Field Reference / Inspection & Maintenance / Refractory Management
Module 06
06 · Inspection & Maintenance

Refractory Management

Covers refractory materials, zones of application, damage recognition, severity assessment, repair decisions, and the critical dry-out procedure following any new installation or wet repair.

⚠
Dry-out is mandatory after any refractory work
Skipping or rushing the dry-out schedule causes explosive steam spalling that can destroy the lining and endanger personnel. Never light full burners on freshly-installed or repaired refractory.

Role of Refractory

Refractory lining serves four functions: it protects the steel shell from radiant heat and flame impingement; it retains heat within the firebox to maximise thermal efficiency; it provides thermal mass that damps temperature excursions; and it protects structural members (arch hangers, floor supports) from direct flame exposure.

Degraded refractory directly increases shell temperatures, accelerates casing corrosion, and raises fuel consumption. Severe failures can expose the pressure boundary — making refractory condition a direct process safety concern, not merely a maintenance one.

Zones & Typical Materials

Different zones operate at very different temperatures and experience different stresses. Material selection is zone-specific.

🔥
Radiant Floor
Hot face: up to 1100 °C
Dense castable / firebrick
High mechanical loading from burner impingement and condensate pooling. Needs abrasion resistance. Typically 150–230 mm thick.
🏛️
Arch / Roof
Hot face: up to 1200 °C
Insulating castable + anchor system
Highest thermal exposure. Relies on stainless steel V-anchors. Spall failures here drop debris onto tubes — immediate hazard.
🧱
Radiant Walls
Hot face: 900 – 1100 °C
Ceramic fibre module or castable
Ceramic fibre offers fast dry-out and low thermal mass (better for cycling units). Dense castable preferred where flame impingement is possible.
〰️
Convection Section
Gas temp: 500 – 850 °C
Insulating castable or ceramic fibre blanket
Lower temperature but subject to flue gas velocity erosion. Baffles and transition pieces see the highest erosion rates.
🔩
Burner Quarls & Tiles
Local: up to 1400 °C
High-density 70% alumina castable
Highest wear zone. Quarl cracking is a primary cause of flame instability. Inspect every shutdown; replace on any through-crack.

Damage Types & Recognition

Knowing the failure mode directs the correct repair. Treating spalling as cracking, or chemical attack as mechanical damage, leads to premature re-failure.

Mechanical
Cracking & Shrinkage
Signs: Hairline to open cracks, regular geometric pattern (shrinkage), irregular branching (thermal shock). Common at anchor points.

Cause: Thermal cycling, oversized panels, inadequate expansion joints, anchor corrosion allowing movement.
Thermal
Spalling & Hot Spots
Signs: Slab-like fragments detached from hot face; glowing or discoloured casing externally (IR camera). Arch debris on tubes.

Cause: Rapid temperature change (wet refractory, emergency shutdown), overfire, or flame impingement. Steam spalling from trapped moisture.
Chemical
Flux Attack & Dissolution
Signs: Glassy, melted or vitrified surface; colour change (dark or green/grey); reduced thickness on hot face.

Cause: Vanadium pentoxide (fuel oil firing), alkali compounds, or process leaks contacting the lining. Attacks alumina-silica binders.
Erosion
Velocity Erosion
Signs: Smooth, scooped or rounded profile loss — especially at flue gas turns, baffle edges, and damper faces. No cracking, just missing material.

Cause: High flue gas velocity carrying particulates. Convection section and stack entry are highest-risk zones.

Severity Assessment

Assess each defect against remaining thickness and operating criticality of the zone. Use the three-tier classification below to determine the response timeline.

Grade 1 — Monitor
Hairline cracks <2 mm wide; <10% hot face area affected; no measurable thickness loss; shell temp normal.
→ Log, re-inspect next shutdown
Grade 2 — Plan Repair
Cracks 2–10 mm, spalled zones exposing insulation or anchor tips, localised hot spot on casing (<50 °C above baseline), thickness loss >20%.
→ Repair at next planned outage; monitor weekly
Grade 3 — Immediate Action
Through-cracking to shell, arch debris on tubes, casing glowing or >200 °C above baseline, quarl through-crack, exposed anchors corroding.
→ Reduce load or shutdown; do not defer
🔴
Arch spall debris on process tubes = immediate Grade 3
Fallen refractory can bridge between tubes, block cooling, and cause localised overheating leading to tube rupture. Inspect tube surfaces after any observed arch spalling — do not continue firing at full rate.

Dry-out Schedule — New & Repaired Refractory

Castable and dense refractory contains significant free and chemically-bound water. Removing this too quickly generates steam pressure inside the matrix, causing explosive spalling. A controlled dry-out curve is non-negotiable.

Typical Dry-out Temperature Profile (castable lining)
Ambient → 110 °C
Free water drive-off
≤ 25 °C / hr
→
110 °C hold
Soak — steam escape
4 – 8 hr hold
→
110 → 300 °C
Chemically bound water
≤ 25 °C / hr
→
300 °C hold
Soak — dehydration
4 – 8 hr hold
→
300 → Normal ops
Ramp to operating temp
≤ 50 °C / hr
ℹ
Ceramic fibre dry-out is simpler
Ceramic fibre modules and blankets contain no chemically-bound water. A single ramp at ≤ 50 °C/hr to operating temperature is typically sufficient, with no holds required. Always follow the supplier's specific data sheet — compositions vary.
Dry-out Pre-checks & Execution
Type: Post-maintenance startup Steps: 10
01
Confirm refractory cure time
Castable requires minimum 24 hr air cure after pour before any heat is applied. For patching mortars follow the product data sheet — some require 48–72 hr. Do not start dry-out early.
02
Remove all combustible material from firebox
Formwork, timber wedges, mixing tools, plastic sheeting. Check drains and sumps are clear — pooled water in contact with hot refractory causes localised steam spalling.
03
Confirm thermocouple coverage
At minimum: one bridge wall TI, one arch TI, one floor TI. Temporary TCs via inspection doors are acceptable if permanent instruments are insufficient. Document calibration.
04
Light minimum burners — pilot only or low-fire
Use the minimum number of burners to achieve a slow, even temperature rise. Avoid burners aimed directly at newly-cast surfaces if alternatives exist.
05
Ramp at ≤ 25 °C/hr to first soak (110 °C)
Log temperatures every 30 minutes. If rate exceeds 25 °C/hr, reduce firing immediately. Do not attempt to compensate with rapid reduction — steady state is better than overcorrection.
06
Hold 110 °C soak for minimum 4 hours
Steam will be visible from expansion joints, peepholes, and drains during this phase — this is normal. Do not advance until steam emission has substantially ceased. Heavy sections may require 8 hr hold.
07
Ramp at ≤ 25 °C/hr to second soak (300 °C)
Chemically-bound water (as gibbsite and boehmite) releases between 150–300 °C. Some additional steam emission is normal in this range.
08
Hold 300 °C soak for minimum 4 hours
For thick or dense castable sections (≥150 mm), extend to 8 hr. Confirm with IR camera that no cold spots remain before advancing.
09
Ramp to normal operating temperature at ≤ 50 °C/hr
Once 300 °C soak is complete, the refractory is substantially dry. Normal ramp rate applies. Continue to log temperatures until stable at operating conditions.
10
Post-dry-out inspection
After first cool-down following dry-out, inspect for: new cracks at anchor points (normal — seal with mortar if >3 mm), delamination, areas of discolouration suggesting incomplete dry-out. Log to maintenance records.

Repair Methods

Match the repair method to the damage extent. Mismatched repair (e.g., gunite over a loose substrate, or patching over chemically-attacked material without removing the affected zone) fails rapidly.

Repair Method Selection Guide
Damage Description Repair Method Dry-out Required? Notes
Hairline cracks, <2 mm, non-through High-temperature refractory mortar trowelled flush No hold needed Damp surface before applying. Brush flush, do not overfill.
Open cracks 2–10 mm, non-through Rout out V-groove; pack with plastic refractory; mortar flush 25°C/hr ramp only Do not seal over wet substrate. Allow mortar to initial-set before firing.
Spalled zone, hot face lost, insulation exposed Remove loose material to sound substrate; form and cast or gunite patch; re-anchor if exposed Full dry-out curve Anchor tips must be embedded ≥ 50 mm. Key patch edges at 45°.
Arch panel delaminated / fallen Full panel replacement; inspect adjacent anchors; may require anchor overlay plate Full dry-out mandatory Do not permit personnel under unsupported arch. Use proprietary anchor systems — do not fabricate in-house.
Chemical attack — vitrified surface Remove all attacked material to uncontaminated refractory; assess for chemical-resistant grade on reinstatement Full dry-out mandatory Standard alumina-silica will re-fail if fuel source (vanadium) is unchanged. Consult refractory supplier for correct material upgrade.
Quarl through-crack Replace quarl block — no patch acceptable Quarl-specific cure Prefabricated quarl blocks must match original geometry. Incorrect quarl geometry changes flame shape and can cause flame instability.
Ceramic fibre module damage Remove damaged modules; install replacement modules to supplier overlap specification Ramp to ops — no soak Fibre debris is a respiratory hazard — RPE and wetting required. Dispose as controlled waste.

Anchor System Integrity

Refractory is only as good as its anchors. V-anchors, Y-anchors, and T-anchors hold castable panels to the casing. Corroded or failed anchors allow panels to detach even when the castable itself appears sound.

⚠
Anchor tips must remain embedded — never exposed
When a spalled zone exposes the tip of a V-anchor, the anchor is no longer functioning as designed. The anchor section between the embedded root and the exposed tip is at elevated temperature, accelerating corrosion. Assess and re-anchor before patching.

During offline inspection, probe suspected anchor zones with a rod. A hollow sound (compared to a solid ring from sound refractory) indicates delamination at the anchor-castable interface. Mark and map all hollow areas on a firebox diagram before deciding repair scope — isolated hollows can be stabilised; widespread delamination across a panel requires full replacement.

Documentation Requirements

Refractory condition should be tracked across shutdowns to identify trends. The minimum record set per shutdown:

← Inspection Checkpoints Tube Assessment →