Tallinn Technical University
Thermal Engineering Department
Tallinn, Estonia

Recovery boiler "Mitsubishi-315" erected at the end 1964 has worked more than 240,000 hours.
Boiler consists of two drums connected with bank tubes, furnace with waterwalls, the first and the second stages of superheater and the vertical economizer with its headers. Some basic elements of boiler have been exchanged during operation the economizer tubes due to corrosion problems two times, the second stage of superheater, and the lower part of waterwalls in the furnace have been reconstructed. The fin tubes on the level below the second oil burners have been changed with smooth tubes so that one upper tube has been connected with two tubes by Y-joints. Original basic parts are drums with bank tubes, the first stage of superheater, upper waterwalls in the furnace and most of headers.
Long-term operation has been ascertained two problems heat transfer surfaces' corrosion and cracking in the drums. The main safety problem of boiler is developing of cracks during the opera-
tion. The reason of forming and developing of cracks is the subject for this paper.