Piping Expansion Joints or Expansion Bellows are highly engineered mechanical devices containing one or more metal/rubber bellows. Expansion Joints are used to absorb dimensional changes caused by thermal expansion or contraction of a pipeline, duct, or vessel while containing the system pressure. The flexible element of the expansion joint that expands or contracts to absorb thermal movement is called Bellows. It consists of one or more convolutions. Expansion joints are successfully used in Refineries, Chemical and Petrochemical Plants, Cryogenic plants, Nuclear power plants, Automotive, Aerospace, heating & cooling systems.
Why Install Piping Expansion Joints?
Piping Expansion Joints serve various purposes when installed in a piping system. Those are:
- To absorb movement (Thermal expansion as well as compression)
- To relieve system stress and strain.
- To reduce mechanical noise and vibration.
- To have a compact design (space constraint)
- To compensate for misalignment.
- To eliminate electrolysis between dissimilar metals.
- To reduce piping loads on equipment nozzles.
Components of an Expansion Joint
The main components which constitutes an expansion joint are as follows:
- Tie Rods
- Shipping Bar
- Protective Covers and Internal Liners
- Special Attachments like Pantographic linkages, etc
Refer to Fig. 1 which shown the main elements of a piping expansion joint.
Bellows are the most important element of a piping expansion joint. This is the basic unit of every expansion joint. They are of two types:
- Formed Bellows
- Formed from a thin-walled tube
- Contains only longitudinal welds
- Single & Multiply
- Fabricated Bellows
- Series of thin gauge discs welded together
- Uses heavier gauge materials than formed bellows
Regardless of the manufacturing method used, all bellows consist of the same basic components as shown in Fig. 2. Bellows can be single-ply or multi-ply design.
Additional components are added to bellow to create expansion joints of increasing complexity and capability which are suitable for a wide range of applications.
Tie Rod of Expansion Joint:
As the name indicates, Ties rods are basically bars or rods, attached to the expansion joint assembly. Tie rods are designed to absorb pressure loads and other extraneous forces like dead weight. Sometimes, Tie rods (Fig. 3) are used as Limit rods to protect the bellows from movements in excess of design that occasionally occur due to plant malfunction or the failure of an anchor.
Expansion Joint End Connections:
Three types of end connections (Fig. 4) are available in a piping expansion joint to attach it with a piping system. They are
- Flanged End including Special flanges, slip-on, or angle flanges.
- Vanstone Ends are a type of modified flanged ends which adds flexibility and resolve the bolt-hole misalignment.
- Welded ends: Any pipe or duct can be directly welded to such bellows.
Protective Covers and Liners for an Expansion Joint:
The expansion joint components are put inside a metallic cover to protect it from external sources as shown in Fig. 1.
Liner is provided to minimize the effect of flowing media on the bellow inner surface. Refer to Fig. 5.
Purge connections are provided along with internal liners to reduce the bellows skin temperature while using in high-temperature applications such as catalytic cracker bellows. Normal air or steam can be used as a purge medium. Purging helps in flushing out the particulate matters in between bellows and the liner. Harmful solid built up in the convolutions can also be stopped by purging.
Pantographic Linkage is a scissors-like device that is a special form of control rod attached to the expansion joint assembly. They are used to positively distribute the thermal movement equally between two bellows of the universal joint throughout its full range of movement. However, note that they are not designed to restrain pressure thrust.
Types of Expansion Joints
Broadly Piping Expansion Joints can be grouped into two categories:
- Unrestrained Expansion Joint and
- Restrained Expansion Joints
Unrestrained Type Expansion Bellows:
Unrestrained Expansion joints are assemblies not capable of restraining the pressure thrust of the system. So, The pressure thrust must be contained using main anchors or equipment. They are of following types:
1. Single Expansion Joint Assemblies are the simplest type of expansion joint consisting of a single bellows element welded to end fittings, either flange or pipe ends.
2. Universal Expansion Joint Assemblies consist of two bellows connected by a center spool piece with flange or pipe ends. The main advantage of universal expansion joint over single expansion joint is that the universal arrangement provides greater absorption of axial, lateral, and angular movements than a Single Expansion Bellows Assembly.
3. Externally Pressurized Pressure Balanced Assemblies are used when a large amount of axial movement is required to absorb and pressure thrust must be absorbed by the expansion joint. The opposing force balancing theory is quite similar to the In-Line Pressure Balanced Expansion joint Assembly. However, for the current case, the opposing forces are generated from pressure acting on the outside of the bellows.
Restrained Type Piping Expansion Joints:
In Restrained type piping expansion joints, the expansion joint hardware is capable of restraining the pressure thrust of the system by their design. Intermediate anchors are installed to withstand the spring force generated due to the deflection of the expansion joint. The main anchors are not required. They are again of various types as mentioned below:
1. Tied Single Expansion Bellows Assemblies add tied rods to a Single Bellows Assembly to increase design flexibility in a piping system. The tie rods are attached to the pipe or flange with lugs. These tie rods carry the pressure thrust generated in the system, eliminating the need for main anchors.
2. Tied Universal Expansion joint Assemblies are mostly similar in construction to a Universal Assembly. The only difference is that tie rods absorb pressure thrust and limit movements to lateral offset and angulation only.
3. Hinged Expansion Bellows Assemblies limit movement to angulation in one plane. Hinged Assemblies (Fig. 9) are normally used in sets of two or three to absorb large amounts of expansion in high-pressure piping systems.
4. Gimbal Expansion Bellows Assemblies are designed to absorb system pressure thrust while allowing angulation in any plane. Gimbal Assemblies (Fig. 9), when used in pairs or with a Single Hinged unit, have the advantage of absorbing movements in multi-planer piping systems.
5. Pressure Balanced Elbow Assemblies are used in applications where space limitations are a concern. A system of tie rods or linkages is used in such a fashion that the pressure thrust acting on the line bellows is equalized by the balancing bellows. The only forces transmitted to equipment are low spring forces created by the axial, lateral, or angular movements. An elbow must be present in the piping network to install this style of an expansion joint.
6. In-Line Pressure Balanced Bellows Assemblies are used if a piping elbow is not present in a piping network to use the pressure balanced elbow assemblies and pressure thrust has to be absorbed by the expansion joint. The inline bellows are tied in the expansion joint through a series of tie rods which equalizes the pressure thrust force. The opposing pressure forces cancel each other leaving only the low spring forces generated from the bellows deflection.
Expansion Joint Materials
Selection of bellows material is another important factor to be considered in the design of a piping expansion joint. Some of the factors which influences the proper expansion bellow material selection process are:
- Corrosion Properties: Process media, surrounding environment, and internal cleaning agents.
- Mechanical Properties: High-temperature service, cryogenic service, and operating stresses.
- Manufacturing Properties: Forming and cold working capabilities and cost & material availability.
Expansion Joints are available in metallic and non-metallic materials. The most common expansion joint materials, widely used in the piping industry are:
- Stainless steel (Austenitic Steel, SS 304 and SS 316)
- High-grade nickel alloy steels (Inconel, Incoloy, Monel, and Hastelloy)
- Teflon or PTFE
- Glass fiber
- an elastomer such as rubber.
Expansion Joint Design Codes and Standards
Expansion Joints used in industry are designed, manufactured, and tested in accordance with the following Codes and Standards
- EJMA, Expansion Joint Manufacturer Association, Inc.
- ASME B31.3 Appendix X, Metallic Bellows Expansion Joints
- ASME Sec. Ⅷ Div.1, App.26.
- EN 14917 Metal Bellows Expansion Joints for Pressure Applications
Expansion Joint or Bellow Manufacturers
There are many organizations who produce piping expansion joints. few of the most reputed expansion joint industry leaders are:
- U. S. Bellows
- Piping Technology and Products
- Senior Flexonics
- United Flexible
- Triad Bellows
- Metraflex, etc.