Expansion joints connect two fixed assemblies or pipes and allow movement due to service load, shock or thermal cycles. They are made of elastomer, fabric or metal materials and feature a bellows-style construction. Convolutions permit misalignment, movement or isolation of the components that are joined together. Expansion joints can have a round, square, or rectangular cross-section. Rounded products often have a neck or collar that permits attachment with a hose clamp. Expansion joints with a flanged end usually attach to components with screws or bolts. Some expansion joints are used in plumbing or piping applications. Others are designed for vacuum or process-gas systems in which vacuum flanges and fittings connect runs of pipes or tubes to other sections of pipes or tubes, as well as to hoses, valves, pumps, chambers and vacuum components. Expansion joints for electrical conduits are also available.
Expansion joints are rated for three types of movement:
- compression, when the distance between the connections is shortened
- extension, when the distance between the connections is lengthened
- deflection, when the connections are moved off center
While they can handle three types of movement, they can only move one way at a time. For example, if a joint is compressed, it must go back to its original position before it can be deflected.
Function and Application
The main function of expansion joints in their various constructions is to compensate for movements in pipe systems, machines and equipment. The movements to be compensated are always relative motions between two parts of a system, caused by temperature differences, misalignment during installation, inertial forces, or foundation lowering. Expansion joints are universally applicable in almost every industrial sector. Particularly in pipeline engineering, they allow space-saving pipe routing for transporting a variety of media such as hot water, steam, fuel, heat transfer fluids, hot gases, and various types of chemical products. Another application field is the apparatus and motor engineering, where the expansion joints decouple vibrations and structure-born noise from diesel engines, turbines, pumps and compressors, preventing their transfer to the continuing lines. At the same time, expansion joints allow nearly force-and torque-free connection of pipes to sensitive fittings, appliances and equipment (e.g. to turbine nozzles). Moreover, expansion joints serve as assembly aids for pipe elements such as valves, where they are used as dismantling pieces or couplings.
Main Elements and Their Functions
Usually, expansion joints have the following components:
- Bellows: They are the flexible element oft the expansion joint and are designed, depending on the requirements, with different numbers of corrugations and layers.
- Guide Sleeves: They protect the bellows against the flowing medium and reduce the flow resistance.
- Protective Tubes, Guiding Tubes: They protect the bellows against mechanical damage and, depending on the design, prevent the expansion joint from lateral deflection (buckling).
- Fittings: They make the connection to the continuing piping. Depending on the design, the following fittings are available: weld ends, ends to be soldered, flanges, threaded nipples.
- Restraint (only for lateral, hinge or pressure balanced types): The restraint transmits the pressure reaction force over the single or, depending on the design, multi bellows. Simultaneously the restraint determines the kinematic flexibility of the expansion joint by incorporating different types of hinge bearings, such as ball joints, single axis bearings with bolts, U-joint or gimbal bearings.
The Bellows and Its Function
The core element of every expansion joint is the metal bellows, which by its corrugation geometry and thin-walled design has a large flexibility in axial, lateral or angular direction, as well as a high pressure resistance. As a condition to be used as an expansion element, the bellows must meet the following basic requirements:
- withstand the operating and test conditions (pressure, temperature) of the pipe system
- be corrosion resistant against internal and external influences
- be able to compensate for flexible expansions or possible oscillations, and achieve a specified life time or number of load cycles
- have sufficient stability against buckling
Using corrugations in form of a lyre is a good compromise between the contradictory requirements for high flexibility combined with high compressive resistance. They are the preferred corrugation shape for standard bellows. By changing the radius, the profile height, the number of layers and the wall thickness, their geometry may be adapted to the requirements on pressure and expansion capability.
Connections
The expansion joints are connected either by welding them to the pipes or container walls or by flanging them on, e.g. to machine sockets. The standard types of connection part are weld ends and flanges. In special cases screwed nipples are used.
Determination of Movement Parameters
Expansion joints compensate for various movements, caused by different sources, such as:
- installation misalignment
- vibrations
- installation gap
- extension caused by pressure force
- soil subsidence
- elongation
Elongation usually causes the highest movement value.
Installation misalignment
Misalignment occurs very often during pipe installation. These imprecisions may be compensated by expansion joints, if they were already considered in the system design. In this case, the expansion joint’s lifetime is hardly affected, because it is a one-time movement. On the other hand a complete or partial blocking of the corrugations may be caused, if short axial expansion joints are installed. The indicated movement compensation would be hindered, leading thus to early failure of the expansion joint.
Vibrations
Vibrations of different frequency and amplitude are caused by rotating or shifting masses in installations such as pumps, piston machines, compressors, etc. These vibrations not only make annoying noise, but stimulate connecting pipes to the extent of fatigue causing early failure. Thus the operating stability and economic efficiency of the installation is at risk.
Installation gap
During the installation of pipe systems, particularly when subsequent removal and replacement of individual components is necessary, an axial installation gap is essential for easy replacement of the modular elements. The so-called disassembly joint may bear a larger movement up to block position of the convolutions, as the frequency of replacement is usually low.
Extension caused by pressure force
Extensions occur in vessels and piping put under pressure force. Their values only have to be considered at larger diameters.
Soil subsidence
Expansion joints may take up larger subsidence movements, because it is a singular occurrence (no stress cycles). The expansion joint may even endure an excessive deformation of the bellows without leakage.
Elongation
Changes in the length of a piping are mainly caused by temperature variations. These changes in length have an insignificant effect in radial direction due to the pipe geometry and can be neglected, since pipe diameter is much smaller than pipe length. However, lengthwise variations of volume deserve close attention, since it can become quite significant when temperature and pipe length increase.
Each material has its own expansion coefficient which for the different types of iron and steel varies in rather narrow range. The differences become more significant for steel alloys such as heat resistant steel, stainless steel or high heat resistant metals and their alloys such as nickel, Monel, titanium, Inconel, Nimonic, etc. Copper and aluminium and their alloys have even bigger expansion coefficients.
For thermoplastic lines the length elongation is more than twice as large as for steel pipes.
Choosing Expansion Joint by Movement Types
Axial Expansion Joint
Axial expansion joints are suited to compensate for axial movements in straight pipeline sections.
Additionally, axial expansion joints are used:
- to dampen mechanical vibrations and reduce sound conducted through solids on pumps and compressors
- as flexible seals at the end of jacketed pipes in district heating systems
- to compensate for thermal movements and vibrations in flue gas ducts of boilers and engines
- as assembly aids for pumps, fittings and plate heat exchangers
- as gas tight wall penetrations of pipelines in nuclear power stations and ships
- in boilers and pressure vessels to compensate for differential expansion
A precondition for the diverse applications of axial expansion joints is the presence of suitable anchors and guides/supports.
Angular Expansion Joint
Angular expansion joints are suited for the compensation of both long pipe sections of district heating systems as well as short boiler and turbine room pipelines in one or more planes. For installations with very limited space one should also check the possibility of the installation of tied universal or pressure balanced expansion joints. Contrary to axial and universal expansion joints that are suited to compensate for movements independently, angular expansion joints are only elements of an expansion system. A minimum of two and a maximum of three angular expansion joints form a static defined system. The function depends on the ability of the bellows to rotate and the amount of rotation is stated in the technical data sheets as permissible angular rotation.
Angular expansion joints are usually installed with 50% pre-stressing. This is accomplished by pre-stressing the entire expansion system after its completion. The pre-stressing amount can be determined from the pre-stressing graph in the section "assembly instructions" taking into account the installation temperature.
Lateral Expansion Joint
A lateral expansion joint works in the same way a hinged expansion joint does. It utilizes the ability of a bellows to rotate in angular direction. The lateral movement capability is the result of the angular rotation of the bellows and their center to center distance. The longer the distance between the bellows, the larger is the movement capacity.
A longer center to center distance also results in lower displacement forces of the expansion joint.
Lateral expansion joints are independent expansion systems in contrast to single hinged expansion joints. They are practically a two pin system. Lateral expansion joints are usually installed with a 50% pre-stressing. This is accomplished by pre-stressing the entire pipe system after the expansion joint is installed. The pre-stressing amount can be determined from the pre-stressing diagram in the section "assembly instructions" taking into account the installation temperature.
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