating, so the overall elongation is relatively small. For prefabricated refractories castables or furnace wall linings poured on site, the total elongation after the furnace wall is heated and expanded is relatively large. If there is no appropriate expansion joint, the furnace lining will be bent and squeezed, and the steel structure at both ends of the furnace wall will be deformed.
2. Expansion value of refractory castables
The design of the overall cast refractory lining expansion joint needs to be specified by the design. When the design does not specify the expansion joint value, the average expansion joint value per meter of lining expansion joint for clay or high alumina castables can be taken from the following data:
① Clay castable is 4~6mm;
② High-aluminum refractory cement castable is 6~8mm;
③ Phosphate refractory material castable is 6~8mm;
④ Water glass castable is 4~6mm;
⑤ Portland refractory cement castable is 5~8mm.
3. Expansion joint reservation
① For furnace body assembled with prefabricated components, if the furnace body is short, there is no need to leave additional expansion joints between the prefabricated parts, and only appropriate gaps need to be left at both ends of the furnace body; if the furnace body is long, according to the temperature of each section of the furnace body, an expansion joint can be left every 5~10m along the length of the furnace body, and the joint can be filled with asbestos rope soaked in clay slurry to prevent smoke and fire from emitting from the furnace at low and medium temperatures.
② Furnace body cast on site. The spacing and width of expansion joints for furnace body cast on site refractory castables. Please refer to Table 7-1. The values in the table are applicable to clay and high-alumina castables.
③ Fixation of fillers. The materials of expansion joints in castables should be fixed in place before pouring or carefully filled during pouring. When pouring in blocks, thecastables should be divided into several blocks according to their expansion joints, and separated and poured from the expansion joints with templates, and the joint fillers should be embedded after initial setting.
④ Prevent displacement. In order to prevent the displacement of expansion joint fillers or local adhesion of castables, the filler layer should be accurately cut and laid tightly. If necessary, nails can be nailed into the castable to stabilize the filler layer.
⑤ Isolation measures. When the filler layer is separated in a cold state and in contact in a hot state under the construction environment, that is, it becomes a sliding surface, it is advisable to add an isolation layer to prevent the displacement of the filler, prevent leakage from sticking or block the expansion gap.
4. Quality requirements
(1) The location, structure and filling of the expansion joint must meet the design requirements.
(2) There must be no debris such as broken bricks and mud in the expansion joint. For this reason, expansion joints should be left during masonry and filled with foam plastic, cardboard or wood chips of appropriate size to prevent debris from falling in. The filling in the joint must be filled tightly and must not be loose.
(3) Measures should be taken to prevent fire from spreading in the expansion joint.
