We have stacked heat exchanger designed with ASME Sec VIII Div 1. Exchanger experiences Thermal cycle with inlet temperature varying from 41 Deg C - 260 Deg C every hour. Fatigue evaluation for the exchanger was therefore requested as per ASME Sec VIII Div II.

Heat exchanger vendor is carring out FEA analysis to carry out fatigue evaluation only for the interconneting nozzle of the stacked heat exchanger.

When requested to carryout fatigue evaluation for all the elements of the exchanger, vendor is reluctant to do so justifying their stand that interconnection section is most stressed part and also their AI has accepted the same. Technically it may be correct but unless all the elements are checked, it is difficult to say which element will govern for fatigue life.

Request your opinion on the same.

Be aware that fatigue is caused by variation of temperature between adjacent parts. It means that if the item increases and decreases in temperature all together there is no fatigue stress. Maybe the part considered is the only where a temperature variation between adjacent parts exists. It is the owner through its data sheet to establish whether fatigue exists.
Mauro