Technical Information
Metal Expansion Joint Types | Accessories | Key Concepts | Movements | Pressure Thrust | Temp. Comp.
Metal Expansion Joint Types
Single Expansion Joint: The simplest form of Expansion Joint, of single bellows construction, for the purpose of absorbing any combination of the three basic movements of the pipe section in which it is installed.
Universal Expansion Joint: A Universal Expansion Joint is one containing two bellows joined by a common connector for the purpose of absorbing any combination of the three basic movements: Axial movement, lateral deflection and angular rotation. Universal Expansion Joints are usually furnished with control rods to distribute the movement between the two bellows of the Expansion Joint and stabilize the common connector. This definition does not imply that only a Universal Expansion Joint can absorb combined movement..
Double (Dual) Expansion Joint: A double Expansion Joint consists of two bellows joined by a common connector which is anchored to some rigid part of the installation by means of an anchor base. The anchor base may be attached to the common connector either at installation or at time of manufacture. Each bellows acts as a single Expansion Joint and absorbs the movement of the pipe section in which it is installed independently of the other bellows. Double Expansion Joints should not be confused with Universal Expansion Joints.
Hinged Expansion Joint: A hinged Expansion Joint contains one bellows and is designed to permit angular rotation in one plane only by the use of a pair of pins through hinge plates attached to the Expansion Joint ends. The hinges and hinge pins must be designed to restrain the thrust of the Expansion Joint due to internal pressure and extraneous forces, where applicable. Hinged Expansion Joints should be used in sets of two or three to function properly.
Gimbal Expansion Joint: A gimbal expansion joint is designed to permit angular rotation in any plane by the used of two pairs of hinges affixed to a common floating gimbal ring. The gimbal ring, hinges and pins must be designed to restrain the thrust of the Expansion Joint due to internal pressure and extraneous forces, where applicable.
In-Line Pressure Balanced Expansion Joint: An in-line pressure balanced Expansion Joint is designed to absorb axial and/or lateral movement while restraining the pressure thrust by means of tie devices interconnecting the line bellows with outboard compensating bellows also subjected to line pressure. Each bellows set is designed to absorb the axial movement and usually the line bellows will absorb the lateral deflection. This type of Expansion Joint is used in a straight run of piping.
Metal Expansion Joint Accessories
Internal Sleeve (Liner): A device which minimizes contact between the inner surface of the bellows of an Expansion Joint and the fluid flowing through it. These devices have also been referred to as liners or baffles.
Cover (Shroud): A device used to provide limited protection of the exterior surface of the bellows of an expansion 
joint from foreign objects or mechanical damage. A cover is sometimes referred to as a shroud.
Testable Bellows Elements: In a two-ply testable bellows, outer ply is a redundant ply that is designed to contain the pressure if the inner ply fails. If the inner ply fails, pressure flows to the test port indicating a failure on customer’s instrumentation.
Equalizing Rings/Reinforcing Rings: Devices used on some expansion joints fitting snugly in the roots of the convolutions. The primary purpose of these devices is to reinforce the bellows against internal pressure. Equalizing rings are made of cast iron, steel, stainless steel or other suitable alloys and are approximately "T" shaped in cross section. Reinforcing or root rings are fabricated from tubing or solid round bars of carbon steel, stainless steel or other suitable alloys.
Control Rods: Devices, usually in the form of rods or bars, attached to the Expansion Joint assembly whose primary function is to distribute the movement between the two bellows of a Universal Expansion Joint. Control rods are not designed to restrain bellows pressure thrust.
Limit Rods: Devices, usually in the form of rods or bars, attached to the expansion joint assembly whose primary function is to restrict the bellows movement range (axial, lateral, and angular) during normal operation. In the event of a main anchor failure, they are designed to prevent bellows over-extension or over-compression while restraining the full pressure loading and dynamic forces generated by the anchor failure.
Tie Rods: Devices, usually in the form of rods or bars attached to the Expansion Joint assembly whose primary function is to continuously restrain the full bellows pressure thrust during normal operation while permitting only lateral deflection. Angular rotation can be accommodated only if two tie rods are used and 90 degrees opposed to the direction of rotation.
Purge Connections: Purge connections, where required, are usually installed at the sealed end of each internal sleeve of an Expansion Joint for the purpose of injecting a liquid or gas between the bellows and the internal sleeve to keep the area clear of erosive and corrosive media and/or solids that could pack the convolutions. Purging may be continuous, intermittent or just on start-up or shut down, as required. These are sometimes called aeration connections..
Pantograph Linkages: A scissors-like device. A special form of control rod attached to the Expansion Joint assembly whose primary function is to positively distribute the movement equally between the two bellows of the universal joint throughout its full range of movement. Pantograph linkages, like control rods, are not designed to restrain pressure thrust.
Key Concepts
Bellows: Flexible element of an expansion joint consisting of one or more convolutions and the end tangents.
Convolution: Smallest flexible unit of a bellows. Total movement capacity and flexibility of a bellows is proportional to the number of convolutions.
Pressure Thrust: Pressure thrust is created by the installation of a flexible unit, such as an expansion joint, into a rigid piping system which is under pressure. Pressure thrust force is a function of the system pressure and mean diameter of the bellows. In cases of internal or positive pressure, bellows are forced to extend in length while the opposite is observed in cases of external or negative pressure. This force is transmitted from the ends of the expansion joint along the pipe.
Shipping Bars: Rigid support devices installed on expansion joint to maintain the overall length of the assembly for shipment and installation. These devices may also be used to pre-compress, pre-extend or laterally offset the bellows. They should not be used to resist pressure thrust during testing.
Movements, Cycle Life, Anchoring and Guiding Axial Movement
Axial Compression: The dimensional shortening of an Expansion Joint along its longitudinal axis. Axial compression has been referred to as axial movement, traverse or compression.
Lateral Movement: This relative displacement of the two ends of an Expansion Joint perpendicular to its longitudinal axis. This has been referred to as lateral offset, lateral movement, parallel misalignment, direct shear or transverse movement.
Angular Movement: This displacement of the longitudinal axis of the Expansion Joint from its initial straight line position into a circular arc. Angular rotation is occasionally referred to as "rotational movement." This is not torsional rotation.
Angular Extension: The dimensional lengthening of an Expansion Joint along its longitudinal axis. Axial extension has been referred to as axial movement, traverse, elongation or extension.
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Cycle Life
The cycle life of an expansion joint is the number of stress cycles endured at operating conditions. A stress cycle if defined as one complete movement of the expansion joint from initial to extreme position and return.
Main Anchor: A main anchor is one which must withstand the full bellows thrust due to pressure, flow, spring forces and all other piping loads.
A main anchor base for connection to the anchor structure can be furnished as an integral part of a single or double Expansion Joint, if desired. The Expansion Joint manufacturer must be advised of the magnitude and direction of all forces and moments which will be imposed upon the anchor base, so that it can be adequately designed to suit the specific application.
Intermediate Anchors: An intermediate anchor is one which must withstand the bellows thrust due to flow, spring forces, and all other piping loads, but not the thrust due to pressure.
An intermediate anchor base for connection to the anchor structure can be furnished as an integral part of a single or double Expansion Joint, if desired. The Expansion Joint manufacturer must be advised of the magnitude and direction of all forces and moments which will be imposed upon the anchor base, so that it can be adequately designed to suit the specific application.
Pipe Guides and Supports: Correct alignment of the pipe adjoining an expansion joint is important to its proper function. Maximum service from expansion joints will be obtained only when the pipeline has recommended number of guides and is anchored and supported in accordance with good piping practice. When locating pipe guides for applications involving axial movement only, it is generally recommended that the expansion joint be located near an anchor and that the first guide be located a maximum of 4 pipe diameters away from the expansion joint. For more information please see EJMA guidelines.
Pressure Thrust
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Temperature Compensation
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