Thermal Fatigue of Pressure Components
Thermal-fatigue mechanisms in pressure equipment, including transient gradients, mixing, stratification, local cycling, thermo-mechanical stress ranges and fatigue assessment.
Thermal fatigue mechanism
Thermal fatigue arises when repeated temperature changes generate repeated structural strain. A pressure component does not need to experience a large change in average temperature; a local surface can cycle rapidly while the surrounding metal remains comparatively stable. The resulting differential expansion produces bending and membrane stress ranges that can initiate cracks at nozzles, welds, branch connections, mixing regions and thick-to-thin transitions. Because the driving quantity is the temperature field through time, steady-state temperature analysis alone cannot establish thermal-fatigue severity.
Common transient sources
Startup and shutdown, hot or cold injection, emergency cooling, intermittent flow, valve leakage, thermal stratification and mixing of streams at different temperatures are common sources. A branch carrying cold fluid into a hot main can create a moving mixing front and highly localised cycling. Dead legs can heat and cool differently from the active line. The relevant transient may therefore occur at a small region that is not obvious from the global operating temperature envelope.
Thermal mixing and striping
When hot and cold streams mix turbulently near a wall, the surface can experience rapid irregular temperature fluctuations often described as thermal striping. The structural penetration depth depends on fluctuation frequency and material diffusivity: high-frequency temperature variation may affect only a shallow surface layer, while slower variation penetrates deeper and creates larger through-thickness bending. CFD, measured temperature spectra or conservative thermal boundary histories may be required where simple one-dimensional conduction cannot represent the spatial and temporal mixing.
Thermal stratification
Partially mixed flow can create a stable hot upper layer and cold lower layer in horizontal piping or vessels. The circumferential temperature difference causes thermal bowing and local bending. If the stratification interface moves with operating condition, the stress field cycles even when pressure is steady. Beam-only pipe models using one temperature per section cannot capture the circumferential gradient directly; a shell or solid thermal-structural model may be required for the critical region.
Mechanical pressure interaction
Thermal and pressure stresses occur simultaneously and can reinforce or oppose one another at different surfaces. The fatigue range should be constructed from complete thermo-mechanical states, preserving sign and timing. A pressure increase during heating may make one surface more tensile, while cooldown at low pressure can reverse the local stress. Independent maxima should not be added without establishing that they are concurrent. The combined range, not the individual peak thermal or pressure stress, drives fatigue usage.
FEA and stress extraction
Run transient heat transfer with time resolution sufficient to capture the steepest temperature changes, then map temperatures into the structural model. Extract stress histories at physically meaningful locations and use a fatigue-compatible measure such as structural stress or local strain according to the selected method. Avoid relying on singular weld-toe peaks. Where local plastic strain occurs, elastic fatigue methods may become inaccurate and a strain-life or nonlinear cyclic approach may be more appropriate.
Inspection and operating evidence
Thermal-fatigue cracks often initiate at predictable geometric and thermal locations. Inspection plans can therefore be informed by the analysis, while thermocouple data can validate the assumed transient. Repeated field cracking despite acceptable nominal calculations usually indicates that the local thermal boundary history or stress concentration is not represented adequately. Operating modifications—slower mixing, preheating, changed injection geometry or revised sequencing—can sometimes reduce the source more effectively than simply increasing thickness.
Verification and life assessment
Verify transient thermal penetration using hand diffusion estimates or simplified one-dimensional models. Demonstrate time-step and spatial mesh convergence for the temperature gradient and resulting structural range. Confirm the cycle count and event severity against operating requirements. Report the temperature history, stress extraction method, fatigue curve and usage contribution by transient family. A defensible thermal-fatigue assessment connects plant operation to local wall temperature, local structural response and accumulated life.
Thermal fatigue is often driven by local temperature cycling that is invisible in the average process temperature. Model the transient boundary condition at the actual hotspot.
Surface heat-transfer uncertainty
Thermal-fatigue prediction can be very sensitive to the heat-transfer coefficient at the wetted surface because that coefficient controls how quickly the metal follows the fluid transient. If the coefficient is uncertain, a single best-estimate value can give false confidence. Bound the plausible range or calibrate against measured metal temperatures. The most damaging structural gradient does not always correspond to the highest coefficient because penetration depth and phase lag also change with the transient.
Crack initiation versus crack growth
A fatigue calculation may address initiation life, crack-propagation life or a code usage factor that implicitly covers both. These are different questions. Once a thermal-fatigue crack is detected, continuing to apply an initiation-style S-N calculation is not sufficient; fracture-mechanics growth and inspection capability become relevant. The assessment should state clearly whether the objective is new-design endurance, continued operation with no known flaw, or management of an existing crack.
Engineering judgement — what can change the conclusion
For Thermal Fatigue of Pressure Components, the harmonised review should concentrate on cyclic plasticity and the distinction between shakedown, alternating plasticity and progressive strain accumulation under repeated pressure/thermal cycles. The engineering value comes from identifying the assumptions that can move the governing margin or failure mode, then testing those assumptions deliberately rather than adding complexity indiscriminately. Where simplified and high-fidelity methods coexist, the simpler method should be used as an independent trend or magnitude check so that agreement is based on physics rather than shared modelling assumptions.