Expansion Joint Layout in a Large Castable-Lined Duct
A rectangular flue-gas duct 14 ft wide by 9 ft high carries 1,600°F gas from a process heater to a stack. The carbon steel shell is lined with 5 in. of dense 70% alumina castable, anchored with 310 stainless steel studs on 9-in. centers. The duct runs 60 ft between supports and includes one 90° turn. The specification required 'expansion joints where required by the installer,' and the installer provided none because the lining was cast in one continuous pour. Eleven months after startup, cracks 1/4 in. wide have opened through the lining at roughly 18–22 ft intervals and diagonally at the corner. Infrared scans show hot spots on the shell at two of the cracks. Installation ambient was 70°F.
1. Determine whether the cracking is expansion-related or anchor-related, and show the calculation that supports your conclusion.
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The pattern is expansion-related, not primarily anchor failure. A dense 70% alumina castable has a mean coefficient of thermal expansion of roughly 4.5 × 10⁻⁶ in/in/°F over this range. The temperature rise from 70°F to 1,600°F is 1,530°F, so free expansion strain is about 4.5 × 10⁻⁶ × 1,530 = 0.0069 in/in, or 0.69%.
Over the 60-ft length (720 in.), that is 720 × 0.0069 = about 5.0 in. of free expansion. The carbon steel shell expands more: 6.5 × 10⁻⁶ × 1,530 = 0.0099 in/in, or about 7.1 in. over the same length. The shell therefore grows roughly 2 in. more than the lining. The anchors tie the lining to a shell that moves farther than the lining wants to, and the lining itself cannot accommodate 5 in. of expansion. It relieves the strain by cracking in tension.
The 18–22 ft crack spacing is a natural crack spacing, not a designed joint spacing. Hot spots confirm the cracks penetrate the full thickness and provide a gas path. Anchor spacing of 9 in. is plausible, but anchors cannot substitute for expansion joints.
2. Specify a joint layout and joint detail for this duct, including a numerical joint width.
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Divide the 60-ft run into panels of about 12 ft, giving four joints plus joints at each end and at the 90° turn. A 12-ft panel expands by 144 in. × 0.0069 = about 1.0 in. Therefore specify a nominal 1-in. compressible ceramic fiber expansion joint board, full lining thickness, placed against the shell and extending to the hot face.
The joint detail should include: fiber board of the correct temperature rating and density, secured to the shell so it stays in place during casting; a slip plane so the castable on each side can close against the board; and a labyrinth or overlapping fiber flap at the hot face so the joint does not become a direct gas bypass to the shell. Joints should also be placed at the corner, at each support, and where the duct penetrates a wall.
Do not simply caulk the existing cracks. The joint width should be confirmed by a thermal expansion calculation submitted by the installer and reviewed by the engineer, because a joint that is too narrow closes and spalls, while one that is too wide creates a gas path and turbulence.
3. What installation and specification changes would you require before the repair is cast?
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First, replace the vague 'as required by installer' language with a calculated joint schedule showing locations, widths, and details, and require the installer to submit the calculation for approval. Second, require the lining to be cast in panels separated by the joint board, not in one continuous pour.
Third, review the anchor system. Because the shell expands about 2 in. more than the lining over 60 ft, anchors must tolerate some differential movement. Verify stud type, length, and pattern for 1,600°F service, and confirm that 310 stainless is suitable for the atmosphere; if the gas contains sulfur or other corrosive species, confirm the alloy choice.
Fourth, control the castable installation: verify water addition, mixing time, vibration, and curing. Fifth, require an as-built inspection with hammer testing and infrared thermography after the first heat-up. Finally, consider whether the corner should be a separate panel, because the diagonal crack indicates concentrated restraint at the turn.