On behalf of STEAG AG, TÜV SÜD tested a critical component of a power station in which a crack had been detected, to ensure it would be safe and reliable in continued operation under more flexible conditions.
Due to the green energy revolution and the intermittent nature of feed-in of electricity from renewable sources, large-scale fossil-fuelled power stations must operate more flexibly. The higher number of startup and shutdown operations and longer part-load service put a strain on materials and components.
In Quierschied-Weiher in the German state of Saarland, Essen-based company STEAG AG operates a modern hard coal-fired power station that supplies electricity and district heat to the region. Non-destructive testing had revealed cracks in the housing of the circulating pump. These cracks were caused by operation and had formed over the past ten years. The circulating pump plays a central role in part-load operation, and is exposed to a high number of load cycles.
TÜV SÜD's experts were commissioned to carry out a fitness-for-service assessment to evaluate the cracks and determine whether continued operation of the component in the power station would be possible for a certain limited period of time. This question had to be assessed reliably in the short timeframe of the power station's turnaround. Key questions were:
- Can spontaneous failure of the component be excluded?
- Can the cracked component be operated reliably over the next two years?
Reliable service life prediction
All indications of cracking were evaluated in accordance with the fitness-for-service standard API 579-1/ASME FFS-1 (Level 3). For this purpose, the experts had to develop an accurate load profile of the circulating pump and extrapolate it to the future operation of the component. To this end, TÜV SÜD's engineers analysed the circulating pump's variable operating parameters (temperature and pressure), as the load profile revealed regular, rapid changes in temperature of up to 100°C. Using a numerical model, the experts then calculated the stresses and used them to run a fracture-mechanics analysis. Apart from brittle fracture analysis, the experts calculated crack propagation on the basis of the load profile. The analysis revealed that as crack depth increases, stresses throughout the wall thickness decrease from the inside to the outside.
"Our analyses have shown crack depth to be far below the critical limit. Crack propagation, too, is modest. Given this, the indicated cracks are within the tolerable limits", explains Dr Johanna Steinbock, Power and Systems, TÜV SÜD. "Provided the circulating pump is exposed to the same strain as in the past, it can be operated reliably and safely up to the next turnaround in two years' time." The new pump housing has already been produced by the manufacturer and will be installed in the 2013 turnaround.