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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.
Tetra was hired by a client to investgate the remaning life of the HEP on 3 units of a thermal power station. The plant intended to operate the units for another 100,000 hours. The scope of work entailed a piping stress analysis of the main steam and hot reheat lines, thermal stress analysis of the SH header outlet/main steam line and remaining life assessment (creep, fatigue, creep-fatigue) of critical areas based on inspection history and results from the pipe stress/finite element analysis. The figure shows time-dependent stresses and temperatures during start up.
During the shutdown of the unit at night, the HP bypass was kept open to maintain a vacuum in the steam turbine condenser in order to guarantee a faster start the next morning. Therefore, a small quantity of high pressure steam from the HP drum was flowing through the SH on the way to the HP bypass valve. The small HP steam flow takes the path of least resistance and only goes through tubes on one side of the unit, affecting their temperature. The temperature of the tubes on the other side change differently and a temperature difference of around 80°C can be seen 2 hours after shutdown. The CCGT owner engaged Tetra Engineering to investigate the impact of these resultant cyclic loads on the fatigue life of the tube to header welds. The study concluded that the temperature difference of 80°C between different tubes does not cause high enough fatigue stresses alone to be considered critical
Tetra Engineering performed a CFD analysis of the HRSG gas path at a CCGT in Asia in collaboration with CFD specialist R&R Consult. The results showed a very uneven flow distribution at base load over the HPHTSH tube sheet, with a higher mass flow located towards the outlet header sections and towards the side walls. The swirling motion of the GT flow results in a different flow pattern for the low load case, compared to the base load case. The flow was more evenly distributed in the longitudinal direction at the SH, but a considerable variation is seen in the span-wise direction. The uneven flow distribution was thought to be a contributing factor to the repeated fatigue failures seen in the boiler over the last couple of years.
Tetra Engineering completed a Tube Failure Analysis of failed SH tubes from a CCGT plant in South East Asia. The results show the following damage mechanisms present: Short-term Creep and Fatigue damage (Creep-Fatigue interaction) Short-term Creep and Stress Corrosion Cracking (SCC) or Hydrogen Embrittlement The fact that short-term creep damage was observed on both of the failed tubes indicates a sudden change in temperature. It was possible that newly discovered damage to the turbine exhaust flow correction device could have allowed a larger mass flow to one side of the boiler (due to swirl influence), increasing the tube temperatures locally and/or increasing local stresses when expansion is prevented, thus accelerating creep damage. The plant had suffered from frequent fatigue failures, with several independent metallurgical analyses over the years confirming thermal fatigue as the previous root cause. No signs of any creep damage had previously been discovered.
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