The thickness of preheater expansion joint castable is not a fixed value because it depends on many factors, such as design pressure, design temperature, material, manufacturing process, etc. In making expansion joints, the material of each layer is usually relatively thin in order to reduce the stress caused by lateral pressure and improve the flexibility of the expansion joints.

However, the specific castable thickness should be determined according to specific design requirements and process parameters. During the pouring process, it is necessary to ensure that the castable can sufficiently fill the internal space of the expansion joint and is compatible with the material of the expansion joint to ensure the performance and service life of the expansion joint.

Therefore, it is not possible to give a specific numerical value for the castable thickness of the preheater expansion joint. It is suggested that the thickness of castable should be determined according to the specific design requirements and process parameters in the actual manufacturing and use process, and sufficient tests and tests should be carried out to ensure the performance and use effect of the expansion joint.