Research On Tubesheet Cracking Reason In Shell-and-tube Heat Exchanger

Shell-and-tube heat exchanger is a common heat-exchange equipment, which is widely used in petroleum, chemical and energy industry. Tubesheet is one of the most important parts in shell-and-tube heat exchanger, and the performance of tubesheet is important for the safety and economics of heat exchanger. The tubesheet and tube cracking reason of two different heat exchanger in a device was mainly analyzed, the main contents as follows:1. More than ten tubes of the first heat exchanger were cracked, and the fracture located at100mm to the tube head. There was much crack at the expansion area of most of tubes, and the crack distributes in the peripheral direction. Some white substance, yellow and black oxides were found in the outside surface of failure tube.The chemical composition analysis of tube material, the macro and micro structure analysis near fracture, and corrosion products of the fracture was carried out.Analysizing the tube cracking reason from the angle of material, it was found that the material of heat exchanger tube met standard requirements. As a conclusion, the inclusions in the material was the proximate cause of the crack of heat exchanger tube.2.Since the second heat exchanger was put into use, the heat exchanger tube and tubesheet cracked. The main failure mode was large area of cracks. Many cracks start from the tube, and extend to the tubesheet along the hoop and radial. By means of burnishing and painting the tubesheet, about one third of weld joints in the tubesheet and tube generate cracks.The chemical composition analysis of tube and tubesheet material, the metallographic structure of heat exchanger tubesheet, the macro and micro analysis of fracture, corrosion products fof the fracture, thermal analysis and stress field of tubesheet were carried out. It was found that the heat exchanger tube and tubesheet material met standard requirements; the low preheating temperature or no-preheating during soldering led to coarsening columnar structure in the weld joints, and the structure is the mainly reason for tubesheet cracking. Working stress and residual stress are the mainly reasons for causing tubesheet cracking, corrosion products promote the tubesheet cracking; the clearance between tubesheet and tube promotes the tube circumferential cracking. In conclusion, tubesheet cracking is the result of stress corrosion, hot corrosion fatigue and tube fluid-induced vibration. Finally, by means of changing segmental baffle, increasing baffle spacing and increasing steam outlet and inlet side of heat exchanger, the fluid-induced vibration impact could be reduced.