Damage Control: Thermal Fatigue Mitigation

Editor’s Note: This regular column offers practical insights into various damage mechanisms affecting equipment in the O&G, petrochemical, chemical, power generation, and related industries. Readers are encouraged to send us suggestions for future topics, comments on the current article, and raise issues of concern. All submissions will be reviewed and used to pick topics and guide the direction of this column. We will treat all submissions as strictly confidential. Only Inspectioneering and the author will know the names and identities of those who submit. Please send your inputs to the author at damagecontrol@inspectioneering.com.

Thermal Fatigue Damage Mitigation Methods

Many parameters influence the thermal-mechanical fatigue performance of metal components, including pressure vessels and piping. They include variables related to cyclic stress (loading), geometry, material properties, and the internal/external environment. The stress parameters include the state of stress, stress range, stress ratio, constant or variable loading, frequency of loading, and maximum stress level.^[1] Other relevant parameters include stress concentrations, component shape/size, severity of metal temperature gradients, and metallurgical/mechanical properties of the base metal and weldments. The internal/external environment parameters include process or ambient temperatures and aggressiveness (corrosivity) of the process stream constituents or outside environment.^[2] The factor that usually has the largest effect on fatigue life is the magnitude of the fluctuation in localized stress or strain. Consequently, reducing the severity of metal temperature gradients is generally the most effective way to mitigate thermal fatigue damage. This includes reducing not only the overall metal temperature difference in a component during a cycle, but also avoiding steep/abrupt metal temperature gradients that occur over small areas or short lengths/spans. While it is often impractical or difficult to reduce the magnitude of the applied stress (process temperature) fluctuations directly, in many cases, a decrease in the severity of stress fluctuations and concentrations can be more easily accomplished through implicit design practices and fabrication techniques.

Design and Analysis Considerations