6. Consideration
In this study, at first a pipe with large disc simplified the part of the boiler drum where feed water is introduced. Thermal stress distribution was obtained by thermal temperature distribution model of a large thickness pipe. Two kinds of heat transfer coefficient derived by Prandtl's formula and Von Karman's formula were used to obtain temperature at the inner edge of the feed water inlet pipe.
The result of this analysis for obtaining stress on the part of the boiler drum where feed water is introduced shows that maximum stress is formed on inner edge of the feed water inlet pipe and the stress decreases immediately as distance from the inner edge increases. The value of maximum stress at the inner edge of the feed water inlet pipe may be recognized to be less than 180 Mpa (N/mm2) under the condition of saturated boiler water pressure of 2.3 Mpa and feed water temperature of 50℃.
According to this result, direct connection of feed water inlet pipe with boiler drum may be permitted for a boiler of design steam pressure less than about 2.0 Mpa, especially less than 1.0 Mpa.
7. Conclusion
Rules of ClassNK requires that the part of the boiler drum where feed water is introduced is to be provided with sleeves or other suitable devices so that an extreme thermal stress may not occur due to direct contact of cold feed water with the drum. According to this study, direct connection of feed water inlet pipe with boiler drum may be possible for a boiler of design steam pressure less than about 2.0 Mpa, especially less than 1.0 Mpa.
Reference
[1] Mechanical Engineering Handbook, The Japanese Society of Mechanical Engineers, 1989, MARUZEN
[2] Kaneyasu Nishikawa & Yasunobu, "Heat Transfer Technology", 1994, RlCOGAKUSYA
[3] Youichiro Takeda, "Thermal Stress", 1971, NISHlN PUBLICATION
