The Connect Way Between Heat Exchanger Tube Sheet With the Tube

Do you know how the heat exchanger tube plates and tubes are connected? What are the characteristics of the different connection methods?

Steel tube and shell heat exchanger is very common in chemical production, whether it is a fixed tube plate or floating head tube plate, U-shaped tube and shell heat exchanger, the connection between the tube and tube plate is a very important structure and link in the heat exchanger.

As the heat exchanger tube and tube plate is the only barrier between the heat exchanger tube and shell process, so the heat exchanger tube and tube plate connection joint quality of the good ring is the shell and tube heat exchanger failure of the most important factors.

Heat exchanger tube plate and tube connection joint type, according to the different conditions of use of the heat exchanger, divided into expansion, welding, expansion plus welding.

1.Expansion joint

First of all, the expander into the tube, so that the tube diameter of the mouth becomes larger, plastic deformation occurs, tightly fit on the tube plate. The tube plate in contact with the orifice will also become larger as the orifice becomes larger, thus undergoing elastic deformation.

When the expander is pulled out, the elastic deformation of the tube plate will return to its previous size, but after the plastic deformation of the orifice still remains large, will not return to its original state, so that the two are tightly connected together!

2.Welding

Welding is divided into two types: strength welding and sealing welding.

The welding process is simple, the strength of the connection is good, the tightness of the connection and the ability to resist pull-off can be guaranteed at high temperatures and pressures, the tube and the thin tube plate should be fixed using welding methods. When the connection is welded, the residual thermal stress and stress concentration in the tube plate and tube may cause stress corrosion and fatigue damage during operation.

In addition, the gap between the tube and the tube plate hole in the existence of non-flowing liquid and the gap outside the liquid has a difference in concentration, but also easy to produce gap corrosion, the current working conditions in the higher requirements of the occasion recommended to use the inner hole welding.

Strength welding

Strength welding refers to ensure the sealing performance of the heat exchanger tube and tube plate connection and tensile strength welding.

Schematic diagrams of strength welding are as follows.

Seal welding

Seal welding refers to welding that ensures the sealing performance of the heat exchanger tube and tube plate connection.

The diagram of seal welding is as follows.

3.Welding plus expansion joint

When the temperature and pressure is high, and in the thermal deformation, thermal shock, thermal corrosion and fluid pressure, the heat exchanger tube and plate connection is very easy to be destroyed, the use of expansion or welding are difficult to ensure the strength of the connection and sealing requirements.

At present, the widely used method is the expansion and welding. Expansion plus welding structure can effectively damp tube beam vibration damage to the weld, can effectively eliminate stress corrosion and gap corrosion, improve the fatigue resistance of the joint, thereby improving the service life of the heat exchanger, than simply expansion or strength welding has a higher strength and sealing.

For ordinary heat exchangers are usually used "paste expansion % strength welding" form; and the use of harsh conditions of heat exchangers require the use of "strength expansion % sealing welding" form. Expansion plus welding according to expansion and welding in the sequence of the process can be divided into first expansion after welding and first welding after the expansion of two.

Expand first and weld later

The lubricating oils used during expansion will penetrate into the joint gap and they are highly sensitive to welding cracks, porosity, etc., thus making the phenomenon of defects in welding more serious. These oils that penetrate into the gap are difficult to remove cleanly, so it is not advisable to use a mechanical expansion process by first expanding and then welding. The use of paste expansion, although not pressure resistant, can eliminate the gap between the tube and the tube plate tube hole, so it can effectively damp the tube bundle vibration to the welded part of the orifice.

However, the use of conventional manual or mechanically controlled expansion methods cannot achieve uniform paste expansion requirements, while the use of computer-controlled expansion pressure of the liquid bag type expansion method can easily and uniformly achieve paste expansion requirements. When welding, due to the influence of high temperature molten metal, the gas in the gap is heated and expands rapidly, these gases with high temperature and pressure can cause some damage to the sealing performance of the strength expansion when leaking out.

Schematic diagram as follows.

Weld first, expand later

For the weld first and then expand process, the primary issue is to control the accuracy of the tube and the tube plate hole and its fit. When the gap between the tube and the tube plate hole is small to a certain value, the expansion process will not damage the quality of the welded joint. But the ability of the weld mouth to withstand shear forces is relatively poor, so the strength of the weld, if the control does not meet the requirements, may cause over-expansion failure or expansion of the welded joint damage.

In the manufacturing process, there is a large gap between the outer diameter of the heat exchanger tube and the tube plate tube hole, and the outer diameter of each heat exchanger tube and the tube plate tube hole gap along the axial direction is not uniform. When the welding is completed after expansion, the tube centre line must coincide with the tube plate hole centre line to ensure the quality of the joint, if the gap is large, due to the greater rigidity of the tube, excessive expansion deformation will produce damage to the welded joint, or even cause the weld off.

The schematic diagram is as follows.