Erosion-Driven Failure in an FCC Cyclone Overhead Line
A 36-in.-ID overhead line downstream of a third-stage FCC cyclone operates at 1,320°F and carries catalyst fines at roughly 18 m/s. The existing 1-in. phosphate-bonded 60% alumina plastic lining has lost about 1/2 in. of thickness after 14 months, with the deepest metal loss on the 45° impingement zone of the elbow. The line cycles between 700°F and 1,320°F about 20 times per year during unit swings. The plant proposes replacing the plastic with a 90% alumina low-cement castable; a vendor alternative is an alumina-silicon carbide gunned mix. The line is 28 ft long, the turnaround window is 9 days, and ambient temperature during installation will be 40–50°F.
1. Evaluate the two candidate materials against the actual damage mechanism, and state what additional data would change your recommendation.
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The damage is thickness loss concentrated at a 45° impingement zone, which is the signature of particulate erosion, not chemical attack or thermal spalling. The existing 60% alumina plastic is comparatively soft and has low erosion resistance. A 90% alumina low-cement castable offers higher strength and hardness but is still an alumina; its erosion resistance improves only modestly. An alumina-silicon carbide gunned mix can provide two to four times the erosion resistance of alumina in fine-particle service because SiC is harder and more resistant to cutting wear.
The trade-off is that SiC-bearing mixes are less forgiving: they are prone to steam spalling during dryout, can oxidize at very high temperature (not a concern at 1,320°F), and may crack under severe thermal cycling. Twenty cycles per year is moderate, so thermal shock is secondary to erosion. My provisional recommendation is the SiC gunned mix, provided dryout can be controlled.
Data that would change this: measured rather than design velocity, particle size distribution and loading, oxygen and moisture content, whether condensation occurs during shutdown, and whether the loss is truly localized. If the loss is confined to the elbow, a localized wear insert is more economical than relining 28 ft.
2. The plant asks whether the 1-in. lining can simply be doubled to 2 in. to extend life. How do you answer?
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Thickness alone does not solve an erosion problem, and doubling it has a hidden penalty. Reducing the inside diameter from 36 in. to 34 in. raises gas velocity by about (36/34)², or roughly 12%. For fine-particle erosion, wear rate scales with velocity to a power of about 2.5 to 3, so a 12% velocity increase raises the erosion rate by roughly 30–40%. Much of the added thickness is consumed by that accelerated wear.
A 2-in. lining also roughly doubles the lining weight and anchor load, lengthens dryout time, increases cost, and makes the elbow geometry worse. Thicker lining does extend life in direct proportion to thickness only if the velocity and impingement angle stay constant, which they do not here.
A better answer is to attack the wear rate: use a harder material, reduce velocity with a larger line or a flow transition, or install a thick wear insert only at the elbow where the velocity penalty is confined to a short length. If local thickening is used, it should be chamfered into the adjacent lining to avoid a step that creates a new impingement point.
3. Identify the principal installation risks for the material you recommend and how you would control them in a 9-day, 40–50°F turnaround.
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For a gunned SiC mix, the dominant risks are water addition, rebound contamination, cold-weather set retardation, and inadequate curing before dryout.
Water control: SiC gunned mixes are sensitive to nozzle water. Require a pre-installation test panel gunned by the production nozzleman, measure density and cold crushing strength on that panel, and adjust water only at the nozzle with a consistent, trained operator. Record the water setting.
Rebound: gunning at 45° would increase rebound and entrapment. Require the nozzle to be held perpendicular to the surface at 3–4 ft, with rebound kept out of the lining. Rebound should be discarded, not reused.
Cold weather: at 40–50°F, hydration is slow. Enclose and heat the work area to at least 50°F, preferably 60–70°F, heat mix water, and protect freshly placed lining from drafts. Do not strip forms or dry out until the material has achieved adequate green strength. Build a slow dryout into the 9-day plan, including holds in the 220–500°F range.