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: What are expansion joints?

Necessity of expansion joints

In the piping systems used for transferring gases and fluids, thermal expansion occurs from the temperature change in the transferred medium. If the heat expansion cannot be absorbed by the piping system itself, it may be damaged because of buckling. Furthermore, the piping system may be damaged by an earthquake, land subsidence, and other reasons, in addition to vibrations transferred from adjacent devices such as an engine. To prevent these dangerous situations, it is necessary to incorporate expansion joints for the purpose into the piping system.

Performance characteristics of expansion joints

The performance characteristics of expansion joints such as flexibility vary depending on bellows parameters such as pitch, corrugation height, and wall thickness.

Performance characteristics of expansion joints

Performance characteristics of expansion joints

ParametersAxial directionDirection
perpendicular to the
axis
Pressure
resistance
Buckling
resistance
FlexibilitySpring
reaction
force
Static
Thrust
FlexibilitySpring
reaction
force
Pitch→Large→Large→SmallNot
relevant
→Large→SmallNot
relevant
→Small
Corrugation height→Large→Large→Small→Large→Large→Small→Small→Small
Wall thickness→Large→Small→Large →Small→Large→Large→Large
Number of corrugati→Large→Large→Small →Large→SmallNot
relevant
→Small
Intermediate pipe
Length→Large
Not
relevant
Not
relevant
Not
relevant
→Large→SmallNot
relevant
Not
relevant
Diameter→LargeNot
relevant
→Large→Large→Small→LargeNot
relevant
→Large

By selecting the optimum structure and the pitch, corrugation height, wall thickness, and the number of corrugations of the bellows, we can provide an expansion joint best suited for the customer’s intended use.

Characteristics of expansion joints in piping systems

Spring reaction force generated by displacement If displacement occurs on the bellows from the thermal expansion of piping, a reaction force is generated in the direction for restoring the original length. This spring reaction force is the product obtained by multiplying the spring constant by the amount of displacement of the bellows.
Thrust generated by internal pressure (thrust reaction force of internal pressure) If internal pressure is applied to a bellows in the free state whose extension in the axial direction is not restricted, the bellows extends in the axial direction. This internal pressure thrust is the product obtained by multiplying the average cross-sectional area of the bellows by internal pressure.
Force applied to the fixed points At the fixed points on the upstream and downstream sides of the expansion joint, the sum of the spring reaction force generated by displacement and thrust generated by the internal pressure is applied. Therefore, fixed points are required to have the strength to withstand this value.
Safety limitations against internal pressure When internal pressure applied to a bellows is gradually increased to reach a certain value, the bellows becomes extremely unstable and cannot retain the axis. In such a case, internal pressure is applied in the horizontal direction or rotational direction, thus causing buckling.
Pressure drop due to turbulent flow If a fluid directly touches the inner surface of the bellows while flowing, it is always a turbulent flow. It generates considerable pressure drop compared to straight pipes with smooth inner surface. This pressure drop occurs in proportion to the number of corrugations and is accelerated in proportion to the square of the flow speed. An internal cylinder is attached to minimize the pressure loss; however, its necessity is determined based on the flow speed and whether the fluid is a liquid or gas.

Expansion joints incorporated into piping systems have unique characteristics; therefore, it is necessary to consider basic piping design by fully taking the characteristics into consideration.

Comparison of characteristics of expansion joints

TypeExpansion characteristicsGeneration
of thrust
LengthOther characteristics
Axial directionDirection
perpendicular
to the axis
Angle
Free typeSingle typeSmallUltra-smallUltra-smallPresentShort The basic type of expansion joint. A duplex type can be used if displacement in the direction perpendicular to the axis is large. On this basic type, thrust generated by internal pressure is applied to fixed points.
Duplex typeMediumLargeUltra-smallPresentMedium/long
Outer pressure typeSingle typeSmallUltra-smallUltra-smallPresentShortThis type is characterized by the structure where the internal pressure of the fluid is applied in the area between the inner surface of the external cylinder and the circumference of the bellows. It has no risk of buckling of the bellows even at high pressure. A drain on the external cylinder makes it possible to easily remove accumulated liquid.
Duplex typeMediumMediumUltra-smallPresentMedium/long
Hinge typeSingle type××MediumNoneShort This type can absorb angular displacement on a single plane. It is designed to restrain the thrust generated by internal pressure by a hinge pin, thus applying no thrust at fixed points.
Z-shaped two-point arrangementMedium××NoneBy combining two to four items on a plane, this product can absorb displacement in the axial direction. As the number of items combined is increased, the product can absorb greater displacement. Moreover, by combining a gimbal type with this type, it becomes possible to absorb displacement not only in the axial direction but also in the direction perpendicular to the axis in all directions.
L-shaped three-point arrangementLarge××None
U-shaped four-point arrangementLarge××None
Gimbal typeSingle type××NoneShort This type can absorb angular displacement in all directions. It is designed to restrain the thrust generated by internal pressure using a gimbal pin, thus applying no thrust at fixed points.
Universal typeDuplex type×Large×NoneMedium/longThis product is the application of the basic duplex type and is equipped with extension bolts that restrain extension in the axial direction. This eliminates thrust applied at fixed points.
Non-welded typeSingle typeSmallUltra-small×PresentShortThis product adopts a structure that the bellows tip is folded back in the shape of collar to attach the bellows to the flange without welding. For this reason, all wetted parts are made of stainless steel.
Straight pipe pressure balanced typeDuplex typeSmallUltra-small×NoneMediumA balance bellows is inserted and connected between two displacement bellows to offset the thrust generated by internal pressure in the expansion joint. This eliminates the thrust applied at fixed points. The outer diameter of the balance bellows is required to be about 1.4 times that of the displacement bellows, which makes the maximum outer diameter of the entire expansion joint larger. Absorption of displacement in the direction perpendicular to the axis is also enabled by adopting a duplex-type displacement bellows.
Single typeMediumSmall×None Long
Curved pipe pressure balanced typeDuplex typeSmallUltra-small×NoneMediumBy connecting a displacement bellows placed at the elbow of a curved pipe and a balance bellows placed behind the elbow, the thrust generated by internal pressure is offset in the expansion joint. This eliminates thrust applied at fixed points. The balance bellow sticks out from the piping system. Absorption of displacement in the direction perpendicular to the axis is also enabled by adopting a duplex-type displacement bellows.
Single typeMediumSmall×NoneLong
Square typeDuplex typeSmallUltra-small×PresentMediumWith the cross section especially designed for use with square ducts, this product is suited for a large volume of flow at low pressure. A duplex type can be used if displacement in the direction perpendicular to the axis is large. The thrust generated by internal pressure is applied at fixed points.
Single typeMediumMedium×PresentLong

Structure of expansion joint

Structure of expansion joint

1Flange
A piping joint used to fasten pipes using bolts and nuts. Select a material and shape suited for the temperature and pressure of fluids. Piping joints other than the flange type can also be used.
2Pipe
A piping material used for transferring fluids. Select a material and shape suited for the temperature and pressure of fluids.
3Bellows
An essential part related to the performance of expansion joints such as flexibility. Select a material and shape suited for the intended use, such as temperature and pressure of fluids and extension volume. (The major materials by different usage are shown in the bellows material table.) The pipe stock is manufactured by hydroforming or roll forming using a cylinder created from a thin steel plate and then welded in the longitudinal direction.
4Reinforcement ring
An external reinforcement ring is used for preventing deformation of the bellows as internal pressure increases. The necessity of this ring is determined from the wall thickness, shape, and other properties of the bellows.
5Internal cylinder
This component is attached to prevent pressure loss and turbulent flow of fluids. The necessity of this component is determined based on the pipe diameter, condition of the fluid, and flow speed. Decide on the wall thickness based on the pipe diameter and length of internal cylinder. It also protects the bellows, which is made of a thin plate, from erosion and corrosion.
6Limitation flange
A flange used for attaching a limitation bolt.
7Limitation bolt
This bolt suppresses excessive extension to protect the bellows. When used with the universal type or pressure balanced type, it absorbs thrust generated by internal pressure.

Bellows material table

Major applicationsBellows material
Flue duct SS400 SS400
Flue (weather resistance) ductS-TEN1 COR-TEN
Gas, air, oilSUS304
High temperature (450°C or above) gasSUS321 SUS316
High temperature (650°C or above) gasSUS309S SUS310S
Water, steamSUS316 SUS316L
PetroleumSUS304L SUS316 SUS316L
For LNG and other gases at extremely low temperature SUS304L SUS316 SUS316L
Erosive fluid and gas
High temperature (650°C or above) gas
INCONEL625 HASTELLOY C276 NIM ONIC75

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