Connection Structure of Heat Exchanger Tube and Tube Plate in the Design of Shell-and-Tube Heat Exchanger
Heat exchangers are widely used in chemical industry, petroleum, medicine, atomic energy and nuclear industry, and their types and structures are also numerous. Shell and tube heat exchangers are the most commonly used. In the design and manufacture of shell and tube heat exchangers, the connection between heat exchanger tubes and tube sheets directly affects the normal operation of the process, and even forces the whole production line to stop production. Therefore, the technical research on the joint type of heat exchanger tube and tube sheet has always been the focus of attention of domestic and foreign technicians.
1 Common Connection Method of Heat Exchanger Tube and Tube Sheet
The connection methods of heat exchanger tube and tube sheet mainly include expansion joint, welding and expansion joint.
1.1 Expansion joint
Expansion is to use the expander to insert the nozzle to rotate and expand the end of the tube which penetrates the hole of the tube sheet, so that the tube can achieve plastic deformation. At the same time, the hole of the tube sheet is expanded to produce elastic deformation. After the tube expander withdraws, the elasticity of the tube sheet recovers, and the contact surface between the tube and the tube sheet produces a great extrusion force, so that the tube and the tube sheet can be firmly combined to achieve both sealing and pull-off resistance purposes. There are two kinds of holes on the tube sheet, one is hole wall grooving, the other is hole wall grooving, as shown in Figure 1.
Figure 1 Expansion joint
At present, the expanding process mainly includes mechanical expanding, hydraulic expanding, explosive expanding, rubber expanding and so on.
Expansion joint is suitable for situations without severe vibration, excessive temperature change and serious stress corrosion. Because of the close connection between the pipe and the hole, the pipe joint can reduce the medium corrosion and withstand the pull-out force.
Expansion joint is simple in structure and easy to repair when medium leakage between shell and tube does not cause adverse consequences. As the plastic deformation occurs at the end of the expansion joint, there exists residual stress. With the increase of temperature, the residual stress gradually disappears, which reduces the effect of sealing and binding force at the end of the expansion joint. Therefore, the expansion structure is limited by pressure and temperature. Generally applicable pressure P < 4 MPa, the limit temperature of residual stress disappearance at the end of the pipe varies with the material. For carbon steel and low alloy steel, the operating temperature can be 300 C when the operating pressure is not high. In order to improve the quality of expansion tube, the hardness of tube sheet material is higher than that of tube end, so as to ensure the expansion strength and tightness.
For the roughness of the joint surface, the size of the pore between the pipe hole and the tube also has a certain impact on the quality of the expansion pipe. If the joint surface is rough, it can produce greater friction. It is not easy to pull off after expansion, but it is easy to pull off if it is too smooth, but it is not easy to leak. The general roughness requirement is Ra12.5. Longitudinal grooves are not allowed on the joint surface in order to ensure that there is no leakage on the joint surface.
There are two kinds of holes: smooth holes and circular grooves. The form of the holes is related to the expansion strength. When the expansion force is small, the smooth holes can be used. When the pull-out force is large, the structure with circular grooves can be used.
Optical hole structure is used in heat exchangers with good material properties. The expanding depth is 3 mm less than the thickness of tube sheet. When the thickness of tube sheet is more than 50 mm, the expanding depth e is generally 50 mm, and the length of tube end is 2-3 mm.
When expanding, the end of the pipe is expanded into a conical shape. Because of the flanging effect, the connection between the pipe and the tube sheet is more firm and the pull-out resistance is higher. When the tube bundle is subjected to compressive stress, the flanging structure is not used.
The purpose of slotting is similar to the flanging of the pipe orifice, which is mainly to improve the pull-out resistance and seal performance. Its structure is to open a small annular groove in the hole, the groove depth is generally 0.4-0.5 mm. When expanding the tube, the material of the tube is squeezed into the groove, so the medium is not easy to leak out. The number of slots in the holes depends on the thickness of the tube sheet. When the tube sheet is less than 30 mm, one slot should be opened. When the thickness of the tube sheet is more than 30 mm, two slots should be opened.
The expansion depth is determined by full expansion and non-expansion. When the tube sheet is not fully expanded, when the thickness of tube sheet is more than 50 mm, the expansion depth is still 50 mm.
The tubesheet is a composite steel plate, and the slotting position can be divided into two cases. When the coating is thin, the slotting position is on the base. If the coating is thick, a slot can be on the top of the cladding, but it is not allowed to slot between the cladding layer and the base layer.
There are two types of welding between heat exchanger tube and tube sheet: end welding and inner hole welding. The typical structure of end welding is shown in Fig. 2.
Fig. 2 The typical structure of end welding
The main application occasions of welding connection between tube bundle and tube sheet are:
- (1) when the pipe spacing is too small or the thin tube sheet cannot be expanded;
- (2) when the thermal cycle is intense and the temperature difference is high;
- (3) when the pressure is high or the connection tightness is strictly required.
It can ensure the tensile strength of welded joints.
End welding belongs to incomplete fusion welding, according to its different requirements, its welding depth can be divided into:
- (1) Strength welding (to ensure the connection strength between heat exchanger tube and tube sheet);
- (2) Sealing welding (only to play a sealing role).
End welded joints have the advantages of easy welding, appearance inspection and maintenance, and are most widely used. However, there is a gap between the tube and the tube sheet, which is easy to cause gap corrosion when used in corrosive media.
1.3 Expansion Welding
When the temperature and pressure are high, and under the action of thermal deformation, thermal shock, thermal corrosion and fluid pressure, the joint of heat exchanger tube and tubesheet is easy to be destroyed. It is difficult to ensure the strength and sealing requirements of the joint by expansion or welding. At present, expansion welding is widely used. Experiments show that the combination of expansion welding can improve the fatigue resistance of joints, effectively eliminate stress corrosion and gap corrosion, and improve their service life. In addition, the heat transfer area of tube-side medium to tube-sheet is many times larger than that of shell-side medium to tube-sheet, especially in the case of thick tube-sheet. This can reduce the temperature difference between the two sides of the tubesheet, reduce the warpage of the tubesheet, and is conducive to the reliability of the tubesheet seal.
Expansion welding can be divided into strength welding + sticking expansion, strength welding + strength swelling, strength swelling + sealing welding, strength swelling + sticking expansion + sealing welding, strength welding + strength swelling + sticking expansion + sealing welding, strength welding + strength swelling + sticking expansion, etc.
2. Welding of Heat Exchanger Tube and Tube Plate Inner Hole
Inner hole welding replaces the end welding of heat exchange pipe and tube sheet with inner hole welding of tube bundle. In the late 1960s, internal hole welding was studied abroad and applied to nuclear equipment in the 1970s. In the mid-1970s, China began to carry out experimental research on internal hole welding, and in the late 1970s began to be applied to nuclear equipment and power plant equipment.
2.1 Joint Type of Internal Hole Welding
There are two types of joints for internal hole welding: one is docking, the other is lapping. In Figure 3, the (a) – (e) type belongs to the inner hole docking. (a), (b), (c) are basically the same. There is a lock edge, the pipe installation is easy to be centered and positioned, and the weld seam has a high strength, which is smooth transition with the tubesheet joint, and improves the static and dynamic load strength of the joint; (a) and (b) the tubesheet joint is flat with the tubesheet, and a stress buffer groove is machined around the hole of the tubesheet, which has a high processing cost. The (b) type tubesheet joints have grooves, so the penetration is improved; (c), (d) and (e) type tubesheet joints extend out of the tubesheet plane, which is conducive to the radiographic detection of welds. Among them, the (d) type adopts groove docking, which is suitable for the occasion of larger diameter and thicker wall of heat exchanger tube. (e) The thinning of the tubesheet joint ensures equal thickness connection with the pipe bundle, improves the stress condition of the joint, and makes the weld leave the edge zone; (f) – (j) type belongs to the overlap of the inner hole. Among them (f) type needs less welding current and is easy to be welded. (g) type needs large welding current, but it is easy to process. (h) and (i) type joints are filled with gaskets, whose function is to control the composition of the weld seam and improve the performance of the joints (j) type is suitable for electron beam welding. When welding, the electron gun is outside the tube sheet, and the electron beam is directly injected into the joint, so the joint can not be limited by the diameter of the tube; (k) type is socket joint, which is between lap and butt, and has both advantages of lap and butt.
The above types of joints have been widely used in foreign countries. All butt welded joints have good mechanical properties. But because of the difficulty of assembly, it is more prevalent only in the United States. The mechanical properties of quasi-butt welded joints with internal holes are not as good as that of full-butt welded joints due to shearing. However, because of its easy assembly, quasi-butt welded joints are prevalent in Western Europe and Japan. Considering the difficulty and cost of manufacturing, the types of joints (a) and (f) that are widely used in China at present are (a) and (f).
2.2 Characteristics of Inner Hole Welding
The main advantages of inner hole welding are:
- (1) The gap of end welding is eliminated, and the ability of anti-gap corrosion and anti-stress corrosion is improved.
- (2) It is a fully penetrated joint type with high vibration fatigue strength and can withstand high temperature and high pressure.
- (3) The aperture of butt-welded inner-hole can be smaller than that of end-welded, which increases the stiffness of tube-plate.
- (4) The stress concentration at the opening of the joint is small, and cracks at the root of the joint are not easy to occur.
- (5) Internal non-destructive testing can be carried out for butt joint. Internal quality of weld can be monitored and controlled, which improves the reliability of weld.
- (6) Although the welding space of inner hole welding is very small, it is easy to realize automation.
- (7) Suitable for welding various materials of tube and tube sheet of heat exchanger. Such as carbon steel, low alloy steel, stainless steel and nickel-based alloy.
Although the inner hole welding is better than the end welding in structure, it also has some shortcomings.
- (1) Internal hole welding requires high processing and assembling accuracy of tubesheet, which improves the cost of equipment.
- (2) Special welding equipment and highly automated welding technology are required.
- (3) It is very difficult to repair the weld when it is unqualified.
2.3 Welding Equipment
Pulsed current tungsten argon arc welding is necessary for inner hole welding equipment, and corresponding pulse current tungsten argon arc welding machine is needed, and the welding process can be programmed. The requirements are:
- (1) It can output the pulse current needed for welding, and can accurately control the magnitude of the current and the output time.
- (2) It can change and precisely control the time of pre-delivery and delayed delivery.
- (3) It can change and precisely control the welding travel, and attenuate automatically after completion of welding.
- (4) The welding process parameters are stable, reliable and reproducible.
- (5) The gun head should be inserted into the hole of the tube plate smoothly, positioned accurately and removed smoothly after welding.
- (6) The relative position of tungsten electrode and welding torch can be adjusted.
- (7) The contact between the gun head and the workpiece has good insulation.
In recent years, special equipment for inner hole welding has been developed in China. For example, the NZA4-250 automatic pulsed tungsten rotary argon arc welding machine for tubesheet produced by Shanghai East China Electric Welding Machine Factory is equipped with inner hole welding torches which can be used for 15-100mm (butt) and 10-100mm (lap) inner diameter of tubesheet, and the JM-200 transistor pulsed tungsten arc welding machine developed by Dalian 523 Factory is equipped with inner hole welding torches with weldable pipe diameters of 13-25 mm. EWA programmable welding power source developed by Kunshan Huaheng Welding Equipment Technology Co., Ltd. is equipped with tube sheet welding which can meet the minimum diameter of 19 mm.
From the point of view of welding equipment, it is still a subject of future research and experiment to improve the precision of regulation of welding power source, the stability of current and the reliability of welding machine.
2.4 Application of Inner Hole Welding Technology
Internationally, Japan and Europe are the representatives. In the aspect of pressure vessel welding, the main target is vessel large-scale. The development of new materials, new steel grades and other related welding technology has been more and more widely used. In order to improve the welding quality, great progress has been made in the development of process equipment and welding materials.
At present, internal hole welding technology is mainly used in nuclear industry and electric power industry in China. For example:
- (1) The joint of heat exchanger tube bundle and tubesheet of Xi’an Nuclear Equipment Co., Ltd. adopts inner-hole welding technology, which improves the service life of the equipment;
- (2) Pulse tungsten argon arc welding machine and welding head manufactured by Shanghai East China Electric Welding Machine Factory are used in Shanghai Power Station Auxiliary Factory to realize inner-hole welding in the welding of high-pressure heating tube and tubesheet;
- (3) Harbin Boiler Works and Shanghai Boiler Works have successively popularized the inner hole welding and applied it to the bottom sealing welding of the inner hole of the joint of boiler header and boiler drum seat, which ensured the complete penetration of the joint of the tube seat, eliminated the defects of the original manual arc welding seam, such as incomplete penetration and slag inclusion, and possibly cancelled the water pressure test;
- (4) Harbin Welding Research The plasma arc welding technology for inner hole is studied, which adds a new welding technology for inner hole welding.
The inner hole welding technology needs to be popularized and applied in other industries.
2.5 Weld Quality Inspection
Welded joints need non-destructive inspection such as appearance inspection, coloring inspection, gas pressure test, nitrogen leak detection, radiography, ultrasonic inspection, eddy current inspection, water pressure test and so on. In addition, metallographic examination, mechanical properties and corrosion tests should be carried out. The corresponding equipment for X-ray and ultrasonic flaw detection has been developed abroad, and the above inspection technology has been mastered. The key problems of X-ray and ultrasonic inspection for inner hole welded joints have not been solved in China. Mainly rely on strict control of welding specification parameters to ensure the accuracy of welding assembly, as well as appearance inspection, coloring, fluorescence detection, water pressure test and sample performance test to ensure the quality of weld. Therefore, it is still a difficult task to develop X-ray source with micro-focus, X-ray flaw detection device and internal X-ray flaw detection device for small diameter tubes.
For these reasons, bore welding has become an indispensable manufacturing technology in nuclear industry and power industry. It is expected to be widely used in heat exchangers with high temperature, high pressure, strong corrosive or radioactive media. With the continuous progress of technology in petroleum, chemical industry and other fields, heat exchangers will develop towards high temperature, high pressure and large-scale, and the requirements for their manufacturing quality will become higher and higher. Inner hole welding technology will be more widely used in these industries.
Source: China Tube Sheet Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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