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Aurecon 360 Issue 8 - Thinking in action

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Unstable steam and thermal fatigue Understanding the risk by Mike Myers Senior Structural Integrity Engineer and Ian Rawlings, CS Energy's Chief Mechanical Engineer Expert structural integrity analysis is helping asset owners such as CS Energy to better understand the risk of damage to their infrastructure and appropriate mitigation measures they can employ to avoid costly equipment replacement. CS Energy, a Queensland Government owned energy provider, whose asset portfolio comprises the Kogan Creek, Callide and Wivenhoe power stations, identified unstable steam temperatures entering the reheater collection headers in the Kogan Creek 750 MW supercritical boiler. Knowing that fluctuating temperatures can cause thermal fatigue damage, CS Energy engaged Aurecon's structural integrity team to perform a detailed investigation. The aim was to determine if CS Energy should be concerned for the structural integrity and future life of the 9Cr-1Mo-V (P91) reheater headers. Understanding thermal fatigue Unstable steam temperatures have been acknowledged by industry as a significant contributor to thermal fatigue damage and cracking, especially in the ligaments between tube holes on the internal surface of collection headers. Stresses are generated by through-wall temperature gradients, which form when the metal surface is heated or cooled relative to the average wall temperature. Experience has shown that the severity of the stress is caused by variables such as tube-hole spacing and ligament dimensions, temperature ramp rate, the material's thermal expansion coefficients and header design thickness. The cracking of ligaments has been a leading cause of header replacements in superheater headers, reheater headers, economiser headers in coal and gas fired boilers, as well as High Pressure, Intermediate Pressure and Low Pressure superheaters and evaporator headers in heat recovery steam generators. Aurecon's approach to investigating the risk of ligament cracking was to: • dentify the typical operating I characteristics of the plant • Simulate the thermal and mechanical stresses acting on the equipment • Calculate the time to crack initiation using creep-fatigue interaction procedures Aurecon engineers accessed CS Energy's data collection systems (PI database) to obtain thermocouple data from key locations associated with the headers. The key locations were tube metal temperature, header steam temperature and header buried (mid-wall) and surface thermocouples. Data was obtained at one minute intervals for a number of two-day blocks of plant operations at different periods since commissioning.

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