Explosive Spalling During Initial Heat-Up of a Castable-Lined Reactor
A 9-ft-ID reactor is lined with 6 in. of 70% alumina low-cement castable, about 12 tons. It cures 48 hours at 70°F and forms are stripped at 24 hours. The dryout schedule is 100°F/h to 350°F, a 4-hour hold, 100°F/h to 1,000°F, a 6-hour hold, then 100°F/h to the 1,750°F operating temperature. At about 620°F, operators hear a loud pop and see steam venting from a manway. After cooldown, a 3 ft by 2 ft spall about 2 in. deep is found near the manway. The lining has no intentional vents, and anchors are 3/8-in. studs on 8-in. centers.
1. Explain the spalling mechanism and why the dryout schedule failed to prevent it.
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This is explosive steam spalling. A 6-in. low-cement castable has low permeability and contains several percent free water after curing. As the hot face is heated, water turns to steam, and the steam must travel through the lining to the cold face or to a vent. With no vents and a thick lining, the steam cannot escape fast enough. The hot face also dehydrates and densifies, further reducing permeability and sealing the lining.
Vapor pressure builds inside the lining until it exceeds the tensile strength of the castable, and a section spalls explosively. The spall occurred at about 620°F, which is consistent with the release of chemically bound water from cement hydrates plus trapped free water.
The schedule failed because 100°F/h is too fast for a 6-in. lining with no vents. A 4-hour hold at 350°F is not enough to dry the full thickness; the critical range for steam generation and hydrate decomposition extends from about 220°F to 800°F. The schedule moved through that range with only one short hold and no venting path.
2. Propose a revised dryout schedule and any lining modifications you would require for the repair.
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For a 6-in. low-cement castable, I would use a slower schedule with holds in the critical range: heat at 25–50°F/h from ambient to about 250°F and hold 6–8 hours; continue at 25–50°F/h to about 500°F and hold 6 hours; then 50°F/h to 800°F and hold 4 hours; then 75–100°F/h to operating temperature. The exact schedule should be checked against the manufacturer's recommendations and the measured shell temperature. Steam should be allowed to escape freely, and the heat-up should be monitored with thermocouples through the lining thickness, not only on the shell.
I would add vents to the repaired lining: 1/4-in. holes on a 12-in. grid to about two-thirds of the lining depth, or vented anchors, to give steam a path. Vents must not penetrate the full thickness and create a gas path to the shell. The repair area should also be checked for anchor condition and for whether the spall exposed anchors.
3. Describe the repair procedure for the spalled area.
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Remove the spall to sound material using light chipping or grinding, and cut the edges square or slightly undercut so the patch is mechanically keyed. Do not feather the edges. Remove all loose material, dust, and oil, and inspect the anchors in the cavity; replace any damaged or corroded anchors and confirm the anchor pattern.
Pre-wet the cavity according to the patch material instructions. Use the same castable or a compatible patching material with demonstrated bond and strength at temperature. Place and compact the patch fully, cure it under the same controlled conditions as the original lining, and then dry it out with a local controlled heat-up that follows the same slow schedule in the critical range.
After dryout, inspect the patch with hammer testing and confirm that the vent holes are open. If the spall was caused by trapped steam, the repair is incomplete unless the venting and schedule changes are applied to the whole lining, not only the patch.