The High Temperature And High Pressure Heat Exchanger Flange Crack Failure Study

This paper is mainly to the research about flange cracking defect on a heat exchanger of Yulin refinery, finds out the reasons why the heat exchanger’s flange crack, and puts forward the countermeasures so as to prevent the similar flange crack and failure to happen again.The cracked welding neck flange’s inner diameter is Φ1200mm, the outer diameter isΦ1495mm, the kind of seal grooves is trapezoid trough, the material is16MnR, sealing gaskets is the octagonal ring. Though the Penetrant testing, find out the creak in the seal groove from the inner wall28-38mm, the most width of crack is4mm. Crack distribution complete turn seal grooves. The Ultrasonic testing show that the crack is most deep about150mm (crack length is about2200mm);. From the test results can be concluded that the flange crack almost throughout the flange thickness, belongs to the serious crack failure.Flange failure analysis related to the original design, flange manufacture, the usage environment, the raw materials used in supply and manufacturing, and many other factors. In order to know the causes of flange failure, this paper research from the above aspects one by one, analysis the detail of each factor, pay attention to identify the cause of the failure. Through chemical composition analysis,mechanical properties testing of materials,fracture surface analysis, It is concluded that chemical composition of the flange is in line with national standards;the flange material does not exist the problem of insufficient strength reserve; The impact power flange material is lower than that of the value of the technical standard.Through the flange design theory, and calculate the flange’s force on the preloaded condition and working condition, and through the flange’s strength calibration to calculate the related stress of flange. Flange strength calibration mainly through calculating the axial bending stress σz between the flange neck and flange joint, the flange ring radial stress σr and the flange ring shear stress σt. Then compare the σz、σr、σt with the flange’s design temperature allowable stress [σ]ft. It is concluded that the flange intensity meetsthe requirements, the original flange design is reasonable.By using the finite element analysis method, construct the flange model, mesh the model, apply loads, confirming boundary conditions, and solve the model. To get the flangeequivalent stress and strain under the preloaded condition and working condition and boltyield, It is concluded that when the preload is too large and make the bolt appears to yield, thestress of the flange seal groove is much greater than the tensile strength of the flange material,and make the seal groove appear crack, this is the main reason of cracking failure of flange.