Technology of connecting tube and tube sheet in shell and tube heat exchanger

What is a heat exchanger?

Heat exchanger is a kind of heat transfer equipment which transfers part of heat from hot fluid to cold fluid. It is widely used in people’s daily life, petroleum, chemical, power, medicine, atomic energy and nuclear industry. It can be used as independent equipment, such as heater, condenser, cooler, etc; It can also be used as part of some process equipment, such as heat exchanger in some chemical equipment.

Especially in the chemical industry with large energy consumption, heat exchanger is an indispensable equipment in the heat exchange and transfer process of chemical production, and also occupies a considerable proportion in the whole chemical production equipment.
From its function, on the one hand, the heat exchanger is to ensure the specific temperature of the medium required by the industrial process, on the other hand, it is also the main equipment to improve the efficiency of energy utilization. According to its structure, there are mainly plate heat exchanger, floating head heat exchanger, fixed tube plate heat exchanger and U-tube heat exchanger. In addition to plate heat exchanger, the others belong to shell and tube heat exchanger.

Because shell and tube heat exchanger has a large heat exchange area per unit volume, good heat exchange effect, strong structure, strong adaptability and mature manufacturing process, it has become the most commonly used typical heat exchanger.

Connection between heat exchange tube and tube sheet in shell and tube heat exchanger

In shell and tube heat exchanger, the heat exchange tube and tube sheet are the only barrier between the tube side and shell side of the heat exchanger. The connection structure and quality between the heat exchange tube and tube sheet determine the quality and service life of the heat exchanger, which is a crucial link in the heat exchanger manufacturing process.
Most of the damage and failure of heat exchanger occurs in the joint of heat exchange tube and tube sheet, and the quality of its joint also directly affects the safety and reliability of chemical equipment and devices. Therefore, the connection process of heat exchange tube and Tubesheet in shell and tube heat exchanger has become the most critical control link in the heat exchanger manufacturing quality assurance system. At present, in the process of heat exchanger manufacturing, the connection between heat exchange tube and tube sheet mainly includes welding, expanding, expanding plus welding and bonding plus expanding.

Welding

When the heat exchange tube and tubesheet are connected by welding, due to the lower requirements for tubesheet processing, simple manufacturing process, good sealing, and convenient welding, appearance inspection and maintenance, it is the most widely used connection method of heat exchange tube and tubesheet in shell and tube heat exchanger.
When using welding connection, there are strength welding to ensure the sealing and tensile strength of welded joint and sealing welding only to ensure the sealing of heat exchange tube and tubesheet connection. For the strength welding, its performance is limited, and it is only suitable for the occasions with small vibration and no interstitial corrosion.
When welding connection is adopted, the distance between heat exchange tubes should not be too close, otherwise the welding quality is not easy to be guaranteed due to the influence of heat. At the same time, there should be a certain distance between tube ends to reduce the welding stress. The length of the heat exchange tube protruding from the tubesheet shall meet the specified requirements to ensure its effective bearing capacity.
In terms of welding method, according to the material of heat exchange tube and tubesheet, the welding methods such as shielded metal arc welding, TIG welding and CO2 welding can be used. TIG welding should be used for heat exchangers with high connection requirements between tube and tubesheet, such as heat exchangers with high design pressure, high design temperature, large temperature variation, heat exchangers with alternating load, thin tubesheet heat exchangers, etc.
Due to the gap between the tube and the tube sheet hole, the conventional welding method is easy to produce gap corrosion and overheating, and the thermal stress generated at the welding joint may also cause stress corrosion and damage, which will make the heat exchanger failure.
At present, the inner hole welding technology has been used in the heat exchanger used in the domestic nuclear industry, electric power industry and other industries. In this connection method, the end welding of the heat exchange tube and the tube sheet is changed to the inner hole welding of the tube bundle, and the full penetration form is adopted, which eliminates the gap of the end welding and improves the ability of resistance to gap corrosion and stress corrosion,
It has high anti vibration fatigue strength, can withstand high temperature and high pressure, and has good mechanical properties of welded joint; The internal nondestructive testing of the joint can be carried out, the internal quality of the weld can be controlled, and the reliability of the weld can be improved.
However, the assembly of inner hole welding technology is difficult, the requirements of welding technology are high, the manufacturing and inspection are complex, and the manufacturing cost is relatively high. With the development of heat exchanger to high temperature, high pressure and large-scale, the requirement of its manufacturing quality is higher and higher, and the inner hole welding technology will be more widely used.

Expansion joint

Expansion joint is a traditional connection method between heat exchange tube and tube sheet, which makes tube sheet and tube produce elastic-plastic deformation and close fit by using expansion equipment, forming a firm connection, so as to achieve the purpose of both sealing and anti pull off. In the manufacturing process of heat exchanger, expansion joint is suitable for the occasions without severe vibration, excessive temperature change and severe stress corrosion.
At present, the expanding technology mainly includes mechanical rolling expanding and hydraulic expanding. Once the connection between tube and tubesheet fails, it is very difficult to repair it by expanding tube; The liquid bag type hydraulic expansion joint is controlled by computer, which has high precision and can ensure the uniformity of expansion joint tightness. The reliability of connection is better than that of mechanical expansion joint. However, the requirements for machining accuracy are strict, and it is difficult to ensure the successful expansion of densely distributed joints. If the joint fails, it is difficult to repair it.

Expansion and welding

When the temperature and pressure are high, and under the action of thermal deformation, thermal shock, thermal corrosion and fluid pressure, the joint between heat exchange tube and tube sheet is easy to be damaged, so it is difficult to ensure the requirements of connection strength and sealing performance by expanding or welding. At present, expanding welding is widely used. The expansion welding structure can effectively damp the damage of tube bundle vibration to the weld, effectively eliminate the stress corrosion and gap corrosion, and improve the fatigue resistance of the joint,
Thus, the service life of the heat exchanger is improved, and the strength and sealing performance are higher than those of simple expansion or strength welding. The common heat exchanger usually adopts the form of “sticking expansion% strength welding”; However, the heat exchanger with severe service conditions requires the form of “strength expansion% seal welding”. According to the sequence of expansion and welding, expansion and welding can be divided into expansion before welding and expansion after welding.
(1) The lubricating oil used in expanding before welding will penetrate into the joint gap, and they are highly sensitive to welding cracks and pores, which makes the phenomenon of defects more serious. It is difficult to remove the oil that penetrates into the gap, so it is not suitable to use mechanical expansion welding instead of expansion welding. It can eliminate the gap between the tube and the hole of the tube sheet, so it can effectively damp the vibration of the tube bundle to the welding part of the pipe orifice.
However, the conventional manual or mechanical expansion method can not meet the requirements of uniform sticking expansion, while the liquid bag expansion method controlled by computer can easily and evenly meet the requirements of sticking expansion. During welding, due to the influence of high-temperature molten metal, the gas in the gap is heated and expands rapidly. When these gases with high temperature and high pressure leak out, they will cause certain damage to the sealing performance.
(2) For the process of welding before expanding, the first problem is to control the precision and matching of the tube and tube sheet hole. When the gap between the tube and the tube sheet is small enough, the quality of the welded joint will not be damaged. However, the ability of the welded joint to bear the shear force is relatively poor, so if the control of the strength welding can not meet the requirements, it may cause over expansion 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 tubesheet hole, and the gap between the outer diameter of each heat exchange tube and the tubesheet hole is uneven along 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 greater rigidity of the pipe, the excessive expansion deformation will damage the welded joint, and even cause the welded joint to break off.

Bonding and expansion

The process of gluing and expanding is helpful to solve the problem of leakage and leakage at the joint of heat exchange tube and tube sheet in heat exchanger. It is important to select the correct gluing agent according to the working conditions of the glued parts. In the process of process implementation, the process parameters should be selected according to the structure and size of heat exchanger, including curing pressure, curing temperature, expansion force, etc., and strictly controlled in the production process. This process is simple, easy to operate and reliable. It has been recognized in the actual use of enterprises and has promotional value.

Summary

• (1) In the connection method of heat exchange tube and tube sheet of shell and tube heat exchanger, it is difficult to ensure the connection strength and sealing requirement by using conventional welding or expansion joint alone.
• (2) The method of expanding and welding is beneficial to ensure the connection strength and sealing between the heat exchange tube and tube sheet, and improve the service life of the heat exchanger.
• (3) The method of gluing and expanding is helpful to solve the problems of leakage and leakage when the heat exchange tube is connected with the tubesheet. The process is simple and reliable.
• (4) As a full penetration welding method, inner hole welding technology has good resistance to gap corrosion, stress corrosion, vibration fatigue and mechanical properties of welded joints; The internal quality of the weld can be controlled, which improves the reliability of the weld. First of all, it is more suitable for promotion and application in high-end products.

Source: China Tube sheet Manufacturer – Yaang Pipe Industry (www.epowermetals.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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