Five connection structures between steel tubes and tube sheets of heat exchangers

Five connection structures between steel tubes and tube sheets of heat exchangers. The connection between steel tubes and tube sheets is a relatively important structural part of the design of shell and tube heat exchangers. It not only requires a large amount of processing work but also requires that each connection be in operation to ensure that the medium does not leak and can withstand the medium pressure.

There are three main types of connection structures between steel tubes and tube sheets:
(1) Expansion
(2) Welding
(3) Expansion welding.
In addition to the inherent characteristics of these structures, these forms have a certain relationship with production conditions and operating techniques during processing.

01 Expansion
When the leakage of the medium between the tube shell will not cause adverse consequences, the expansion structure is simple and the steel tube is easy to repair. Since the expansion tube end produces plastic deformation during expansion, there is residual stress. As the temperature rises, the residual stress gradually disappears, which reduces the sealing and bonding effect at the tube end. Therefore, this expansion structure is subject to certain restrictions on pressure and temperature. Generally, the applicable pressure P0≤4MPa. The limit temperature at which the residual stress disappears at the pipe end varies with the material. For carbon steel and low alloy steel, when the operating pressure is not high, the operating temperature can be up to 300℃. To improve the quality of expansion, the hardness of the tube sheet material is required to be higher than the hardness of the steel pipe end, to ensure the expansion strength and tightness. The roughness of the joint surface and the size of the pores between the tube hole and the steel pipe also have a certain influence on the quality of expansion. If the joint surface is rough, it can produce a large friction force, and it is not easy to pull off after expansion. If it is too smooth, it is easy to pull off, but it is not easy to leak. The general roughness requirement is Ra12.5. To ensure that there is no leakage on the joint surface, longitudinal groove marks are not allowed on the joint surface.

There are two types of tube holes: smooth holes and holes with annular grooves. The form of the tube hole is related to the expansion strength. When the pull-off force on the expansion mouth is small, a smooth hole can be used, and when the pull-off force is large, a structure with annular grooves can be used. The light-hole structure is used for heat exchangers with better material properties. The expansion depth is the thickness of the tube sheet minus 3mm. When the thickness of the tube sheet is greater than 50mm, the expansion depth e is generally 50mm, and the tube end protrudes 2-3mm. When expanding, the tube end is expanded into a cone. Due to the effect of flanging, the steel tube and the tube sheet can be more firmly combined, and the ability to resist pull-off is higher. When the tube bundle is subjected to compressive stress, the flanging structure is not adopted. The purpose of grooving the tube hole is similar to the flanging of the tube mouth, which is mainly to improve the pull-off resistance and enhance the sealing. Its structural form is to open a small annular groove in the tube hole. The groove depth is generally 0.4~0.5mm. When expanding the tube, the steel tube material is squeezed into the groove, so the medium is not easy to leak out. The number of grooves in the tube hole depends on the thickness of the tube sheet. When the tube sheet is less than 30mm, one groove is opened, and when the thickness of the tube sheet is ≥30mm, two grooves are opened. The expansion depth is determined by full expansion or non-expansion. When the tube sheet adopts non-full expansion when the thickness of the tube sheet is greater than 50 mm, the expansion depth is still 50 mm. The tube sheet is a composite steel plate, and the slot position is divided into two cases. When the coating is thin, the slot position is on the base layer. If the coating is thick, a slot can be opened on the composite layer, but it is not allowed to open the slot between the coating and the base layer.

02 Welding of welded steel pipes and tube sheets
Currently, it is widely used. Since the tube holes do not need to be slotted, and the roughness requirements of the tube holes are not high, the ends of the steel pipes do not need to be annealed and polished, so the manufacturing and processing are simple. The welded structure has high strength and strong pull-off resistance. When the welded part leaks, it can be repaired. If the steel pipe needs to be replaced, a special tool can be used to disassemble the welded leaking pipe, which is more convenient than disassembling the expanded pipe. The shear section of the weld between the steel pipe and the tube sheet should not be less than 1.25 times the section of the steel pipe.

Stainless steel pipes and tube sheets generally adopt welding structures, regardless of their pressure and temperature. To avoid fluid stagnation on the tube sheet after parking, compensate for the special situation of pressure loss at the entrance of the steel pipe, and reduce the resistance of the tube mouth, the steel pipe can be shrunk to a certain position inside the tube sheet hole, but this structure has high welding technology requirements, and generally requires an automatic argon arc welding machine to ensure quality. The tube mouth is easy to be blocked during welding, especially for small-diameter steel pipes, so attention should be paid during welding. Sometimes, to reduce welding stress, a concave groove surface can be machined downward at the tube sheet hole. This structure is generally used for welding stainless steel and tube sheets. Grooving around the tube hole is troublesome and labor-intensive. The groove has been eliminated in the current construction.

03 Expansion welding combination
For high pressure, strong permeability, or corrosive media on one side, to ensure that the leakage does not contaminate the material on the other side, this requires that the connection between the steel pipe and the tube sheet is leak-proof, or to avoid the impact of vibration on the weld during shipping and operation, or to avoid the possibility of crevice corrosion, etc. From the perspective of processing technology, the structure of expansion welding combination has several forms, such as expansion before welding, welding before expansion, welding after expansion, and expansion after welding.

Expanding before welding, expanding the tube before welding can improve the fatigue resistance of the weld because the tube wall is close to the tube plate hole wall after expansion, which can prevent cracks during welding. However, when expanding the tube, the lubricating oil enters the gap of the joint due to the use of lubricating oil. These residual oil stains exist and the air in the gap is heated expanded and gasified. During the welding process of the joint, gas is generated under the action of high temperature and escapes from the welding surface, causing pores in the weld, which seriously affects the quality of the weld. Therefore, these residual oil stains must be cleaned before welding.
Welding before expansion: The above phenomenon can be eliminated by welding before expansion, but welding before expansion may cause cracks in the weld during expansion. To prevent this phenomenon, in addition to careful operation and proper control during expansion, the distance between the tube end, that is, the first groove and the tube sheet surface should be considered larger, about 16 mm, and expansion should not be performed within the range of 10 to 12 mm from the tube sheet surface to avoid damage to the weld during expansion. The advantage of welding first and then expanding is that there is no need to clean the oil residue after expansion, but the expansion position during expansion after welding is required to be high, and it must be ensured that expansion is not performed within the range of 10 to 12 mm, otherwise, it is easy to damage the weld.
Expand first and then weld or weld first and then expand. For the welding part, there are two welding forms: sealing welding and strength welding. For the expansion part, there are strength expansion and sticking expansion. For example, the structure combined with expansion and sealing welding is that the expansion bears the force, while the sealing welding ensures the sealing. The height of the sealing weld is generally 1 to 2 mm, which will not affect the expansion strength, but the oil stains at the joint must be cleaned during welding. The structure of strength welding and expansion (sticking and expansion) is to bear the force by welding, and the purpose of sticking and expansion is only to eliminate the gap between the steel pipe and the tube sheet to prevent the gap from being corroded by corrosive media.
Expansion and expansion after welding: expansion and expansion after welding are generally suitable for heat exchange equipment with higher pressure. The welding part is to strengthen the sealing welding, and the welding waist height is 2.8 mm. The expansion part bears the force. When the expansion fails, the strengthening sealing welding can bear the force. The sticking and expansion part is to eliminate the corrosion of the gap.

Under what conditions does the structure of welding and expansion adopt first welding and then expansion or first expansion and then welding? There is no unified regulation at present, but it is generally preferred to weld first and then expand. At present, due to the different processing technology and equipment conditions of each manufacturer, they are all accustomed to the production method of this factory.

04 Inner hole welding Inner hole welding
The tube hole forms a butt structure on the shell side. The heat exchange tube needs special welding equipment to butt weld it. Inner hole welding is a form of butt welding between the tube sheet and the heat exchange tube after processing. Special equipment is required to penetrate the welding gun from the tube hole on the tube sheet side into the weld for welding (from the original intersection joint to the butt joint), which optimizes the stress state at the connection between the heat exchange tube and the tube sheet and greatly reduces the edge stress. It is very practical for heat exchangers with stress corrosion or gap corrosion media. However, inner hole welding requires a high welding technology level and is difficult. If welding defects occur, they cannot be repaired, which will cause the entire heat exchanger to be scrapped. To ensure qualified welding, welding, and testing must be carried out strictly by the construction process parameters.

05 Explosive expansion
The explosive expansion method is used to connect steel pipes and tube sheets. It has begun to be used abroad. This is a new process developed in recent years. Due to the use of explosive expansion plus sealing welding or strength welding methods, not only the connection strength is high, but also the expansion efficiency is greatly improved. No lubricating oil is required during explosive expansion, and there is no oil stain on the pipe end, which is very beneficial for welding after expansion. An explosive expansion joint uses explosives to deform the inside of the tube under the action of high-pressure gas shock waves in a very short time so that the tube is firmly attached to the tube sheet hole. An explosive expansion joint is suitable for the expansion of thin-walled tubes, thick-walled small-diameter tubes, and thick tube sheets. The advantages of explosive expansion joints are high pull-out resistance, small axial elongation, and deformation of steel pipes. When the end of the steel pipe leaks and cannot be repaired by mechanical expansion, an explosive expansion joint is used for repair.