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Expansion joint is any device containing one or more flexible element. Expansion joints are used to absorb dimensional changes in piping systems or vessels where flexibility is needed. Expansion joints are most commonly used to absorb thermal expansion/contraction, vibration, mechanical movements and ground settlements.

Expansion joints consist of bellows, accessories, and end connections. Bellows are the flexible elements. Convolutions are parts making up a bellows. Layers/plies are sheets making up a bellows

There are many advantages of using an expansion joint:

• They can absorb large movements in a small space.
• They are very flexible, lower forces on the piping system or vessel walls (except pressure force) are applied.
• They are maintenance free.
• If correctly designed, they can last a long time.
• They can absorb movements in all directions (except torsion).

Expansion joint covers are used to provide limited protection to the bellows exterior surface. Using covers is a relatively inexpensive way for protecting bellows from mechanical damage that can happen during shipping, handling and installation and is strongly recommended for critical expansion joints. Standard practice is to ask for removable shrouds. Removable shrouds let customers inspect the bellows before installation.

Expansion joint absorb movements by bellows convolution walls bending. Each convolution is flexible and absorbs the movement it is designed for. Expansion Joints are used to absorb axial, lateral and angular movements. Torsion should not be applied on expansion joints. Advantages of a multi ply expansion joint are lower spring rates  (better for pipe stresses) and higher cycle life.

Expansion Joints are required to add flexibility to the piping systems, to eliminate vibration, to reduce noise, to absorb ground settlements or earthquake, wind displacements, to reduce stresses on equipment nozzles.

Most commonly used materials for metal bellows are austenitic stainless steels such as 304 / 304L, 316 / 316L, 321ss. However, many other stainless steels and nickel alloys can be used. Other commonly used materials are, 309S, 310S, super duplex 2507, duplex 2205, alloy 625, alloy 600, alloy 800H, alloy C22, alloy C276, SMO254, 904L. Theoretically any weldable material that has sufficient elongation can be used to manufacture bellows.

For the other parts of the Expansion joint carbon steels such as S235, P265, P355, A36, A516-70, nickel alloys, stainless steels and high alloys such as P11, P12, P22, 16Mo3 can be used.

There are several steps in manufacturing of an expansion joint

1. Shearing: Shears and rollers used for carbon steels must not be used for stainless steel materials to avoid cross contamination
2. Bellows Tube Welding: After bellows sheets are cut they are rolled and longitudinally welded with automated welding process. This process is repeated for each layer/ply
3. Bellows Forming: Punch forming, hydroforming or roll forming are different methods of bellows forming. In punch forming, convolutions are formed by an expanding mandrel one by one. This is the most commonly used method and it gives good cold work to bellows. Wide variety of tools and shapes available giving flexibility in manufacturing. It is good for custom design and low quantity of orders. In hydroforming, convolutions are formed by applying liquid pressure inside the tube. Tube ends are sealed and held with tools outside the tubes. Then liquid pressure is applied and tube is pressed from each side to form convolutions. This process gives little cold work to bellows and it has limited tooling flexibility It is mostly good for very small diameters and for large quantities and re-rolling after process is NOT required. It is good for standard type of bellows. Bellows may need to be heated to remove any moisture between plies. In roll forming, convolutions are formed by placing the tube between rolling disks. Disks are both inside and outside the tube. Then disks are moved to form the convolutions. This process gives good cold work to bellows and wide variety of tools and shapes available giving flexibility in manufacturing. It is good for very small and very large diameters. In this case re-rolling after process is NOT required.
4. Bellows Re-rolling : After bellows are punch formed, convolution shapes are not final. Re-rolling gives convolutions their final shape
5. Welding of end connections.

There are various methods used for testing expansion joints:

• Hydro testing: Tests expansion joint integrity and leaks
• Pneumatic Testing: Tests expansion joint integrity and leaks
• Testing under water: Testing leaks
• Dye penetrant testing: Tests weld imperfection and may detect leaks
• X-ray testing: Tests weld imperfection and may detect leaks
• Air-jet leak test: Detects leaks

Bellows is the flexible part of the expansion joint. An Expansion joint has end connections and hardware such as inner sleeve, outer cover, tie rods, ginges or gimbals in addition to the bellows.

A bellows type Expansion joint has a bellows type (U shape EJMA type) flexible element. Other types of Expansion Joints are; Lens type Expansion Joints, rubber Expansion Joints, fabric Expansion Joints, slip type Expansion Joints.

Metal Expansion Joints are mostly custom designed per customer’s needs. Other factors that affect the price of an Expansion joint are; Variety of materials used, wide range of diameters, various accessories, design conditions such as movements, pressure and temperature.

There are many probable causes of bellows on metal expansion joints. Excessive vibration of piping exceeded cycle life of bellows, higher movements of expansion joint than it is designed for, improper handling or installation, incorrect material selection, chlorides present in the system, exposure to high temperatures, excessive movements in the system or incorrect design or installation may be the source of cracking of bellows. The solutions to cracking differ on each case.

• If there is excessive vibration, then check the piping system to make sure there is no vibration.
• If there is vibration, make sure the bellows is multi-ply bellows. If the bellows is installed, check manufacturing drawings. If the bellows is uninstalled, check the longitudinal weld of bellows inside and outside. If the welds are at different positions, it is a minimum of 2-layer bellows.
• If the bellows is a single ply bellows, offer multi ply bellows for better vibration absorption. If the bellows is double ply offer more layers and try to learn the vibration frequency of the system
• If the cycle life of bellows are exceeded, then offer new bellows
• If the expansion joint sees higher movements than it is designed for then convolutions can be touching each other or lost their form due to excessive movement. Offer to redesign the expansion joint.
• Keep in mind that thin layer bellows cannot be repaired.

Cracking may be due to improper handling or installation. Bellows are frequently damaged from being struck by hard objects such as tools, chains, forklifts, and other adjacent structures as a result of improper handling. Any denting or gouging of the bellows can severely affect the performance of the bellows and lead to premature failure. Weld splatter can also damage the thin bellows material. In this case how much bellows life is affected cannot be determined. In this case bellows need to be replaced since these damages cannot be repaired.

If rust is observed on expansion joints then the probable cause of cracking is incorrect material selection or chlorides present in the system. Bellows have to be re-manufactured with the correct bellows material suitable for the internal and external medium. Process engineers or customer’s engineers have to choose the material. If chlorides are present in the system use of stainless steel bellows is not recommended due to stress corrosion cracking concerns. Bellows need to be re-manufactured with correct bellows material.

If color change is observed on bellows the probable cause is exposure to high temperatures. It is likely that the bellows is exposed directly to high temperatures that the material may not be suitable for. Offer to use an inner sleeve and even better insulation between bellows and inner sleeve. A high temperature resistant material may also be suggested. How much bellows performance is effected cannot be determined. In this case bellows need to be re-manufactured with correct bellows material and/or expansion joint needs to be redesigned with inner sleeve and insulation

If inner sleeve is installed in the opposite direction of flow expansion joint must be installed with the liner in the flow direction immediately. Failure to do so may result in catastrophic failure and damage to other system components.

If hardware such as external cover, rods or hinges are touching bellows then the probable cause of cracking is the excessive movements in the system or incorrect design or installation. It is possible that the system sees more movements than it is designed for. It is also possible that installers forced the expansion joint to take a movement that it is not designed for exceeding installation tolerances. Another possibility is that the manufacturer did not take into consideration all movements at the design stage. How much bellows performance is effected cannot be determined. In this case contact manufacturer and ask about their opinion providing all the available data.

If there is excessive movement of bellow and/or convolutions stretched out or touching each other than the probable cause of cracking is excessive movements in the system or incorrect design or installation. It is possible that the system sees more movements than it is designed for. It is also possible that installers forced the expansion joint to take a movement that it is not designed for exceeding installation tolerances. Another possibility is that the manufacturer did not take into consideration all movements at the design stage. It is also possible that guiding and anchoring of the piping system is not done correctly. First guide should be within 4 x ND of pipe and second guide should be within 14 x ND of pipe. If these guides are not installed and designed correctly this may cause the expansion joint to fail. Also, fixed anchors in the system should be located and designed correctly. It is the responsibility of the piping designer to do these. Most likely bellows need to be replaced immediately.