Fig. 1
Temperature changes in piping during plant start-up

Description:
Leakage was detected at the welding seam of a main-steam collector outlet. Metallographic analysis of the fracture surface identified thermal cycling as the cause of failure. However, neither manufacturer nor operator could provide an explanation.  

Failure cause:
Hochtemperatur-Dehnungs- und Temperaturmessungen bestätigten jeweils beim Start der Turbine schnell verlaufende Temperaturabsenkungen von ca. 250°C auf der äußeren Rohroberfläche. Dies führte zu hohen Wechselbeanspruchungen im Rohrquerschnitt.

Remedy:
Due to the design of the main steam collector, condensate collected in the lower pipe sections after each turbine shutdown. When the turbine was started up, the relatively cold condensate was sprayed onto the already hot internal pipe surface. Provision of a drainage means in the pipe solved the problem.

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Repair-related outage and loss in utilization is thus a thing of the past.

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