fired-heater-field-issues

name	fired-heater-field-issues
description	Fired heater and furnace field troubleshooting — tube failures (hot spots, overheating, creep, carburization, sulfidation, oxidation), refractory failures (spalling, cracking, hot face erosion, anchor failure), burner issues (flame impingement, incomplete combustion, CO, flame instability, pulsation), tube coking (thermal cracking of hydrocarbon), fireside corrosion (vanadium attack, sulfuric acid dew point), tube metal temperature monitoring (tube skin thermocouples), draft and combustion air control, stack losses, thermal efficiency, API 560 fired heaters, API RP 535 burner management.
metadata	{"priority":8,"promptSignals":{"phrases":["fired heater","furnace tube","burner","refractory","tube hot spot","flame impingement","carburization"],"minScore":2}}

Fired Heater Field Issues — Complete Troubleshooting Guide

Tube Failures

Overheating and Hot Spots

Causes of hot spots:

• Flame impingement: burner flame directly contacts tube → localized heat flux 5–10× radiant average → tube metal T exceeds design
• Flow maldistribution: one tube passes less flow (fouled, partial blockage) → less cooling → tube wall overheats
• Coking inside tube: coke layer insulates → process side heat transfer resistance increases → tube wall T rises
• Flame length too long: burners firing too close to convection section → tubes beyond radiant design zone

Tube metal temperature (TMT) calculation: T_tube_metal = T_fluid + Q/(h_i × A_i) + Q × ln(D_o/D_i)/(2πkL) [Q = heat absorbed; h_i = inside film coefficient; A_i = inside area; k = tube conductivity] API 560: design TMT = maximum expected TMT plus a margin (typically 15–25°C)

Creep failure (high temperature): Above ~0.4–0.5 T_melt, creep dominates: ε̇ = A × σⁿ × exp(−Q_c/RT) Larson-Miller parameter: P = T × (C + log₁₀(t_r)) × 10⁻³ [T in Rankine; t_r = rupture time; C ≈ 20 for carbon steel] Sigma_rupture from Larson-Miller plot; if actual stress + temperature above curve → creep rupture inevitable Typical tube life: 100,000 hours design; each 10°C over design temperature → 50% life reduction (approximate)

Symptom progression: tube bulge (creep cavitation expanding tube diameter) → tube thinning at hot spot → sudden rupture Measure tube OD annually: API 560 allows 0.5% bulge (diameter increase); retire at 1%

Carburization

Mechanism: Carbon-rich combustion products (CO, hydrocarbons) at high temperature → C diffuses into steel → carbide formation → embrittlement Affected: austenitic SS tubes in direct-fired reformers, ethylene cracking furnaces Temperature: onset >850°C; severe above 950°C; carburized zone brittle (impact toughness drops to near zero)

Alloy selection: HP alloy (25Cr-35Ni+microalloying): highly carburization resistant; silicon additions (1–2.5%); mixed oxides barrier HK-40 (25Cr-20Ni): older standard; acceptable to 950°C

Detection: Hardness increase (carbides raise hardness); metallographic examination (carbide precipitation at grain boundaries); eddy current testing (conductivity decreases with carburization)

Sulfidation

Mechanism: H₂S or sulfur compounds + metal → metal sulfide (FeS) → porous, non-protective scale → rapid wall thinning Rate increases exponentially with temperature: significant above 260°C; severe above 370°C (Nelson-Couper curve for H₂S service)

Alloy resistance: Plain carbon steel: high sulfidation rate above 300°C 5Cr-1Mo (P5): good to 450°C 9Cr-1Mo (P9): good to 500°C Type 304 SS: good to 500°C; 310SS: good to 650°C Inspection: UT wall thickness measurement; significant thinning → replacement

---

Refractory Failures

Spalling

Mechanism: Thermal gradient → differential thermal expansion → spall (surface layer separates) Caused by: rapid temperature cycling (startups/shutdowns); steam impregnation (moisture in lining → steam pressure on heat-up); chemical attack (alkali vapor weakens silica brick)

Castable refractory: Hydraulic-bonded castable: low thermal shock resistance; spalls easily if heated too fast Phosphate-bonded castable: better thermal shock resistance Low-cement castable (LCC) and ultra-low cement (ULCC): excellent high-temperature strength; cure schedule critical (slow dry-out required)

Dry-out curve (curing): After castable placement: hold at 120°C for 24h (drive off free water); 250°C for 12h (chemically bound water); then ramp at ≤ 30°C/h to operating temperature Too-fast initial heat-up → steam pressure in castable → explosive spalling

Anchor failures: Metal anchors (stainless Vee, Y-shape) hold castable; oxidation corrosion of anchors → anchor volume change → lining cracked Use: Alloy 601, 310SS anchors for T > 900°C; ensure sufficient anchor density and spacing

Hot Face Erosion

Gas velocity at hot face: high-velocity combustion products erode refractory surface Fine particles from burner (catalyst dust, fly ash) → abrasive erosion Prevention: refractory hardness selection; use dense castable or silicon carbide hot face for abrasive services

---

Burner Issues

Flame Impingement

Cause: Flame length exceeds designed tube clearance; flame deflection from cross-draft; wrong fuel gas composition (heavier fuel → longer flame); over-firing (excess fuel rate)

Detection: Tube skin thermocouple alarm; visual through peepholes (flame touching tube); tube color (bright orange-white at contact zone)

Consequence: 10×–50× local heat flux → rapid tube overheating → immediate failure risk if not corrected

Fix: Reduce firing rate immediately; adjust flame pattern (quarl angle, register settings); check fuel composition; reduce air preheat if available; burner inspection/cleaning (tip fouling changes flame pattern)

Incomplete Combustion and CO

Excess air and combustion equation: Theoretical air for natural gas (CH₄): 9.55 m³ air/m³ CH₄ Stoichiometric products: CO₂ + H₂O + N₂ Excess air EA = (O₂_flue / (21% − O₂_flue)) × 100% [from flue gas O₂ analysis; approximate]

CO formation: Insufficient air (< 105% of stoichiometric) → CO in flue gas Target: O₂ in flue gas = 2–5%; CO < 50 ppm (typical process heater) High CO: incomplete combustion → wasted fuel; also CO is toxic; refractory can smoulder if CO accumulates and re-ignites (pulsation, explosion risk)

Air register problems: Damper stuck → cannot increase air → CO; burner tip coked → air maldistribution → localized rich zones → CO and soot

Burner Pulsation

Mechanism: Combustion instability → pressure oscillation couples with furnace acoustic mode → resonance → loud pulsation, flame blowout risk Similar to thermoacoustic instability

Triggers: Low firing rate (turndown ratio exceeded); high CO or fuel-lean zones; flame proximity to cold surface; pressure fluctuations in fuel supply (poorly regulated fuel gas header)

Fix: Increase firing rate if at low turndown; adjust pilot flame; inspect fuel gas pressure regulation; check burner register opening

---

Fireside Corrosion

Vanadium Attack (Oil Firing)

Mechanism: Heavy fuel oil contains V, Na, S → V₂O₅ (vanadium pentoxide) melts at ~625°C; liquid V₂O₅ dissolves protective Cr₂O₃ on tube surface → aggressive corrosion Significant above 650°C metal temperature in oil-fired furnaces

Prevention: Use vanadium inhibitors (magnesium compounds, dolomite) → forms high-melting-point MgO-V₂O₅ complex → no liquid slag Switch to lower-V fuel; reduce TMT below V₂O₅ melt point

Sulfuric Acid Dew Point Corrosion

Acid dew point: SO₃ (from oxidation of SO₂ in combustion) + H₂O → H₂SO₄ vapor; condenses at dew point temperature Acid dew point T_dp ≈ 160–170°C for typical fuel oil (increases with S content and excess air) Convection section tubes, economizer, air preheater: if T_metal < T_dp → H₂SO₄ condensation → severe corrosion (carbon steel: 10–20 mm/year)

Prevention: Keep minimum metal temperature above T_dp + 15°C margin Use acid-resistant alloy (weathering steel, 11Cr ferritic) or coatings in cold zones Monitor: acid dew point analyzer on flue gas; adjust air preheater bypass if needed

---

Thermal Efficiency and Stack Losses

Heat Balance

Fired heater efficiency (LHV basis): η = (Q_absorbed / Q_fired) × 100% Q_fired = fuel mass flow × LHV [LHV = lower heating value] Q_absorbed = process duty (from process flow, T_in, T_out, Cp)

Stack loss components: Sensible heat loss: Q_stack = m_flue × Cp_flue × (T_stack − T_ambient) [m_flue = flue gas mass flow] Excess air loss: higher EA → more N₂ heated → larger sensible loss; each 1% excess air → ~0.06% efficiency loss (approximate) Radiation loss: 1–3% of fired duty for typical heater

Stack temperature optimization: Lower T_stack → higher efficiency; but T_stack > T_dp + margin (acid dew point limit) Typical T_stack: 200–350°C; each 20°C reduction in T_stack ≈ 1% efficiency gain

O₂ trim control: Flue gas O₂ analyzer → trim excess air to minimize → maintain O₂ = 2–4%; significant fuel savings

---

Draft Control

Natural draft furnace: Stack creates negative pressure (draft) drawing combustion air through burner registers Draft = ρ_air × g × H_stack × (1 − T_ambient/T_flue) [H_stack = stack height] Too much draft: excess air infiltration → heat loss; too little → CO, positive pressure → dangerous

Forced draft (FD) fan: Blower forces air into firebox under positive pressure; constant air supply regardless of wind FD failure → loss of combustion air → burner trips (BMS: flame detectors confirm flame loss)

---

Standards

Standard	Scope
API 560	Fired heaters for general refinery services
API RP 535	Burner design and safety for process heaters
API 573	Inspection of fired boilers and heaters
API 579	FFS assessment of tubes with bulging or carburization
ASME Section I	Power boilers
NFPA 86	Ovens and furnaces
ISO 13705	Petroleum/petrochemical — fired heaters

Output

Identify failure category. For each: Tube hot spot: TMT measured [°C] vs. design [°C]; bulge [%]; Larson-Miller creep life [hours remaining]; flame impingement? reduce firing immediately Carburization: tube material; T [°C] > 850°C?; hardness check; wall thickness remaining [mm]; replace or FFS per API 579 Sulfidation: H₂S content [ppm]; T [°C] vs. Nelson-Couper curve; wall thinning rate [mm/year]; alloy upgrade required Refractory spalling: type; dry-out curve followed?; anchor condition; hot face erosion rate; repair method (castable or brick) Burner CO: flue gas O₂ [%]; CO [ppm]; EA [%]; adjust air register; clean burner tip Flame impingement: flame length vs. tube spacing; burner quarl angle; immediate action: reduce firing rate Efficiency: T_stack [°C]; O₂ flue gas [%]; η [%]; stack loss [%]; optimize excess air Acid dew point: T_dp [°C]; T_stack_minimum [°C] = T_dp + 15; cold section protection? Applicable API 560/573 + priority action (immediate shutdown / next turnaround / monitor).