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Avec plus de 30 ans d'expérience dans les services de production d'électricité et de vapeur industrielle, Tetra Engineering a une longue histoire de projets.
During the 2018 shutdown at a CCGT in Europe several of the HRSG HP drum riser nozzle and downcomer nozzle welds were checked and all were found to be cracked on the inside of the drum, at the weld toe or between weld beads, with the cracks following the fusion line back to the nozzle. The drum feedwater inlet and steam outlet nozzles (above drum waterline) were both undamaged. The drums were removed and all welds were repaired in the workshops. Tetra performed Finite Element Analysis (FEA) and operational data analysis and could conclude that the operational stresses were high enough given the current cycling regime to cause the cracking observed. The quality and design of the original welds is also likely to have contributed to the cracking.
Tetra was engaged by a client to investigate the recently observed cracking on the diverter damper actuator. The design documentation and failure mode was reviewed. From this initial review it could be concluded that the shaft cracking is most likely a result of the operating procedure, where the damper is kept in an open position for an extended period of time. The exposure to hot flue gas in combination with the load acting on the toggle arms is the most likely source of the cracking
An assessment of the susceptibility of HRSG piping components was performed according to Tetra Engineering’s FACRisk™ methodology which includes the use of thermal modelling simulation software (PPSD). From each system, sub groups are ranked using both time to minimum wall thickness and wear rate. The highest risk components (such as elbows, tees and valves) are identified. Overall, due to relatively high operating pH level (>9.2), the overall risk of FAC failures at the CCGT plant in question was considered to be relatively low, with few specific areas showing increased risk. Three separate load cases were simulated and analysed to determine the potential FAC risk. The results of each simulation were incorporated into the final locations recommended for inspection.
After several tube failures in recent years in the 1st row of LTRH hanger tubes, Tetra was tasked with performing a Root Cause Analysis (RCA) to determine the underlying cause of the failures. A detailed review of operating data, failure reports and design information was carried out and potential causes investigated. The tube failure mechanism as determined by metallurgy was short-term overheating, with temperatures likely exceeding 700°C prior to failures. Two contributing causes were identified by elimination of all other possibilities: temporary loss of steam flow in affected leading row tubes coupled with high flue gas temperatures in certain tubes at the side of the boiler gas path. Unfortunately, the underlying root cause of the loss of flow could not be confirmed, whereas the asymmetric flue gas temperature distribution is a known issue since commissioning.
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