3 Key Allowances in Ceramic Refractory Anchoring Design to Prevent Failures
To accommodate expansion movement within anchoring systems,
vital allowances must be incorporated
Research shows that over 40% of monolithic refractory failures are the result of improper anchor design and installation. Failures commonly attributed to the refractory component are instead often caused by shortcomings in the actual anchoring system.
Like any material, refractories will expand and contract due to changes in temperatures. Expansion in refractories can be broken down into two different types: Permanent Linear Change (PLC), and reversible expansion. PLC is a type of expansion stress that occurs during the initial heat-up when moisture is released from the refractory. Reversible expansion happens when the refractories go through thermal cycling, or in more detail, from an extremely high temperature to room temperature, and then back again to high temperatures.
Both types of expansion, PLC and reversable, are accounted for in mathematical terms as the rate of expansion, per inch, for every degree that the temperature increases. Engineers and refractory installers can find expansion rate numbers listed on technical data sheets.
Ceramic anchors, as an integrated part of the overall refractory lining, will move with the monolithic refractories as it experiences these thermal cycling and initial heat-up PLC stresses.
So, a critical consideration when designing an anchoring layout for a suspended refractory application is factoring in movement of the anchor. If not accommodated, this movement can cause the ceramic anchor to fail, leading to a section of refractory dislodging. In the extreme, refractory sections could fall out and leave a major gap or hole in the lining.
We Live in a Three-Dimensional World
Thermal expansion happens in all three dimensions: length, width, and height. For anchor designing purposes, the “height” aspect refers to the thickness of the installation material and is a minor consideration when designing for expansion allowance. Instead, the larger focus should be on the other two anchoring dimensions since the installation is far more likely to move in the length and width directions. Therefore, it is critical that allowances be made in the anchoring system to accommodate this movement.
Illustrated in Figure #1, the forces that act on a ceramic anchor in one of the directions of expansion. Arrows signify the pressure distributed across the various faces of a ceramic anchor, while the top of the anchor is constrained in place. Note that there is a concentration of stresses at approximately the point where an anchor clip would normally contact the anchor. If this clip or the beam that supports the clip does not allow the anchor to move to relieve this stress, the anchor will very likely fail.
Preventing Anchoring System Failures
To create a strong anchoring system that accommodates movement, here are three simple steps that should be followed in the design and planning of a ceramic anchoring system:
- Determine how much expansion the installation requires, including reversible expansion due to cycling, and the PLC that happens only during initial bake-out.
- Choose an appropriate expansion scheme.
- Select an anchoring system that will accommodate these expansion requirements, including … number of anchors, anchor spacing, anchor shape, size and material. This point is commonly overlooked.
A strong anchoring system is key to maintaining monolithic refractory lining integrity and preventing cracking, dislodging, or a total structural collapse. And essential to a strong anchoring system is movement accommodation. By carefully determining expansion, and then designing the suspension system to accommodate for movement, you’ll greatly improve refractory life and prevent any possible catastrophic refractory lining failures due to a rigid and poorly designed anchoring system.
For more information on optimal ceramic refractory anchoring design, including a short video, check out the Plibrico Company’s YouTube Channel: https://plibri.co/Plibrico_YouTube. You can also contact Plibrico at 312-337-9000, or firstname.lastname@example.org