Case study 2: Failure Caused by Overheating During Secondary Soldering – Damage to Brazed Structure

Failure Caused by Overheating During Secondary Soldering – Damage to Brazed Structure at Nozzle Connection

This technical note explains a common failure mechanism in copper-brazed plate heat exchangers (BPHE) where uncontrolled secondary soldering at the nozzle connection introduces localized overheating and weakens the original brazed joints, leading to leakage.

Overview

Video evidence: leakage location and brazed joint damage pattern after secondary nozzle soldering.

In copper-brazed plate heat exchangers, the internal plate stack is joined through a controlled vacuum brazing process. The integrity of the unit depends on maintaining the metallurgical stability of these brazed joints.

However, secondary soldering or welding operations performed at the nozzle connection can introduce excessive localized heat. Without temperature protection, overheating can re-melt or weaken the original brazed copper bond near the nozzle area, resulting in leakage.

Equipment type Copper-brazed plate heat exchanger (BPHE)

Failure location Nozzle connection / end-plate brazed zone

Primary trigger Uncontrolled secondary soldering / localized overheating

Typical timing Shortly after commissioning

Observed Failure Characteristics

Based on inspection and leakage pattern, the key observations were:

Leakage at the nozzle interface
No signs of corrosion-induced degradation
No evidence of pressure overload
Localized joint weakening around the connection area

The failure mode is consistent with thermal degradation of the brazed structure rather than a material or manufacturing defect.

Root Cause Analysis

1) Copper-Brazed Plate Structure

Copper-brazed plate heat exchangers are manufactured in a vacuum furnace under controlled conditions (typically around 1100°C). The copper bonds:

Join the plates
Seal the internal channels
Provide structural integrity

After brazing, the copper joints remain stable only within defined thermal limits during field operations and modifications.

2) Secondary Soldering Without Thermal Protection

During external nozzle soldering, the following risk factors were present:

Direct flame heating was applied
No heat sink or cooling protection was used
Local temperature exceeded the safe threshold

Excessive localized heat can:

Re-melt or soften internal brazed copper
Disturb capillary bonding
Create micro-voids
Reduce joint strength

This compromises the seal between the plate pack and end plate near the nozzle area.

3) Mechanism of Leakage

When the brazed seam close to the nozzle is thermally disturbed:

Micro-cracks develop
Capillary seals weaken
Internal cross-leakage or external seepage appears

Key takeaway: This failure mechanism is strongly associated with post-production overheating during secondary soldering and typically occurs soon after commissioning.

Why This Is Not a Manufacturing Defect

The original brazed structure is stable under specified operating conditions. The observed failure pattern matches:

Localized overheating introduced after production
Metallurgical weakening of the brazed joint near the nozzle
Failure concentrated at the modified connection zone

Preventive Recommendations

To prevent overheating-related leakage during nozzle work:

Use temperature-controlled soldering techniques
Apply heat sinks or cooling wraps during nozzle soldering
Avoid prolonged direct flame exposure
Monitor surface temperature during the operation
Prefer factory-assembled connection options when possible

Practical note: If field soldering is unavoidable, implement a controlled procedure (heat sink + temperature monitoring) to protect the internal brazed structure.

Engineering Conclusion

Copper-brazed plate heat exchangers are sensitive to uncontrolled secondary heating at nozzle connection points.

This case is consistent with overheating during soldering without adequate thermal protection, which degraded the original brazed joints and caused leakage near the nozzle area.

Proper temperature management during field modifications is essential to maintain brazed joint integrity and long-term reliability.