Expansion joint is any device containing one or more flexible elements. 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
- Mechanical movements
- Ground settlements
Parts of an Expansion Joint
Expansion joint consists of bellows, accessories and end connections.
- Bellows – Flexible element
- Convolution – Parts making up a bellows
- Layer / Ply – Sheets making up a bellows
- End connections – Parts that connect bellows to piping system
Why are Expansion Joints preferred?
There are many reasons to use expansion joints.
- 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)
Alternatives to Expansion Joints
There are alternatives to expansion joints but they all have many disadvantages.
Pipe Loops are the most basic way of supplying flexibility to a piping system.
- Pipe loops are maintenance free
- Pipe loops can be used where space is of no concern
Disadvantages of Pipe Loops are:
- If space is limited pipe loops cannot be used
- Limited flexibility
- They can absorb movements which are in line with the pipe
- May not be good for using pigs
- They can cause pressure losses
Slip Type Expansion Joint
Slip type expansion joints can absorb large axial movements but they are not suitable for lateral or angular movements. They require maintenance and they can have sealing problems at high pressures
Types of Expansion Joints
Universal and Single Expansion Joint
Single expansion joint consists of one bellows. It is used to absorb all 3 types of movements. Universal expansion joint consists of 2 bellows. It can absorb all 3 types of movements.
Universal Expansion Joint
Universal expansion joints are most useful in cases requiring greater amounts of lateral movement than a single expansion joint can absorb. The longer the space between bellows the more lateral movement they can absorb.
Tie Rods and Limit Rods
Tie rods and limit rods are devices in the form of rods or bars attached to the expansion joint assembly. They are designed to hold pressure forces of the bellows and any externally applied forces specified by the customer.
Difference between Tie Rods and Limit Rods
Tie rods continuously restrain full bellows pressure thrust during normal operation while permitting only lateral movements. Conical washers must be used to allow lateral only movements. Control rods are more like safety devices. In the event of a main anchor failure they prevent bellows over-extension or over-compression.
Pressure Balanced Expansion Joint
Pressure balanced expansion joints continuously restrain pressure forces of the bellows while allowing for axial movements.
Hinged Expansion Joint
Hinged expansion joint is designed to allow angular rotation in one plane only while continuously restraining pressure force of the bellows.
Gimbal Expansion Joint
Gimbal expansion joint is designed to allow angular rotation in any plane while continuously restraining pressure force of the bellows.
Externally Pressurized Expansion Joint
When large axial movements are needed it is necessary to increase number of convolutions to accommodate these movements and offer good cycle life. However, increasing the number of convolutions lowers lateral spring rate and that makes bellows less resistant to internal pressure, causing buckling, column squirm. To overcome this problem, we externally pressurize the bellows to prevent column squirm. In this configuration, medium is inside the pipe but outside the bellows. So, bellows is pressurized from outside preventing bellows to buckle. When expansion joint compresses, bellows extends. When expansion joint extends, bellows compresses.
Definition of 1 CYCLE per EJMA
Expansion joint starts at neutral position and moves to first design position. Then moves to second design position going through the starting point. Then moves back to starting position. If there is more than one type of movement, it goes through all of them. This is 1 cycle with concurrent movements. If only one type of movement at a time is considered for 1 cycle – It is called as non-concurrent movement
Expansion Joint Connection Types
Connection type must be specified by the customer.
Floating (Rotating) Flanges (Vanstone)
Floating flanges are ideal for:
- Low pressures
- Where medium is corrosive and flanges need to be isolated from the medium
- Installations in existing piping systems where flange bolt alignment can be a problem
- Cheaper solution for large diameters needed
- Welding in the field is not needed
Floating Flanges are not recommended for high pressure applications. In this configuration, bellows or pipe is not welded on the flange. Flange is free to rotate around its axis.
Fixed (Welded) Flanges
Fixed flanges are used where:
- Sealing of the flange is of concern
- Medium is not corrosive or flange material is selected accordingly
- Flange bolt hole alignment is not of concern
- Welding in the field is not preferred
In this configuration, bellows or pipe is welded on the flange. Flange is not free to rotate around its axis.
Welding Necks (Pipe Ends)
Welding necks are used where:
- Pressures and temperatures are high
- Welding is not a problem
- Leakages are of concern
They are less expensive since pipe is cheaper than flange. No gaskets are needed. In this configuration, bellows is welded on a pipe
Liner (Inner Sleeve)
Liners are usually manufactured from stainless steels rolled and longitudinally welded. They can also be manufactured from thicker carbon steel material or materials such as Hardox or nickel alloys.
Liners serve many purposes:
- If medium is erosive they are can protect bellows from the medium
- They prevent a possible flow induced vibration of bellows by keeping the flow (not medium) away from the bellows
- They lower pressure losses by smoothing the flow
However, liners can cause pressure drop if flow area is reduced too much.
Liner thickness is mainly effected by:
- Flow velocity
- Turbulence of flow
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
Reinforcing rings are used in-between convolutions. They tightly fit in the roots. Their primary purpose is to reinforce bellows against internal pressure.
Each layer is designed for full pressure. If the inner ply fails, leak detection device alerts the user for the inner ply failure, unit continues to operate safely while a new unit is ordered.
Pre-Setting of Expansion Joints
Pre-setting of expansion joints means installing the bellows at a length or shape different than its neutral length or shape. Pre-setting allows expansion joints to absorb more movement than it can if it is installed at neutral length. It is very useful at large displacement applications. Pre-setting is most commonly applied by using rods.
Pre-Setting of Expansion Joints – Example
- An axial unit must be designed to absorb 200mm compression.
- Bellows neutral length is 400mm.
In this case bellows set length can be extended to 400+100=500mm. It is shipped and installed in this position. When in operation, bellows compresses 200mm and comes to (500mm-200mm) 300mm set length. This is only 100mm (400mm-300mm) less than neutral length although bellows saw 200mm compression. Cycle life is still calculated for 200mm compression.
Squirm is the instability of bellows. Squirm can greatly reduce bellows performance by reducing fatigue life and pressure carrying capacity.
There are two types of squirm
- In-plane – Seen mostly in large diameter short length bellows
- Column squirm – Seen mostly in small diameter long length bellows
In-plane squirm’s most important characteristic is that the convolutions are no longer perpendicular to the axis of bellows due to warping or tilting of one or more convolutions. Convolutions are no longer parallel to each other. They look like potato chips
Column squirm is defined as a gross lateral shift of the center section of the bellows. For single bellows this is the center of the bellows. For universal expansion joints the center spool shifts or rotates.
Vibration of Bellows
Metal bellows can be used in applications where vibration is High Frequency and Low Amplitude. They are not suitable for Low Frequency and High Amplitude in applications such as reciprocating machines. Also, where flow velocities are high, turbulent flow generated within the bellows section may induce vibration leading to flow induced vibration. This can greatly reduce bellows cycle life. Flow induced vibration can easily be prevented by using a liner.
Resonance of Bellows
All objects tend to vibrate at a specific rate called as the object’s natural frequency. An object will vibrate strongly when it is subjected to vibrations or regular impulses at a frequency equal to or very close to its natural frequency. This phenomenon is called resonance. Mechanical resonance can produce vibrations strong enough to destroy the object in which they occur. Formulas exist to design bellows to avoid resonance due to system vibrations.
Lens Expansion Joints
Characteristics of lens expansion joints are:
- Relatively large convolution height and pitch
- Single ply and wall thickness from 2 mm and thicker
- Manufactured from S-shape half convolution and joined with a continuous circumferential weld
- Manufacturing capacity up to 5mm for carbon steel depending on diameter and shape
Advantages of Lens Expansion Joints
- Dents create stress risers in thin ply bellows which result in fatigue cracks over time. Lens bellows holds up to mechanical damage better than thin wall bellows.
- Weld repair can be performed on thick walled bellows
- Thicker wall of lens bellows holds up better to corrosion attacks.
- Use of carbon steel material is possible
Selecting correct material for bellows element is very important. This ply bellows are not designed to have any corrosion allowance. Since bellows layers are already thin there is no room for corrosion. It is very important to choose the bellows material correctly It is the customer’s responsibility to specify materials of construction.
Rubber Expansion Joints
Rubber expansion joints help pipe systems to absorb movements, vibrations or noise. They prolong service life of the piping system and connected equipment. They are flexible units manufactured from natural or synthetic elastomers or fluoroplastics. In case of necessity, a reinforcement of the bellows (steel, nylon or aramid) are added. Power stations, process plants, pulp and paper plants, heating and ventilating systems, sanitary installations, shipbuilding, off-shore installations, flue gas cleaning systems, chemical or petrochemical plants may benefit from rubber expansion joints applications.
Advantages of Rubber Expansion Joints
- Best solution to vibration, noise and misalignment problems
- Suitable up to 16 bar pressure and 110°C temperature working conditions
- Optimum solution for narrow spaces, still able to absorb large movements due to the flexible bellows
- Absorb axial, lateral and angular movements
- Safe, reliable, durable and maintenance free
- Chemical and corrosion resistant
- Can accommodate piping misalignment
Weld End Expansion Joints
In places where axial movement is requested weld end expansion joints are used. They are equipped with carbon steel or stainless steel pipe connections. Even though they can absorb movements in any direction, they are mainly used for axial movements. They may have a double bellows in order to absorb higher lateral movements.Weld end expansion joints are used in hot and cold water pipelines, district heating pipelines, steam pipelines, ship building and exhaust systems, geothermal water applications, chemical, iron and steel, pulp and paper industries.
Advantages of Weld End Expansion Joints
- Economical compared with other flange end expansion joints
- No need for gaskets
Fixed Flanged Expansion Joints
They are equipped with welded carbon steel or stainless steel flanges. They are available for various industries including exhaust gas, liquid medium and steam. Bellows are designed following latest EJMA standards. Fixed flanged type expansion joints may have a double bellows which are designed for absorbing the higher lateral movements.
Advantages of Fixed Flanged Expansion Joints
• Quick connection
• Easy installation
• Higher pressures than floating expansion joints
• Easily replaced
Floating Flanged Expansion Joints
Floating flanged expansion joints are equipped with carbon steel or stainless steel flanges (EN, ASME or as requested). They are available for various industries including exhaust gas, liquid medium and steam. Bellows are designed following latest EJMA standards. Fixed flanged type expansion joints may have a double bellows which are designed for absorbing the higher lateral movements.
Advantages of Floating Flanged Expansion Joints
• Quick connection
• Easy installation for matching existing flange centering
• Easily replaced
• Carbon steel flange is protected from media via bellows
Rectangular Expansion Joints
In low pressure applications rectangular metal expansions can be used to absorb three dimensional movements. Rectangular expansion joints are mostly used in ventilation systems, ducts and exhaust systems.
- They can be designed and manufactured in various corner types in accordance with customer spesifications
- Thermal expansion compensation
- Working temperature up to 750°C
For piping, ducting and vessel thermal growth problems lens bellows may be the right solution compared to conventional thin walled metal bellows. Thick-wall expansion joints are preferred in heat exchangers and large diameter piping systems where thin-wall expansion joints are not sufficient. Chemical, mining, iron and steel industries, power stations, cement factories benefit from the application of lens bellows.
Advantages of Lens Bellows
• Lens bellows hold up to mechanical damage better than thin wall bellows
• Thicker wall of lens bellows holds up better to corrosion attacks
• Plant maintenance staff can weld repair on thick walled bellows
• In order to prevent condensate build up drain couplings may be added to the bottom of the convolution
• Limitless convolution height
• Thick walled, high convolution is durable and lasts for a long time
How We Process Expansion Joint Orders
- Expansion joint order processing starts when an order is received from a customer.
- Order is checked if it corresponds to our offer. (Commercially, technically or delivery date)
- If there are discrepancies between offer and order customer is contacted to clarify the discrepancies
- Order is entered in the ERP system.
- Order confirmation is sent to the customer.
- Customer must check /review the order confirmation to make sure it is in accordance with the order.
- Parts will be manufactured as specified in the confirmation.