The causes leading to header's supports and tube socket cracking on superheater

and the entire header shifting upwards

which were discovered in the major repair of 125 MW unit(SG-400-50411 type)

have been analysed.It is believed that the main causes are as follows: the secondary stress of steam conduit tube exceeding set standard;insufficient flexibility of the steam conduit tube;unreasonal designed structure of the orginal sliding supports

etc.For these

some remedial measures

such as retrofitling the sliding structure of supports

increasing the flexibility of steam conduit tube

adopting steam conduit tube of small diameter and thin wall etc.

have been put forward.The experiences can be used as reference for repair and operation personnel.