Possible root cause for the boiler superheater tubes failure

🔍 1. Observation Summary

• Boiler type: Water tube boiler

• Operating pressure: 32 bar

• Failure location: Superheater tube

  • At weld joint to header

  • At bend section of superheater tube

• Leak nature: Crack/leak

---

⚙️ 2. Possible Root Causes

A. Thermal Fatigue / Thermal Stress Cracking

Most common cause at weld joints and bends.

Mechanism:

• Frequent start-stop cycles, load fluctuations, or poor control of superheater steam temperature cause expansion and contraction of tubes.

• At bend and weld areas, stress concentration is higher.

• Over time, micro-cracks form due to repeated thermal cycling → grow into visible cracks → leakage.

Signs:

• Fine cracks at the heat-affected zone (HAZ) near weld.

• No heavy corrosion inside/outside.

• Failure usually along tube circumference at bend radius.

Contributing factors:

• Inadequate flexibility in tube support/hanger system.

• Uneven heat distribution due to poor gas flow design.

• Sudden steam load changes.

---

B. Overheating due to Steam Flow Restriction

Second most common cause in superheaters.

Mechanism:

• Partial choking/blockage of tube (by scale, oil, or magnetite).

• Steam flow reduced → poor internal cooling.

• Metal temperature rises above design (e.g. >540°C).

• Tube softens → creep or rupture, especially at bends and welds.

Indicators:

• Tube surface oxidized, bulged, or deformed.

• Discoloration (blue, dull grey) shows overheating.

• Localized crack near hottest region (bend).

---

C. Poor Welding Quality / Residual Stress

Welding-related failure at header joint.

Mechanism:

• Improper welding (lack of fusion, undercut, wrong filler material, inadequate PWHT).

• High residual stress trapped in HAZ.

• Crack initiates during service when combined with pressure + temperature cycles.

Clues:

• Crack starts exactly at weld toe or fusion line.

• Metallurgical exam shows brittle fracture or grain boundary oxidation.

---

D. Corrosion-Fatigue / Oxidation at Bend

When external environment or flue gas is aggressive.

Mechanism:

• External oxidation due to flue gas + high metal temp.

• Scale forms → reduces heat transfer → overheating.

• Vibration or pulsation adds fatigue → crack grows.

Typical in:

• Units burning high-sulphur fuel or with poor combustion air distribution.

---

E. Creep Failure (Long-Term Overtemperature)

If boiler runs continuously at high temperature for long hours.

Mechanism:

• Prolonged metal exposure to high temperature causes grain elongation.

• Tube wall thins, bulges, then cracks.

• Found mostly near bend or outlet header (hottest zones).

Indications:

• Bulged tube with longitudinal crack.

• Creep voids under microscope.

---

🧩 3. Most Probable Root Cause Combination

For your case (superheater at 32 bar, crack at header weld & bend):

Primary root cause: Thermal fatigue and localized overheating.
Contributing factors:

• Uneven steam flow / temperature distribution.

• Possible poor weld quality or residual stress.

• Inadequate stress relieving during fabrication or repair.

• Rapid temperature cycling during startup/shutdown.

---

🧰 4. Recommendations / Preventive Actions

✅ Inspection & Metallurgical Analysis

• Perform metallographic examination (microstructure, hardness, grain growth).

• Use replica or SEM test to confirm thermal fatigue or creep pattern.

✅ Boiler Operation

• Avoid sudden steam load changes.

• Maintain steady firing rate during startup/shutdown.

• Ensure proper superheater drain during startup to prevent water carryover.

✅ Design & Maintenance

• Verify tube support alignment to reduce vibration.

• Ensure adequate expansion gaps in tube bank.

• Regular NDT (UT/MT/PT) at header weld joints.

• Check for steam flow balancing and internal cleanliness (no scale build-up).

✅ Repair

• Cut out and replace cracked section.

• Apply proper preheat and post-weld heat treatment (PWHT).

• Use compatible filler metal and qualified welding procedure (ASME IX).