Jun 10, 2020 · Thermal stress in a boiler works in much the same way. Thermal stress or thermal shock, also known as thermally induced stress cycling, occurs when your boiler undergoes a rapid change in temperature, such as when cold water flows into a hot boiler.
PREVENTING THERMAL SHOCK - Thermal Tech, Inc.: Boiler
In a boiler and feedwater system, acidic attack can take the form of general thinning, or it can be localized at areas of high stress such as drum baffles, "U" bolts, acorn nuts, and tube ends.
Stresses may be associated with vibration due to flue gas flow or sootblowers (high-frequency, low-amplitude stresses), or with boiler cycling (low-frequency, high-amplitude stress mechanism). Components typically affected: Fatigue failures will most often occur at areas of constraint such as tube penetrations, welds, or supports.
The stress in a steam boiler shell can be calculated as σ = P D / 2 t (1)
Stress in Steam Boiler Shells from Boiler Pressure
Thermally induced cyclic stresses are due to the resistance of the boiler structure to the movement caused by the thermal expansions and contractions within the boiler. The stresses occur during every firing cycle of the burner, a cycle defined as burner on, burner off. Failures of this type appear as leaks from sectional cracking.
The problem of thermally induced stress cycling (loosely referred to as thermal shock) has been apparent for many years, particularly in boilers installed in hot-water heating systems. The term "shock" suggests a sudden impact type failure, which in the vast majority of cases is far from what actually happens.
Finding the Root Cause of Boiler Tube Failures