Gunned Lining Density Dispute on a Regenerator
A 22-ft-ID FCC regenerator receives a 4-in. gunned 90% alumina lining. The specification requires 155 pcf minimum density and 6,000 psi cold crushing strength. The contractor guns from scaffold with the nozzle about 5 ft from the surface at a 45° angle and reports 6% rebound. An 18-in. square test panel gunned at the start gives 143 pcf. Two 2-in. diameter cores cut from the shell give 141 and 146 pcf. Visual inspection shows faint horizontal laminations in two areas. The turnaround has 3 days remaining, and the vessel must be back in service.
1. Is the lining acceptable? Support your answer with the data and with the significance of density in this service.
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On the stated specification, the lining is not acceptable. Both the test panel at 143 pcf and the cores at 141 and 146 pcf are below the 155 pcf minimum. The cores are 6–9% below specification, which is not measurement scatter. The laminations are additional evidence of poor gunning practice.
Density matters in a regenerator because the lining sees hot catalyst-bearing gas and thermal cycling. Low density generally means higher porosity, lower strength, lower erosion resistance, and higher permeability. Higher permeability allows gas and catalyst fines to penetrate the lining and can lead to anchor corrosion and spalling. The 6,000 psi cold crushing strength requirement is consistent with a dense, erosion-resistant lining; a lining at 143 pcf is unlikely to meet it.
The fact that the test panel is also low is important. Panels are often gunned more carefully than production lining, so a low panel suggests the production lining is at least as low, not better.
2. What does the density shortfall indicate about the contractor's gunning technique?
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The density shortfall points to excess water at the nozzle, poor compaction, and rebound entrapment. Gunning at 5 ft is at the far end of the acceptable distance range; the further the nozzle from the surface, the more the stream slows and the more water must be added to keep the material flowing. A 45° impingement angle also increases rebound and reduces compaction compared with a perpendicular nozzle.
The reported 6% rebound is not extreme, but the laminations suggest that rebound was not fully cleared or that the nozzleman made passes that trapped partially set material. Laminations act as weak planes and gas paths. The low test panel indicates the problem is systematic, not a single bad area, which implicates water control and nozzle technique rather than one localized defect.
The contractor should have adjusted water at the nozzle, held the nozzle perpendicular at 3–4 ft, and gunned in a pattern that avoids trapping rebound.
3. Given the 3-day turnaround, what testing and disposition would you require before accepting the lining?
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I would not accept the lining on the present data. I would require a minimum of one additional core per 50–100 ft² of lining, or a statistically defensible grid, plus hammer testing of the full surface to detect delamination and laminations. Cores should be tested for density and cold crushing strength, and the results plotted by location so that any acceptable areas can be distinguished from unacceptable ones.
If the additional cores confirm widespread low density, the affected lining should be removed and re-gunned. If low density is confined to identifiable areas, those areas can be removed and replaced, but the contractor must first demonstrate on a new test panel that the revised technique can meet 155 pcf and 6,000 psi.
A fitness-for-service acceptance would require an engineering evaluation showing that the lower density and strength are adequate for the actual thermal and erosion loads. Given that this is a regenerator with catalyst erosion, I would be reluctant to accept a lining 6–9% below density specification without strong justification.