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Learn how to connect the heat exchanger tube sheet to the tube
2022-06-28
Steel shell and tube heat exchangers are widely used in chemical production. Whether it is a fixed tube sheet, a floating head tube sheet, or a U-shaped shell and tube heat exchanger, the connection between the tube and the tube sheet is a very important structure in the heat exchanger. and link.
Since the heat exchange tube and the tube sheet are the barriers between the tube side and the shell side of the heat exchanger, the good quality of the connection joint between the heat exchange tube and the tube sheet is the main factor for the failure of the shell and tube heat exchanger.
The connection type of the heat exchanger tube sheet and the tube is divided into expansion joint, welding, expansion joint and welding according to the different use conditions of the heat exchanger.
First, put the tube expander into the tube, so that the diameter of the tube becomes larger, plastic deformation occurs, and it is tightly attached to the tube sheet. And the tube sheet in contact with the nozzle will be elastically deformed as the nozzle becomes larger.
When the expander is pulled out, the elastic deformation of the tube sheet will return to the previous size, but the nozzle after plastic deformation will remain enlarged and will not return to its original state, so the two are tightly connected.
The quality of the expansion port mainly depends on the radial residual compressive stress on the pipe end, and its value is the same as the material and size of the pipe and the tube sheet. factor related.
In order to obtain good and stable expansion performance, in addition to strictly controlling the machining accuracy of the tube sheet and ensuring the appropriate hardness difference between the tube sheet material and the tube material, it is also necessary to correctly select the tube expander, the power and control means to ensure proper expansion. degree and adopt a reasonable expansion sequence.
Roller Expansion
Three small-diameter rollers are embedded in a frame, and there is a tube expander with a tapered mandrel in the middle. When expanding the tube, the cylindrical part of the tube expander is inserted into the tube hole, and the mandrel is rotated by electric power, pneumatic power, etc. , Through the roller rotating along the mandrel in the circumferential direction, the mandrel is squeezed into the inner surface of the tube and the tube is forced to expand to achieve a certain degree of expansion, so that the tube is tightly expanded to the hole of the tube sheet.
The tube expansion operation can be divided into two types: forward type and backward type. The forward type is to insert the frame into the tube, rotate the mandrel, and squeeze forward. After reaching the set tightening degree, the motor reverses, and the tube is pulled out from the tube to complete the expansion process. .
The reverse type rotates the mandrel forward like the forward type. After reaching the original tightening degree, the motor s. At the same time, the clutch of the reverse device is engaged and reversed. The relative position of the roller and the mandrel remains unchanged. Parallel expansion is carried out continuously and uniformly from the depth to the inlet.
Since this expansion process is from the inside to the outside, the elongation of the pipe occurs on the outside of the tube sheet, which can eliminate the stress state of the tube bundle and improve the quality of the product, so it is used for connections with an expansion length greater than 60cm.
Explosion process
Using high-energy explosives, the huge pressure of the shock wave generated at the moment of explosion (10 × 10-6 ~ 12 × 10-6s) forces the tube to produce high-speed plastic deformation, thereby expanding the tube and the tube sheet together. To realize the connection of the tube to the tube sheet.
The schematic diagram of the process is as follows:
The cylindrical explosive is placed in the center of the tube end. In order to prevent the shock wave from damaging the tube wall, there is a tubular buffer packing (viscous or plastic) around the explosive, so that the pressure can be evenly transmitted to the tube wall.
Hydraulic expansion process
Hydraulic expansion is a new expansion technology, which is expanded to the inner surface of the plate hole by applying high hydraulic pressure to the inner surface of the tube to plastically deform the tube.
The expansion head of the hydraulic expansion joint is a section of mandrel with a diameter slightly smaller than the inner diameter of the tube. There are multiple seals on the outer surface of the two ends of the mandrel, and an oil inlet hole is arranged in the middle of the mandrel. The high pressure is applied to the pipe section to make the pipe plastically expand and deform to realize the expansion joint.
The use of hydraulic expansion pipes in high-speed manufacturing has obvious advantages over mechanical expansion pipes, especially for products that only require sealing and expansion requirements, it not only effectively improves work efficiency, but more importantly, the quality of expansion joints has been significantly improved. The improvement, when the power plant is running, avoids or reduces the leakage of the pipe end, reduces the economic loss caused by the high pressure shutdown to the power plant, and has significant social benefits.
Rubber tube expansion process
The new rubber expansion technology is developed on the basis of the deformation of rubber under force. It uses the radial pressure generated by the axial compression of the rubber elastic body to expand the tube to the tube plate.
When the loading rod exerts a tensile force, the expanding rubber is axially compressed and radially expanded at the same time, and the expanding force is sufficient to deform the tube material, thereby realizing the connection between the tube and the tube sheet.
Expanding rubber is made of materials with high elasticity and high strength. In order to prevent the axial movement of the rubber under high pressure, special hard rubber sealing rings are installed at both ends of the expansion head.
The tie rod of the rubber expansion tube is made of high-strength steel. It is loaded on the tie rod through the pressure water or oil of about 20MPa. Since the pulling force is balanced against the pressure ring, it forms an internal force system without other supports or constraints.
Welding is divided into strength welding and sealing welding.
The welding process is simple, the connection strength is good, and the tightness and pull-off resistance of the connection can be ensured at high temperature and high pressure. The fixing of the pipe and the thin tube sheet should adopt the welding method. After the joint is welded, the residual thermal stress and stress concentration in the tube sheet and the tube may cause stress corrosion and fatigue damage during operation.
In addition, the stagnant liquid in the gap between the tube and the tube sheet hole has a difference in concentration with the liquid outside the gap, and it is also prone to crevice corrosion. Currently, inner hole welding is recommended for occasions with high working conditions.
Strength Welding
Strength welding refers to welding that ensures the sealing performance and tensile strength of the connection between the heat exchange tube and the tube sheet.
After the heat exchange tube and the tube sheet are welded, the residual thermal stress and stress concentration in the tube sheet and the tube may cause stress corrosion and fatigue during operation. In addition, the stagnant liquid in the gap between the tube and the tube sheet hole has a large concentration difference with the liquid outside the gap, which is also prone to crevice corrosion.
In addition to the occasions with large vibration and crevice corrosion, as long as the material has good weldability, strength welding can be used in other occasions. The connection between the tube and the thin tube sheet shall be welded.
Sealing Welding
Bee welding refers to the welding that ensures the sealing performance of the connection between the heat exchange tube and the tube sheet.
Seal welding effectively seals the connection between the heat exchange tube and the tube sheet, and strengthens the weld penetration with appropriate welding methods and welding process parameters, so that it has a certain weld strength.
When the heat exchange tube and the tube sheet are connected by welding, due to the low processing requirements of the tube sheet, the manufacturing process is simple, the sealing performance is good, and the welding, appearance inspection and maintenance are very convenient. It is a widely used connection method for connecting medium heat exchange tubes and tube sheets.
When using welded connections, there are strength welding to ensure the tightness and pull-off strength of the welded joints, and sealing welding to only ensure the tightness of the connection between the heat exchange tube and the tube sheet. For strength welding, its performance is limited, and it is only suitable for occasions with small vibration and no crevice corrosion.
When welding is used, the distance between the heat exchange tubes should not be too close, otherwise due to the influence of heat, the quality of the welding seam will not be guaranteed. The length of the heat exchange tube extending out of the tube sheet should meet the specified requirements to ensure its effective bearing capacity.
In the welding method, according to the material of the heat exchange tube and the tube sheet, welding can be carried out by electrode arc welding, TIG welding, CO2 welding and other methods. For heat exchangers with high connection requirements between heat exchange tubes and tube sheets, such as high design pressure, high design temperature, large temperature change, and heat exchangers under alternating loads, thin tube sheet heat exchangers, etc., TIG welding should be used. .
When the temperature and pressure are high, and under the action of thermal deformation, thermal shock, thermal corrosion and fluid pressure, the connection between the heat exchange tube and the tube sheet is easily damaged, and it is difficult to ensure the connection strength and tightness by expansion or welding. Require.
At present, the method of expansion welding is widely used. The expansion joint and welding structure can effectively damp the damage of the tube bundle vibration to the weld, can effectively eliminate stress corrosion and crevice corrosion, improve the fatigue resistance of the joint, and thus improve the service life of the heat exchanger. Strength welds have higher strength and tightness.
For ordinary heat exchangers, the form of "strength expansion % strength welding" is usually used; while the heat exchangers with severe operating conditions require the form of "strength expansion % sealing welding". According to the sequence of expansion and welding in the process, expansion and welding can be divided into two types: first expansion and then welding and first welding and then expansion.
Expansion before welding
The lubricating oil used during expansion will penetrate into the joint gap, and they have a strong sensitivity to welding cracks, pores, etc., so that the phenomenon of welding defects is more serious. It is difficult to remove these oil stains that penetrate into the gap, so the process of first expansion and then welding is adopted, and mechanical expansion is not suitable. Although it is not pressure-resistant, it can eliminate the gap between the tube and the tube hole of the tube sheet, so it can effectively damp the vibration of the tube bundle to the welding part of the nozzle.
However, the conventional manual or mechanically controlled expansion method cannot achieve uniform expansion requirements, while the liquid bag expansion method in which the expansion pressure is controlled by a computer can easily and uniformly achieve the expansion requirements. During welding, due to the influence of molten metal at high temperature, the gas in the gap is heated and expands rapidly. These gases with high temperature and high pressure will cause certain damage to the sealing performance of the strength expansion when they leak out.
Weld first and then expand
For the welding-before-expansion process, the primary issue is to control the precision and fit of the tube and the tube sheet hole. When the gap between the tube and the tube hole of the tube sheet is small to a certain value, the expansion process will not damage the quality of the welded joint. However, the ability of the welded joint to withstand shearing force is relatively poor, so during strength welding, if the control does not meet the requirements, it may cause overexpansion failure or damage to the welded joint.
In the manufacturing process, there is a large gap between the outer diameter of the heat exchange tube and the tube hole of the tube sheet, and the gap between the outer diameter of each heat exchange tube and the tube hole of the tube sheet is not uniform in the axial direction. When the expansion joint is completed after welding, the center line of the pipe must coincide with the center line of the tube hole of the tube sheet to ensure the quality of the joint. If the gap is large, due to the large rigidity of the pipe, excessive expansion deformation will cause the welded joint damage, and even cause the solder joint to be desoldered.
According to the processing conditions, welding expansion joint can be divided into first welding and then expanding, first expanding and then welding. The advantages and disadvantages are as follows:
The expansion-before-welding manufacturing process requires a high degree of cleanliness of the tubes and tube sheets, otherwise manufacturing defects are likely to occur. However, the cleanliness of the tube sheet and the tube is not high after the expansion after welding.
In the process of first expansion and then welding, the welding has an adverse effect on the expansion joint, and it is easy to cause relaxation of the expansion joint. The gas generated during welding is not easy to be eliminated, and weld defects are prone to occur, and this situation can be fundamentally avoided by first welding and then expanding.
From the point of view of weld quality and use effect, the welding first and then expanding process is also much better than the first expanding and then welding process.
Since the heat exchange tube and the tube sheet are the barriers between the tube side and the shell side of the heat exchanger, the good quality of the connection joint between the heat exchange tube and the tube sheet is the main factor for the failure of the shell and tube heat exchanger.
The connection type of the heat exchanger tube sheet and the tube is divided into expansion joint, welding, expansion joint and welding according to the different use conditions of the heat exchanger.
Expansion
First, put the tube expander into the tube, so that the diameter of the tube becomes larger, plastic deformation occurs, and it is tightly attached to the tube sheet. And the tube sheet in contact with the nozzle will be elastically deformed as the nozzle becomes larger.
When the expander is pulled out, the elastic deformation of the tube sheet will return to the previous size, but the nozzle after plastic deformation will remain enlarged and will not return to its original state, so the two are tightly connected.
The quality of the expansion port mainly depends on the radial residual compressive stress on the pipe end, and its value is the same as the material and size of the pipe and the tube sheet. factor related.
In order to obtain good and stable expansion performance, in addition to strictly controlling the machining accuracy of the tube sheet and ensuring the appropriate hardness difference between the tube sheet material and the tube material, it is also necessary to correctly select the tube expander, the power and control means to ensure proper expansion. degree and adopt a reasonable expansion sequence.
Traditional expansion technique
Roller Expansion
Three small-diameter rollers are embedded in a frame, and there is a tube expander with a tapered mandrel in the middle. When expanding the tube, the cylindrical part of the tube expander is inserted into the tube hole, and the mandrel is rotated by electric power, pneumatic power, etc. , Through the roller rotating along the mandrel in the circumferential direction, the mandrel is squeezed into the inner surface of the tube and the tube is forced to expand to achieve a certain degree of expansion, so that the tube is tightly expanded to the hole of the tube sheet.
The tube expansion operation can be divided into two types: forward type and backward type. The forward type is to insert the frame into the tube, rotate the mandrel, and squeeze forward. After reaching the set tightening degree, the motor reverses, and the tube is pulled out from the tube to complete the expansion process. .
The reverse type rotates the mandrel forward like the forward type. After reaching the original tightening degree, the motor s. At the same time, the clutch of the reverse device is engaged and reversed. The relative position of the roller and the mandrel remains unchanged. Parallel expansion is carried out continuously and uniformly from the depth to the inlet.
Since this expansion process is from the inside to the outside, the elongation of the pipe occurs on the outside of the tube sheet, which can eliminate the stress state of the tube bundle and improve the quality of the product, so it is used for connections with an expansion length greater than 60cm.
Explosion process
Using high-energy explosives, the huge pressure of the shock wave generated at the moment of explosion (10 × 10-6 ~ 12 × 10-6s) forces the tube to produce high-speed plastic deformation, thereby expanding the tube and the tube sheet together. To realize the connection of the tube to the tube sheet.
The schematic diagram of the process is as follows:
The cylindrical explosive is placed in the center of the tube end. In order to prevent the shock wave from damaging the tube wall, there is a tubular buffer packing (viscous or plastic) around the explosive, so that the pressure can be evenly transmitted to the tube wall.
New technology for expansion
Hydraulic expansion process
Hydraulic expansion is a new expansion technology, which is expanded to the inner surface of the plate hole by applying high hydraulic pressure to the inner surface of the tube to plastically deform the tube.
The expansion head of the hydraulic expansion joint is a section of mandrel with a diameter slightly smaller than the inner diameter of the tube. There are multiple seals on the outer surface of the two ends of the mandrel, and an oil inlet hole is arranged in the middle of the mandrel. The high pressure is applied to the pipe section to make the pipe plastically expand and deform to realize the expansion joint.
The use of hydraulic expansion pipes in high-speed manufacturing has obvious advantages over mechanical expansion pipes, especially for products that only require sealing and expansion requirements, it not only effectively improves work efficiency, but more importantly, the quality of expansion joints has been significantly improved. The improvement, when the power plant is running, avoids or reduces the leakage of the pipe end, reduces the economic loss caused by the high pressure shutdown to the power plant, and has significant social benefits.
Rubber tube expansion process
The new rubber expansion technology is developed on the basis of the deformation of rubber under force. It uses the radial pressure generated by the axial compression of the rubber elastic body to expand the tube to the tube plate.
When the loading rod exerts a tensile force, the expanding rubber is axially compressed and radially expanded at the same time, and the expanding force is sufficient to deform the tube material, thereby realizing the connection between the tube and the tube sheet.
Expanding rubber is made of materials with high elasticity and high strength. In order to prevent the axial movement of the rubber under high pressure, special hard rubber sealing rings are installed at both ends of the expansion head.
The tie rod of the rubber expansion tube is made of high-strength steel. It is loaded on the tie rod through the pressure water or oil of about 20MPa. Since the pulling force is balanced against the pressure ring, it forms an internal force system without other supports or constraints.
Welding
Welding is divided into strength welding and sealing welding.
The welding process is simple, the connection strength is good, and the tightness and pull-off resistance of the connection can be ensured at high temperature and high pressure. The fixing of the pipe and the thin tube sheet should adopt the welding method. After the joint is welded, the residual thermal stress and stress concentration in the tube sheet and the tube may cause stress corrosion and fatigue damage during operation.
In addition, the stagnant liquid in the gap between the tube and the tube sheet hole has a difference in concentration with the liquid outside the gap, and it is also prone to crevice corrosion. Currently, inner hole welding is recommended for occasions with high working conditions.
Strength Welding
Strength welding refers to welding that ensures the sealing performance and tensile strength of the connection between the heat exchange tube and the tube sheet.
After the heat exchange tube and the tube sheet are welded, the residual thermal stress and stress concentration in the tube sheet and the tube may cause stress corrosion and fatigue during operation. In addition, the stagnant liquid in the gap between the tube and the tube sheet hole has a large concentration difference with the liquid outside the gap, which is also prone to crevice corrosion.
In addition to the occasions with large vibration and crevice corrosion, as long as the material has good weldability, strength welding can be used in other occasions. The connection between the tube and the thin tube sheet shall be welded.
Sealing Welding
Bee welding refers to the welding that ensures the sealing performance of the connection between the heat exchange tube and the tube sheet.
Seal welding effectively seals the connection between the heat exchange tube and the tube sheet, and strengthens the weld penetration with appropriate welding methods and welding process parameters, so that it has a certain weld strength.
When the heat exchange tube and the tube sheet are connected by welding, due to the low processing requirements of the tube sheet, the manufacturing process is simple, the sealing performance is good, and the welding, appearance inspection and maintenance are very convenient. It is a widely used connection method for connecting medium heat exchange tubes and tube sheets.
When using welded connections, there are strength welding to ensure the tightness and pull-off strength of the welded joints, and sealing welding to only ensure the tightness of the connection between the heat exchange tube and the tube sheet. For strength welding, its performance is limited, and it is only suitable for occasions with small vibration and no crevice corrosion.
When welding is used, the distance between the heat exchange tubes should not be too close, otherwise due to the influence of heat, the quality of the welding seam will not be guaranteed. The length of the heat exchange tube extending out of the tube sheet should meet the specified requirements to ensure its effective bearing capacity.
In the welding method, according to the material of the heat exchange tube and the tube sheet, welding can be carried out by electrode arc welding, TIG welding, CO2 welding and other methods. For heat exchangers with high connection requirements between heat exchange tubes and tube sheets, such as high design pressure, high design temperature, large temperature change, and heat exchangers under alternating loads, thin tube sheet heat exchangers, etc., TIG welding should be used. .
Welding expansion joints
When the temperature and pressure are high, and under the action of thermal deformation, thermal shock, thermal corrosion and fluid pressure, the connection between the heat exchange tube and the tube sheet is easily damaged, and it is difficult to ensure the connection strength and tightness by expansion or welding. Require.
At present, the method of expansion welding is widely used. The expansion joint and welding structure can effectively damp the damage of the tube bundle vibration to the weld, can effectively eliminate stress corrosion and crevice corrosion, improve the fatigue resistance of the joint, and thus improve the service life of the heat exchanger. Strength welds have higher strength and tightness.
For ordinary heat exchangers, the form of "strength expansion % strength welding" is usually used; while the heat exchangers with severe operating conditions require the form of "strength expansion % sealing welding". According to the sequence of expansion and welding in the process, expansion and welding can be divided into two types: first expansion and then welding and first welding and then expansion.
Expansion before welding
The lubricating oil used during expansion will penetrate into the joint gap, and they have a strong sensitivity to welding cracks, pores, etc., so that the phenomenon of welding defects is more serious. It is difficult to remove these oil stains that penetrate into the gap, so the process of first expansion and then welding is adopted, and mechanical expansion is not suitable. Although it is not pressure-resistant, it can eliminate the gap between the tube and the tube hole of the tube sheet, so it can effectively damp the vibration of the tube bundle to the welding part of the nozzle.
However, the conventional manual or mechanically controlled expansion method cannot achieve uniform expansion requirements, while the liquid bag expansion method in which the expansion pressure is controlled by a computer can easily and uniformly achieve the expansion requirements. During welding, due to the influence of molten metal at high temperature, the gas in the gap is heated and expands rapidly. These gases with high temperature and high pressure will cause certain damage to the sealing performance of the strength expansion when they leak out.
Weld first and then expand
For the welding-before-expansion process, the primary issue is to control the precision and fit of the tube and the tube sheet hole. When the gap between the tube and the tube hole of the tube sheet is small to a certain value, the expansion process will not damage the quality of the welded joint. However, the ability of the welded joint to withstand shearing force is relatively poor, so during strength welding, if the control does not meet the requirements, it may cause overexpansion failure or damage to the welded joint.
In the manufacturing process, there is a large gap between the outer diameter of the heat exchange tube and the tube hole of the tube sheet, and the gap between the outer diameter of each heat exchange tube and the tube hole of the tube sheet is not uniform in the axial direction. When the expansion joint is completed after welding, the center line of the pipe must coincide with the center line of the tube hole of the tube sheet to ensure the quality of the joint. If the gap is large, due to the large rigidity of the pipe, excessive expansion deformation will cause the welded joint damage, and even cause the solder joint to be desoldered.
According to the processing conditions, welding expansion joint can be divided into first welding and then expanding, first expanding and then welding. The advantages and disadvantages are as follows:
The expansion-before-welding manufacturing process requires a high degree of cleanliness of the tubes and tube sheets, otherwise manufacturing defects are likely to occur. However, the cleanliness of the tube sheet and the tube is not high after the expansion after welding.
In the process of first expansion and then welding, the welding has an adverse effect on the expansion joint, and it is easy to cause relaxation of the expansion joint. The gas generated during welding is not easy to be eliminated, and weld defects are prone to occur, and this situation can be fundamentally avoided by first welding and then expanding.
From the point of view of weld quality and use effect, the welding first and then expanding process is also much better than the first expanding and then welding process.
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