Connection of heat exchange tubes and tube sheets in shell and tube heat exchangers(Ⅰ)

In the shell-and-tube heat exchanger, the heat exchange tube and the Tube Sheet are the only barrier between the tube side and the shell side of the heat exchanger. The connection structure and connection quality between the heat exchange tube and the tube sheet determine the quality of the heat exchanger. The pros and cons and service life are a crucial link in the heat exchanger manufacturing process.

The damage and failure of most heat exchangers occur at the connection between the heat exchange tube and the tube sheet, and the quality of the joint also directly affects the safety and reliability of chemical equipment and devices. The connection process of heat pipe and tube sheet has become the most critical control link in the quality assurance system of heat exchanger manufacturing. At present, in the heat exchanger manufacturing process, the connection between the heat exchange tube and the tube sheet mainly includes: welding, expansion joint, expansion joint plus welding, glue joint plus expansion joint and other methods.

1. Welding

When the heat exchange tube and the tube sheet are connected by welding, due to the lower requirements for the processing of the tube sheet, the manufacturing process is simple, the sealing is good, and the welding, appearance inspection and maintenance are very convenient. It is the current Shell And Tube Heat Exchanger. It is the most widely used connection method for heat exchange tubes and tube sheets. When welded connections are used, there are strength welds that ensure the tightness of welded joints and pull-off strength, and seal welds that only guarantee the sealability of the connection between heat exchange tubes and tube sheets. For strength welding, its performance is limited, and it is only suitable for occasions with small vibration and no crevice corrosion.

When using welding connection, the distance between the heat exchange tubes should not be too close, otherwise the quality of the weld seam is not easy to be guaranteed due to the influence of heat. At the same time, a certain distance should be left at the ends of the tubes to reduce the welding stress between them. The length of the heat exchange tube protruding from the tube sheet must meet the specified requirements to ensure its effective carrying capacity. In terms of welding methods, according to the materials of heat exchange tubes and tube sheets, electrode arc welding, TIG welding, CO2 welding and other methods can be used for welding. TIG welding should be used for heat exchangers with high requirements for the connection between heat exchange tubes and tube sheets, such as high design pressure, high design temperature, large temperature changes, heat exchangers subject to alternating loads, and thin tube sheet heat exchangers. .

The conventional welding connection method, due to the gap between the tube and the tube plate hole, is prone to crevice corrosion and overheating, and the thermal stress at the welded joint may also cause stress corrosion and damage, which will cause the heat exchanger to fail. At present, in the heat exchangers used in the domestic nuclear industry, electric power industry and other industries, the connection between the heat exchange tube and the tube sheet has begun to use the inner hole welding technology. This connection method changes the end welding of the heat exchange tube and the tube sheet to The inner hole welding of the tube bundle adopts the form of full penetration, which eliminates the gap of the end welding and improves the ability of resisting crevice 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 joints; internal non-destructive testing can be performed on joints, and the internal quality of welds can be controlled, which improves the reliability of welds. However, the assembly of the inner hole welding technology is difficult, the requirements for welding technology are high, the manufacturing and inspection are complicated, and the manufacturing cost is relatively high. With the development of high temperature, high pressure and large-scale heat exchangers, the requirements for its manufacturing quality are getting higher and higher, and the inner hole welding technology will be more widely used.

2. Expansion

Expansion joint is a traditional connection method between heat exchange tubes and tube sheets. The tube sheet and tubes are elastic-plastically deformed to form a firm connection by using tube expansion equipment to achieve the purpose of sealing and resistance to pull-off. In the manufacturing process of heat exchangers, expansion joints are suitable for occasions where there is no severe vibration, no excessive temperature changes, and no severe stress corrosion.

The expansion joints currently used mainly include mechanical rolling and hydraulic expansion. The mechanical rolling expansion joint is uneven, and once the connection between the tube and the tube sheet fails, it is very difficult to repair it with the expansion tube; the liquid bag hydraulic expansion joint is controlled by the computer, with high precision, and can ensure the uniform tightness of the expansion joint. The reliability of the connection is better than mechanical expansion joints. However, there are strict requirements on machining accuracy, and it is difficult to ensure the success of expansion joints for densely distributed joints. If it fails, it is also difficult to expand and repair it.