The connecting weld between the emergency drain pipe of the steam drum and the valve of Unit#2 in a power plant cracked during operation,resulting in the non stop of the unit.In order to find out the causes,the layout of the emergency drainage pipe system,the material of the cracked weld,the valve selection,assembly and welding process were comprehensively analyzed.The results show that due to improper valve selection of emergency drain pipe,the weld type is unreasonable,the assembly process is poor,and the welding materials are selected incorrectly,resulting in the internal stress caused by thermal expansion of components sustained by the fillet weld connecting the valve and the pipe for a long time.At the same time,the operating temperature of the boiler startup and shutdown pipeline changes.With the accumulation of pipeline vibration cyclic loading,cracks initiate and expand under the action of alternating stress.Finally,the weld joint cracks and fails under the superposition of pipeline working stress,thermal stress and internal medium pressure.The valve was replaced and operated for 35000 hours,and no abnormality was found during reinspection.
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