Long-Term High-Temperature Longevity Testing of Thermosyphons with Actual Dimensions

The results of a 20-year-long longevity bench testing of nineteen full-size thermosyphons (TSs) made of steel 20 at a steam-water medium temperature of 240–265°C are presented. For the manufacture of TSs, various methods for processing the inner surface and different compositions of filling aqueous solutions have been used. The thermosyphons were periodically removed from the tests and cooled for monitoring the conditional relative vacuum \({{p}_{{{\text{vac}}}}} = 1 - {{{{p}_{0}}} \mathord{\left/ {\vphantom {{{{p}_{0}}} {{{p}_{{{\text{atm}}}}}}}} \right. \kern-0em} {{{p}_{{{\text{atm}}}}}}},\) where \({{p}_{0}}\) and \({{p}_{{{\text{atm}}}}}\) are the absolute pressure in the steam-gas volume of the thermosyphon and the atmospheric pressure, respectively. The value of \({{p}_{0}}\) was determined using a thermal method described in this paper, which does not require thermosyphon depressurization. After the last inspection of p_vac, four TSs were removed from the tests. The solution contained therein was taken for the chemical analysis of its composition. For the inspection of the inner surface, samples were cut out of the pipes. For the samples of one thermosyphon, metallographic studies were performed to assess changes in the structure and mechanical properties of the pipe metal in the course of hydrogen diffusion through the pipe wall. It was revealed that 16 of 19 TSs exhibit a decrease in p_vac less than 6% during the test period. A high performance was also obtained for TSs manufactured according to the economical technology reported in this paper. The oxygen-free environment inherent in the thermosyphons provide self-passivation of the thermosyphon internal surface with the formation of a protective layer of magnetite. This layer reduces the corrosion rate with releasing hydrogen up to the rate of hydrogen removal via diffusion through the wall of a carbon steel pipe. Retaining the initial vacuum, reducing the wall thickness of the thermosyphon less than by 0.1 mm, and the positive results of metallographic studies confirmed the potentialities of long-term thermosyphon operation with retaining high heat-transfer characteristics and with the absence of hydrogen influence on the structure and mechanical properties of the thermosyphon metal.