Titanium Condenser Tube Failure Analysis And Solution

Condenser tubes are the key components of condensers that condense steams into water in large-scale gas turbines. Once the tubes were failed, the cooling water would leak and then pollute the circulating water, seriously influencing the normal operation of the whole equipment. Thus, material selection of the tubes plays an important role in reliability and service lifetime of the condensers. Thanks to its advantages like superior mechanical properties and corrosion resistance, the industrially pure titanium is the most-widely used matrix material for condenser tubes. However under complicated service conditions, the pure titanium tubes are also possibly encountered with failures as wear, corrosion, etc, and will consequently result in leakage of cooling water, and finally bring about safety problems. As for the condensers of the two heavy-water nuclear power facilities in Third Qinshan Nuclear Power Plant, the titanium tubes were failed with severe wall-thinning after8-year service, greatly shorter than their expected lifetime20years. If the actual causes of this premature failure were not found, and the effective countermeasures were not taken, such titanium tubes would inevitably leaked, not only causing great economic losses, but also inducing safety accidents.Hence, fundamental theories of failure analysis and series of characterization methods were comprehensively utilized to investigate this failure incident from two aspects. On one hand, the chemical compositions, the metallographic structures, and the mechanical properties of the in-service titanium tubes were inspected, and the fractographs of the tensile test samples were also observed. The results showed that these titanium tubes met the requirements of corresponding standards, in other words, the failure was induced by external factors rather than the tubes themselves.On the other hand, the abnormally wall-thinned titanium tubes were sampled for analysis. Microscopic morphologies and chemical compositions of the worn surfaces were observed and detected by SEM (scanning electron microscope),3D-OM (3-dimensional optical microscope), and EDS (energy disperse spectroscopy); then the deposits on the tube surfaces were collected and studied by TGA (thermogravimetry analysis), FTIR (Fourier transform infrared spectroscopy), and XRD (X-ray diffraction). The results showed that the unqualified processing of the plate holes, and the contact wear effects between the titanium tubes and the iron oxides corrosion substances from the plate surface were the main causes of tubes’ wall thinning. What’s more, the three-body contact wear due to the foreign particles, and the thermal stresses that were generated when the turbine started up, etc, were all the other factors leading to this failure.Besides, some failed tubes were also found corrosion pits on their surfaces. After sampling and then analyzing by SEM, EDS,3D-OM, as well as considering their actual service conditions, it was determined that these pits were brought about by cavitation effect from the neighboring steams.In summary, this premature failure of the condenser titanium tubes were caused by the interactions among the unqualified processing of the plate hole, the introduction of iron oxides corrosion products and foreign particles into the plate holes, the thermal stress from the turbine’s starting up, and the influence of neighboring steams and so on. Then, pertinent suggestions as polishing the plate holes and clearing the corrosion products on the plate surface were proposed. After adopting this quick, simplified, and effective failure analysis method and its countermeasures, recurrences of similar failures were successfully prevented, economic losses were greatly reduced, and safe operation of the condensers was ensured.