The connection between tube and tube plate is an important part in the design of shell and tube heat exchanger.It is not only the processing workload, and must make each connection in the operation of the equipment, to ensure the media without leakage and withstand the media pressure capacity
For the connection structure of pipe and tube plate, there are mainly the following three types: (1) expansion, (2) welding, (3) expansion welding.These forms in addition to the inherent characteristics of their own structure, in the processing, production conditions, operating technology have a certain relationship.
1. expanded joint
It is used in the case that the leakage of medium between the shell and tube will not cause bad consequences.As the plastic deformation occurs at the end of the expansion joint, residual stress exists. With the rise of temperature, the residual stress gradually disappears, thus reducing the effect of sealing and binding force at the end of the tube.Therefore, the expansion structure is limited by pressure and temperature.Generally applicable to pressure P0≤4MPa, the limit temperature of residual stress disappearance at the end of the pipe, varies with the material, for carbon steel, low alloy steel when the operating pressure is not high, the operating temperature can be up to 300℃.In order to improve the quality of the expanding tube, the hardness of the tubesheet material should be higher than that of the tube end, so as to ensure the strength and tightness of the expanding joint.
For the roughness of the joint surface, the size of the pores between the pipe and the pipe also has a certain effect on the quality of the expansion tube. For example, if the joint surface is rough, greater friction can be generated, and it is not easy to pull after the expansion. If it is too smooth, it is easy to pull, but not easy to leak.Longitudinal grooves are not allowed on the bonding surface to ensure no leakage.
There are two kinds of pipe holes: light hole and ring slot hole. The form of pipe hole is related to the expansion strength. Light hole can be used when the tension at the expansion is small, while the structure with ring slot can be used when the tension is large.
The optical hole structure is used for heat exchangers with good material properties. The depth of tube expansion is 3mm less than the thickness of tube plate. When the thickness of tube plate is greater than 50mm, the expansion depth e is generally 50mm and the length of tube end extension is 2-3 mm.
When expanding, the end of the tube will be expanded into a conical shape, because of the role of flanging, the tube and tube plate can be combined more firmly, the ability to resist the pull force is higher.When the tube bundle is under compressive stress, the flanged structure is not adopted.
The purpose of grooving the pipe hole is similar to the flanging of the pipe mouth, mainly to improve the tensile force and enhance the tightness.Its structure is to open a small circular groove in the tube hole, the groove depth is generally 0.4~ 0.5mm, when the expansion of the tube, the tube material is squeezed into the groove, so the medium is not easy to leak out.The number of grooves in the tube hole is determined according to the thickness of the tube plate. When the tube plate is less than 30mm, one groove is made; when the thickness of the tube plate is ≥ 30mm, two grooves are made.
The expansion depth is determined according to the full expansion type and the non-expansion type. When the tube plate is not full expansion type, when the thickness of the tube plate is greater than 50 mm, the expansion depth is still 50 mm.
The tube plate is a composite steel plate, and the grooving position is divided into two conditions. When the cladding layer is thin, the grooving position is on the base layer. If the cladding layer is thick, a groove can be opened on the cladding layer, but it is not allowed to open the groove between the cladding layer and the base layer.
At present, the welding of pipe and tubesheet is widely used, because the tube hole does not need grooves, and the roughness of the tube hole is not high, the end of the pipe does not need annealing and polishing, so the manufacturing is simple.High strength welding structure, strong tensile force, when the welding part leakage, you can repair welding, if you need to replace the pipe, can use special tools to remove welding broken leakage pipe, but more convenient than disassembly tube expansion.
For the welding of pipe and tubeplate, the shear section of the weld shall not be less than 1.25 times of the pipe section.
Stainless steel tube and tube plate, generally adopts the welding structure, regardless of its pressure, temperature.To after parking, avoid the stagnation of fluid tube plate, and compensate the special case of the pipe pressure loss at the entrance, reduce the pipe resistance, can be in pipe shrinkage tube plate holes within a certain position, but this kind of structure welding technology demand is higher, generally need to use automatic argon arc welding machine, the quality can be guaranteed, in the process of welding nozzle blocking, especially for small diameter pipe, should pay attention when welding.Sometimes in order to reduce the welding stress, the tube plate orifice can be machined downward concave groove surface, the structure is generally used for stainless steel and tube plate welding.The grooves around the pipe holes are difficult to process and the workload is large. The grooves have been removed in the current construction.
3. expansion welding combination
For high pressure, strong permeability, or corrosive medium on one side, in order to ensure that the other side of the material will not be contaminated after leakage, this requires that the connection between the pipe and tubescreen absolutely not leak, or to avoid the impact of vibration in the process of loading and operation on the weld, or to avoid the possibility of crack corrosion.
The structure of the combination of expansion and welding, from the point of view of the processing process, there are first expansion after welding, welding after expansion, welding after expansion and sticking expansion and other forms
After welding, expanding the tube first before welding can improve the anti-fatigue performance of the welding seam, because after expanding the tube, the tube is close to the hole wall of the tube plate, which can prevent the generation of cracks during welding.But due to the use by the tube expanding the aperture of lubricating oil into the joint, the residual oil and clearance of the air heat expansion and gasification, generated in the process of welding joint under the action of high temperature gas, escape from the weld surface, to cause weld porosity, seriously affect the quality of weld, so must be before welding the residual oil washed away.
After welding first expansion: the use of the first welding after expansion can eliminate the above phenomenon, but the use of the first welding after expansion may cause weld cracks during expansion.In order to prevent the occurrence of this phenomenon, in addition to careful operation and proper control during the expansion, the distance between the tube end, namely the first groove, and the surface of the tube plate should be considered larger, about 16mm, and the expansion should not be carried out within the range of 10-12mm from the surface of the tube plate, so as to avoid damage to the welding seam during the expansion.The advantage of first welding and then expansion lies in that there is no need to clean up the residual oil after the expansion tube, but the position of the expansion tube during the expansion tube after welding requires higher requirements, so no expansion must be carried out within the range of 10 ~ 12 mm, otherwise it is easy to damage the welding seam.
First expansion followed by welding or first welding followed by expansion, for the welding part: there are seal welding and strength welding two types of welding differences, for the expansion part, there are strength expansion and sticking expansion differences.For example, the combination of expansion and sealing welding structure, so that the expansion to bear the force, and sealing welding to ensure sealing.Sealing welding height is generally 1 ~ 2 mm, so will not affect the strength of expansion, but in the welding must clean the joint oil.The combination of strength welding and expansion (expansion) is designed to withstand the force of welding, and the purpose of expansion is only to eliminate the gap between the tube and the tubesheet, so as to prevent the gap from being eroded by corrosive media.
Expansion and sticking: expansion and sticking after welding are generally used in heat exchange equipment with high pressure. The welding part is to strengthen seal welding, and the welding waist height is 2.8mm. The expansion part bears the force.
Under what condition is the structure that welds bilge to combine, use after welding first bilge or bilge first hind weld, still do not have unified regulation at present, but generally tend to be bilge after welding first is advisable.At present, due to the different technology, equipment conditions, are used to the factory's production methods.
4. inner hole welding
Internal hole welding is the butt structure formed by the pipe hole at the shell side. The butt welding of the heat exchange pipe requires special welding equipment.Internal hole welding is the form of butt weld formed between the tube plate and the heat exchange tube after processing. Special equipment is required to weld the torch from the tube hole at the side of the tube plate to the welding seam (from the original joint to the butt joint), optimize the stress state at the connection between the heat exchange tube and the tube plate, and greatly reduce the edge stress.It is very practical for heat exchangers with stress corrosion or gap corrosion medium.
However, the internal hole welding requires a high level of welding technology and is difficult to repair, which will lead to the scrap of the whole heat exchanger.In order to ensure qualified welding, welding and testing shall be conducted strictly in accordance with the construction process parameters.
5. Explosion expansion
This is a new technology developed in recent years. Due to the use of explosive expansion and sealing welding or strength welding, not only the connection strength is high, but also the expansion efficiency is greatly improved.There is no lubricating oil in the explosion expansion and no oil at the end of the pipe, which is of great benefit to the welding after expansion.
Explosive expansion is the use of explosives, in a very short period of time, the tube under the action of high pressure gas shock wave, deformation, so that the tube escape firmly on the tube plate hole.Explosion expansion is suitable for expansion of thin wall tube, thick wall tube with small diameter and large thickness tube plate.The advantages of explosive expansion are large tensile force, small axial elongation and deformation of the pipe, when the pipe end leakage, when mechanical expansion can not be repaired, the use of explosive expansion repair effect is very good.