The leakage failure occurs for the head flange connection system of a reforming reactor after the insulation layer is replaced and the reforming reactor has been running for six months.In order to solve this problem,the failure mechanism considering elevated temperature creep of flanges and bolts was studied by transient thermalstructure analysis.Results show that the elevated temperature creep of bolts and flanges has a significant effect on the contact stress of gasket,but the maximum contact stress of gasket of the head flange connection system is greater than the operating specific pressure of gasket after six months’ service,therefore the elevated temperature creep of bolts and flanges is not the main cause of leakage.Cooling amplitude on the outer surface of the flange joint when replacing the insulation layer has a significant influence on contact stress of the gasket.With the increasing of cooling amplitude,the contact stress of gasket is gradually reduced,and the effective sealing width is also reduced.When the cooling amplitude is too large,the contact stress of gasket will decrease to be less than the operating specific pressure of gasket,and the gasket will lose the sealing capacity.Therefore,the excessive cooling amplitude when replacing the insulation layer is the main cause of leakage.