Common Problems in Design and Manufacture of Expansion Joints
The expansion joint is a flexible component installed on the shell of the fixed tube sheet heat exchanger. It compensates the expansion deformation difference between the tube bundle and the shell by relying on the flexible structure that is easy to deform to eliminate the temperature difference stress between the shell and the tube bundle. Expansion joints are widely used in industrial sectors due to their reliable operation, good performance, compact structure and other advantages. From the shape of wave, there are U-shaped, Ω shaped and C-shaped expansion joints, among which U-shaped expansion joints are most widely used. Now I will elaborate on the common problems in the design and manufacture of U-shaped single wave expansion joint, so as to remind the designer and reduce errors.
When to set expansion joints
In a fixed tubesheet heat exchanger, the shell and the heat exchange tubes are rigidly connected. Under operating conditions, the temperature difference between the tube wall and the shell wall will cause temperature difference stress, which may cause the heat exchange tube to pull off or the tube bundle to lose stability. Therefore, when the thermal expansion deformation difference is large, the expansion difference compensation element – expansion joint must be set.
Whether to set expansion joints or not, it is necessary to weigh the stress level obtained through calculation according to the specific design parameters of the heat exchanger, and then make a decision. In some historical data, due to the limitation of calculation means, it is unreasonable to judge whether to set expansion joints by simply and roughly calculating whether the axial stress of the cylinder or heat exchange tube exceeds a specified value. They assume that the tubesheet is absolutely rigid, and each heat exchange tube in the tube bundle is in the same tension and compression state, which is far from the actual stress condition of the tubesheet and tube bundle. In addition, some designers even roughly judge whether an expansion joint is required by whether the operating temperature difference of the tube and shell side or the design temperature difference exceeds a specified value (such as more than 50 ℃), which is also incorrect, because the operating temperature difference and the design temperature difference are not equal to the metal temperature difference, nor can they replace the thermal expansion difference between the heat exchange tube and the shell side cylinder. Especially when the two materials are different (for example, when the heat exchange tube is made of stainless steel and the shell side cylinder is made of carbon steel), due to the large difference between their linear expansion coefficients, it may occur in extreme cases: the metal temperature difference between the tube and shell side is large, but the thermal expansion difference is zero; The metal temperature difference between the tube and shell sides is very small or zero, while the thermal expansion difference is very large. Therefore, the axial stress of the shell is calculated according to various working conditions with temperature difference in the calculation of the tubesheet σ c. Axial stress of heat exchange tube σ t. When one of the pull-out forces q between the heat exchange tube and the tube sheet fails to meet the strength (or stability) conditions, an expansion joint is required. When expansion joints are set in fixed tubesheet heat exchangers, the axial stiffness of the shell is greatly reduced, thereby greatly reducing the transverse shear force and bending moment around the tubesheet, so as to greatly alleviate the stress of the tubesheet. The smaller thickness of tube sheet can meet the design requirements. This method is also very effective in reducing the pulling force between the tube sheet and the tube. Therefore, it is very necessary and economical to set up expansion joints when the thermal expansion difference of the shell is large.
In the tubesheet strength check calculation, when the tubesheet thickness is determined and no expansion joint is set, sometimes the tubesheet strength is not enough, and the tubesheet thickness may meet the requirements after the expansion joint is set. At this time, an expansion joint can be set to reduce the tubesheet thickness, but it should be determined from the comprehensive evaluation of material consumption, manufacturing difficulty, safety and economic effects.
Position setting of expansion joint
When adding expansion joints to the fixed tubesheet heat exchanger supported by ear type supports, where is the appropriate position for adding expansion joints? If the expansion joint is added on the support and the support position is low, the expansion joint will bear the additional axial force generated by the dead weight of the heat exchanger and reduce the stability of the equipment. The reasonable design is that the expansion joint should be located below the support. If conditions permit, the center of gravity of the equipment should be located below the support plane of the support to improve the stability of the equipment.
Number of longitudinal welds of expansion joint
When the corrugated pipe is formed by integral method, the number of longitudinal welds shall refer to Table 1.
|Nominal diameter DN/mm||≤600||650 – 1200||1 300 – 1800||1900 – 2000|
|Number of longitudinal welds||1||≤3||≤4||S5|
The number of longitudinal welds shall refer to the provisions in Table 2 when the plate is split, welded and half wave integral stamping.
|Nominal diameter DN/mm||≤600||650~900||1000||1100||1200||1300||1400||1500||1600||1700||1800||1900||2000|
|Austenitic stainless steel||S<5mm||0||≤4||≤5||≤6||≤7|
Note: No matter what method is used for forming corrugated pipes, the number of longitudinal welds shall be the minimum, and the distance between two adjacent longitudinal welds shall not be less than 5mm.
Corrosion allowance of expansion joint
For carbon steel and low alloy steel, C2 ≤ 1mm; When C2>1mm, austenitic stainless steel shall be selected (when stainless steel is not suitable, C2 can be selected according to the vessel design requirements); For austenitic stainless steel, when the medium is extremely corrosive, C2=0mm.
Fatigue life check of expansion joint
For austenitic stainless steel bellows, when σ R≤ σ St, the problem of low cycle fatigue may not be considered, otherwise the fatigue life shall be checked. In GB16749-1997, the displacement of austenitic stainless steel expansion joints in Appendix A is based on the design temperature of 300 ℃ and the number of fatigue failures N=4.5 × 104 calculations. The safety factor is 15. The allowable number of cycles [N]=3000. When the design temperature is 300 ℃ and the cumulative allowable cycles in actual operation are equal to 3000, the displacement in the table in Appendix A can be used. If it is not at the design temperature of 300 ℃ or the actual cumulative allowable number of cycles is not equal to 3000 (the allowable number of cycles is 50~5000). The displacement shall be corrected by multiplying the value specified in the table in Appendix A by the coefficient CN or Ct. When calculating with SW6 software, if the material of the expansion joint is stainless steel, it is also necessary to choose the type of fatigue cycle load, which is temperature difference cycle load or mechanical cycle load.
Other problems needing attention in expansion joint design
- (1) In order to reduce the wear of the expansion joint, prevent vibration and reduce fluid resistance, a liner can be set inside the expansion joint if necessary. The liner should be welded with the equipment barrel (or end pipe) at one end facing the medium flow direction. When the medium is steam or liquid and the flow direction is vertically upward, 4 diameters (90 ° apart) shall be set at the lower end of the inner bushing φ There are 4 intermittent welds (the length of the fracture is 20mm) on the weld between the 6mm semi-circular drain hole or the inner bushing and the equipment barrel (or end pipe).
- (2) The materials used to manufacture expansion joints shall have good plasticity, mechanical properties and corrosion resistance. For expansion joints operated at high temperature, materials with good thermal strength and performance shall also be selected. The materials of expansion joints are generally the same as those of equipment cylinders. When stainless steel is used, post welding heat treatment is required for conditions with intergranular corrosion.
The expansion joint is a relatively weak link in the fixed tube sheet heat exchanger, and its design and manufacturing have certain difficulties. However, as an important part of the equipment, the expansion joint requires us to strictly follow the provisions of the standards and specifications in the design and manufacturing to ensure the safe operation of the equipment.
You’ve probably heard of a lot of different things that you can use to get high quality expansion joints. However, it’s much easier if you just follow a few simple steps and know what to look for in an expansion joint.
Check the size
The size of the expansion joint is the first thing to consider when deciding on a product. The main purpose of an expansion joint is to allow pipes, hoses and other items to move without damaging them or causing leaks. This means that there needs to be enough space between the pipe and its surroundings for movement. According to ASCE 7-10, for steel piping systems with lengths less than 50 feet (15 meters), no more than 0.25 inches (6 mm) should be used as a minimum clearance for thermal movement plus additional 0.25 inches (6 mm) for safety factors when calculating pipe diameter size based on system design conditions such as pressure, temperature and temperature range etc…
The second factor you will want consider is whether or not you need any additional protection from corrosion caused by water in your environment or if it’s designed primarily outdoors where it will face extreme weather conditions such as snow/ice build up which can cause pipes expand due freezing temperatures while also corrode them over time depending on what type material they made out
Look for the right materials
For high-quality expansion joints, you’ll want to look for stainless steel. Corrosion-resistant metals like stainless steel are strong and durable enough to withstand constant use without breaking down over time. Stainless steel is also resistant to rusting and corrosion, so it won’t degrade when exposed to moisture or other elements that could damage the pipe under normal conditions.
Check the warranty
Check the warranty. The warranty is an important part of any purchasing decision, especially when it comes to home renovation and construction. Go with a reputable company that offers a long-term warranty on their product. A good minimum is 10 years, but even better if you can get a lifetime guarantee or even better than that—a guarantee that covers replacement parts as needed over the life of your product.
Investigate the company’s reputation
Before you buy anything, you should always do your homework. You want to make sure the company has a good reputation, and that they’re providing high quality products. In order to do that, there are several steps you can take:
Ask your friends and family for their input. They may have used expansion joints from a particular manufacturer before and be able to tell you how satisfied they were with their purchase or installation process.
Look at reviews on the internet. There are many websites dedicated to reviewing different brands and types of expansion joints as well as contractors who install them. These sites can be incredibly helpful when trying to decide whether or not it’s worth using an expansion joint in your project or business venture.* Go over social media accounts like Facebook, Twitter, Instagram etc.. These platforms allow companies like ours direct access so we can engage with customers directly through comments/replies etc.. This also gives us insight into any issues which may arise during use such as leaking seals (which means replacing rather than repairing).
You should make sure that you get an expansion joint that is the right size, made from high quality materials and that comes with a good warranty.
When you are looking to get an expansion joint, it is important that you make sure that the product will be large enough for your needs. You should also ensure that the material used in making the expansion joint is high quality and comes with a good warranty.
Some expansion joints are made from metal while others are made from plastic. Both can last a long time but it depends on how much they will be used and what kind of weather conditions they will be exposed too.
You should also make sure that the manufacturer offers a warranty on their products so if there are any defects with your purchase then you can have them repaired or replaced by them without having to pay extra fees for doing so.
We recommend that you invest in a high quality expansion joint that is made from durable materials. This way, you will get the best value for your money and be able to use it for many years without having any problems.
Source: China expansion joints manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.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.)
If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]