Thermal Shock in Boilers

Thermal shock is a leading cause of boiler failure and can occur at any stage—during initial operation or after years of use. It happens when boiler metal experiences a rapid and significant temperature change, creating stress that leads to cracking.

Key Factors of Thermal Shock

1. Temperature Change

   • Sudden temperature differences (e.g., cold water hitting hot metal) create stress.

   • Example: Ice cracking when warm water is poured on it.

   • Boiler steel, like ice, is vulnerable to cracking under such stress.

2. Material Thickness

   • Thicker parts (tube sheets, shells, drums) are more susceptible.

   • Thin materials distribute heat faster and are less affected.

3. Frequency of Stress

   • A single severe shock may damage the boiler.

   • Repeated small shocks lead to microfissures that grow into large cracks over time.

Causes

• Water-Side Thermal Shock:

  • Occurs when cold water returns suddenly to a hot boiler, common in hydronic heating systems or service water heating systems.

• Fire-Side Thermal Shock (Firing Shock):

  • Happens when boilers trip at high fire and are immediately purged with cold air.

  • Fire tube boilers, especially the ends of fire tubes in the second pass, are most vulnerable.

Common Scenarios

• Improper adjustment of firing rate controls causing shutdown while at high fire.

• Automatic controls in hydronic systems returning cold water to hot boilers.

• Service water heating systems cycling water on/off instead of maintaining constant flow.

Conclusion

Thermal shock damage may be immediate or develop gradually. Preventing it requires:

• Proper control adjustments,

• Avoiding sudden temperature changes,

• Using constant water flow in sensitive systems.