The tubesheet is forged with 20R, the tubesheet diameter is 1.5m, and the tubesheet thickness is 200mm. The connection between tube plate and tube shall be welded after expansion. Spiral plate heat exchanger There are many cracks on the tubesheet, and the cracks on the tubesheet are concentrated in the upper half. The cracks mainly occurred in the gap between the tube sheet and the tube bundle joint. The crack propagated from the tube bundle to the tube sheet in a circular and radial manner.

Corrosive substances mainly include Fe, Al, C, S, Si, O and Cu. Among them, Fe and Cu mainly come from steel itself. C and S are derived from oil, and O, Si and Al are derived from aluminosilicate cracking catalysts. If the tube sheet is broken, the oil in the tube will enter the crack and cause great corrosion.

The tube sheet has flat fracture, the color is gray, and there is no plastic deformation, which is brittle fracture. The cracks originate from grooves and diverge outward in a dendritic shape. The tail is sharp, the macro fracture of the transient fault zone is flat, and the appearance is clear. The macroscopic morphology of the crack has the typical morphological characteristics of stress corrosion cracking.

Scanning the fracture with a scanning electron microscope shows that its plane is a fracture unit. In the crack propagation zone, the crack originates from a hard point in the non inclusion or grain. The fracture pattern is like a river pattern, extending from the center to the surrounding area, short and bent, with fewer branches, and continuous expansion. The tube sheet of the spiral plate heat exchanger is mainly dendritic, The crack tail is sharp; The crack morphology is mainly formed by grains, accompanied by a small amount of intergranular, indicating that the fracture is close to stress corrosion fracture.

When the tubesheet is under stress, the defects or inclusions on the metal surface form microcrack sources, and the solution inside and outside the hole is not easy to convection and diffusion. Due to the increase of metal ions and hydrogen ions in the crack, in order to maintain electrical neutrality, the sulfur anion outside the crack enters the crack to form corrosive acid, which makes the corrosion inside the crack continue in an autocatalytic manner.

　　 Spiral plate heat exchanger There are some defects such as band inclusions in the tube sheet of. Due to the existence of temperature difference stress, manufacturing stress, working stress and residual stress, there are a lot of residual stresses in the tubesheet. The analysis of corrosion products shows that when the tube sheet cracks, the oil in the pipeline enters the cracks, further accelerating the corrosion.