Expansion joints in the design and manufacture of common problems

Listed U-shaped single wave expansion joint design, manufacture of common problems, to remind the designer’s attention.

An expansion joint is installed in the fixed tube plate heat exchanger shell on the flexible components, relying on the deformation of the flexible structure, the tube bundle, and shell expansion deformation between the difference in compensation to eliminate the temperature difference between the shell and the tube bundle stress. Expansion joints are widely used in the industrial sector due to their reliable operation, good performance, compact structure, and other advantages. From the wave shape to points, there are U-shaped, Ω-shaped, and C-shaped expansion joints, among which U-shaped expansion joints are the most widely used. Below I U-shaped single wave expansion joint design, manufacture common problems to elaborate to remind the designer and reduce errors.

1. When to set the expansion joint

In the fixed tube and plate heat exchanger, shell and heat exchanger tube is a rigid connection between the operating conditions due to the temperature difference between the tube wall and shell wall temperature difference will cause temperature difference stress, temperature difference stress may cause heat exchanger tube pull off or tube bundle instability, therefore, when the thermal expansion deformation difference is very large, it is necessary to set up the expansion difference compensation element – expansion joints.
Whether to set the expansion joint and need for heat exchanger specific design parameters, through the calculation, the stress level to weigh, and then decided. Historically, some information, subject to the limitations of the means of calculation, through a simple rough calculation of the cylinder or heat exchanger tube axial stress exceeds a specified value to determine whether to set the expansion joints, which is unreasonable, they assume that the tube sheet is rigid, each heat exchanger in the bundle of tubes are in the same state of pull, pressure, which is far from the actual stress on the tube plate and the bundle of tubes. In addition, some designers even rougher through the tube, shell program operating temperature difference, or design temperature difference is more than a specified value (such as more than 50 ℃) to determine whether the need to set the expansion joint, which is also incorrect, because the operating temperature difference and the design temperature difference is not the same as the metal temperature difference, and cannot be replaced by the heat exchanger tube and the shell program between the cylinder of the thermal expansion difference. Especially in the two materials at the same time (such as heat exchanger tube for stainless steel, shell cylinder for carbon steel), because the difference between the two coefficients of linear expansion is very large; in the extreme case may occur: the metal temperature difference between the tube and shell program is very large, but the difference in thermal expansion is zero; tube and shell program is a very small difference in the temperature of the metal or zero, and its thermal expansion is very large. Therefore, in the calculation of the tube plate according to the temperature difference between the various conditions calculated shell axial stress σ_c, heat transfer tube axial stress σ_t, heat transfer tube and tube plate between the pull-off force q, there is a cannot meet the strength (or stability) conditions, there is a need to set up expansion joints. Expansion joints in the fixed tube and plate heat exchanger, the axial stiffness of the shell is greatly reduced, thereby greatly reducing the transverse shear force and bending moment around the tube and plate to achieve a great easing of the purpose of the stress of the tube and plate. Can make the smaller tube plate thickness to meet the design requirements. This method also effectively reduces the pull-off force between the tube plate and the tube. Therefore, in the case of large thermal expansion of the shell, setting the expansion joint is very necessary and economical.
In the tube plate strength checking calculations, when the tube plate thickness is determined, not set the expansion joint, sometimes the tube plate strength is not enough, set the expansion joint, and the tube plate thickness may meet the requirements at this time, can be set to thin the expansion joint tube plate, but from the consumption of materials, manufacturing difficulties, safety and economic effects of the comprehensive assessment.

2. Expansion joint position setting problems

Ear support fixed tube plate heat exchanger with expansion joints, expansion joints at the end to which position is more appropriate? If the expansion joint is added to the support above and the support position is low, the expansion joint to withstand the additional axial force generated by the weight of the heat exchanger and reduces the stability of the equipment. Reasonable design is the expansion joint should be located in the support of the lower; when conditions allow, the center of gravity of the equipment should try to make it located in the support plane below the support to improve the stability of the equipment.

3. Expansion joints longitudinal welding seam number

When the bellows use the whole method of molding, the number of longitudinal welds refers to the provisions of Table 1.
Table.1

Nominal diameter DN/mm	≤600	650-1200	1300-1800	1900-2000
Number of longitudinal welds	1	≤3	≤4	≤5

Plate sub-flap welding half-wave overall stamping, the number of longitudinal welds refers to the provisions of Table 2.
Table.2

Nomminal diammeter DN/mm		≤600	650-900	1000	1100	1200	1300	1400	1500	1600	1700	1800	1900	2000
Carbon steel	S<5mm	0		≤4				≤5				≤6
	S≥5mm	0						≤3			≤4
Austenitic stainless steel	S<5mm	0	≤4					≤6
	S≥5mm	0				≤4						≤5

Note: bellows regardless of what method of forming, the number of longitudinal welds should be the principle of minimum, and the distance between the two adjacent longitudinal welds should not be less than 5mm.

4. Expansion joint corrosion margin issues

For carbon steel and low alloy steel, take C₂ ≤ 1mm; C₂> 1mm should be selected for austenitic stainless steel (when it is not appropriate to use stainless steel, C₂ can be selected according to the container design requirements); for austenitic stainless steel, when the medium corrosivity is very slight, take C₂ = 0mm.

5. Expansion joints fatigue life calibration

Austenitic stainless steel material bellows, when σ_R ≤ σ_s^t, may not consider the problem of low fatigue; otherwise, fatigue life should be calibrated. In GB16749-1997, Appendix A in the austenitic stainless steel expansion joint displacement is based on the design temperature of 300 ℃, the number of fatigue damage N = 4.5 × 10⁴ times calculated. Safety factor 15, the number of permissible cycles [N] = 3000 times. When the design temperature of 300 ℃, the actual operation of the cumulative number of permissible cycles is equal to 3000 times and can be selected according to the displacement of the table in Appendix A. If not in the design temperature of 300 ℃ or the actual need for the cumulative number of permissible cycles is not equal to 3000 times (permissible cycle selection range of 50 – 5000 times). Then the displacement should be corrected by multiplying the coefficient C_N or C_t based on the specified values in the table in Appendix A. SW6 software in the calculation; if the joint expansion material is stainless steel, you also need to choose the type of fatigue cycle load is the temperature difference cycle load or mechanical cycle load.

6. Expansion joints need to pay attention to the design of other issues

• (1) To reduce wear and tear expansion joints, prevent vibration and reduce fluid resistance, if necessary, in the expansion joints on the inside to set up a liner tube, the liner tube should be facing the direction of media flow at one end of the cylinder and equipment (or end pipe) welding. When the medium for steam or liquid and flow direction vertically upward should be set up in the liner sleeve at the lower end of 4 (90 °) diameter φ6mm semicircular liquid discharge hole or in the liner sleeve and equipment cylinder (or end pipe) welded weld on the weld with 4 intermittent welds (fracture size of 20mm).
• (2) The manufacture of expansion joints of the material, the requirements of good plasticity, good mechanical properties, and good corrosion resistance. For the expansion joints operating at high temperatures, but also need to choose a good thermal performance of the material, the expansion joints are generally used with the same materials and equipment cylinder when using stainless steel, intergranular corrosion conditions, the need for post-weld heat treatment.

7. Conclusion

An expansion joint is a fixed tube plate heat exchanger in the weaker links, the design and manufacture of which are difficult. Still, the expansion joint is an important part of the equipment we are required to design and manufacture and should be strictly by the provisions of the standards and specifications to ensure the safe operation of the equipment.
Author: Li Zhen