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DME encourages the use of the following technical definitions concerning Expansion Joint components and related equipment as published by the Expansion Joint Manufacturers Association. Click on a word below to see the corresponding definition
ANCHOR, DIRECTIONAL – A directional or sliding anchor, is one which is designed to absorb loading in one direction while permitting motion in another. It may be either a main or intermediate anchor, depending upon the application involved. When designed for the purpose, a directional anchor may also function as a pipe alignment guide. In the design of a directional anchor, an effort should be made to minimize the friction between its moving or sliding parts, since this will reduce the loading on the piping and equipment and insure proper functioning of the anchor.
ANCHOR, INTERMEDIATE – 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.
ANCHOR, MAIN – A main anchor is one which must withstand the full bellows thrust due to pressure, flow, spring forces, and all other piping loads.
ANGULAR ROTATION – The displacement of the longitudinal axis of the Expansion Joint from its initial straight line position into a circular arc. Angular rotations occasionally referred to as “rotational movement.” This is not torsional rotation.
AXIAL COMPRESSION – The dimensional shortening of an Expansion Joint along its longitudinal axis. Axial compression has been referred to as axial movement, traverse, compression.
AXIAL 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.
AXIAL SPRING RATE – Force reflected in lbs/inch required to compress or extend bellows.
BELLOWS – The flexible element of an expansion joint consisting of one or more convolutions and the end tangents with: Bellows Convoluted Length (in.) ∕ Inside Diameter of Cylindrical Tangent and Bellows Convolutions (in.) ≤ 3 , with no more than five plies.
COMBINED MOVEMENTS – A combination of axial, lateral or angular movements that occur in the same expansion joint.
CONCURRENT MOVEMENTS – Combination of two or more types of axial or lateral movements.
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.
CONVOLUTION – The smallest flexible unit of a bellows. The total movement capacity of a bellows is proportional to the number of convolutions.
COVER – 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.
CYCLE – One complete movement of an expansion joint from initial to extreme position and return to neutral.
CYCLE LIFE – Total number of cycles an expansion joint will absorb at rated movement.
DEFLECTION FORCE – Amount of force required to cause movement in an expansion joint.
DOUBLE EXPANSION JOINTS – A double expansion joint consists of two bellows joined by a common connector which is anchored to some rigid part on 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.
EQUALIZING AND 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, carbon steel, stainless steel or other suitable alloys and are approximately “T” shaped in cross section. Reinforcing rings are fabricated from tubing or solid round bars of carbon steel, stainless steel, or other suitable alloys.
EXPANSION JOINTS – Any device containing one or more bellows used to absorb dimensional changes, such as those caused by thermal expansion or contraction of a pipeline, duct, or vessel.
EXTERNALLY PRESSURIZED EXPANSION JOINTS – Design to absorb large amount of axial movement under relatively high pressure, pressure apply to the external surface of bellows prevents the phenomenon called squirm. Flexible element is totally protected by outer casing.
FLANGED ENDS – The ends of an expansion joint equipped with flanges for the purpose of bolting the expansion joint to the mating flanges of adjacent equipment or piping.
GIMBAL EXPANSION JOINTS – A gimbal expansion joint is designed to permit angular rotation in any plane by the use 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.
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.
IN-LINE PRESSURE BALANCED EXPANSION JOINT – An in-line pressure balanced Expansion Joint is designed to absorb axial movement and/or lateral deflection 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.
INTERNAL SLEEVE – A device which minimizes contact between the inner surface of the bellows of an expansion joint and the media flowing through it. These devices have also been referred to as liners or baffles.
INTERNALLY GUIDED EXPANSION JOINT – An internally guided expansion joint is designed to provide axial guiding within the Expansion Joint by incorporating a heavy internal guide sleeve, with or without the use of bearing rings. The use of such Expansion Joints will assure installation without initial lateral or angular misalignment and can be installed in pipelines where reverse flow will be encountered. The use of an internally guided expansion joint does not eliminate the necessity of using adequate external pipe guides.
LATERAL DEFLECTION – The 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.
LATERAL SPRING RATE – Force reflected in lbs/inch required to laterally deflect bellows.
LIFTING LUGS – A lifting device that is attached to the expansion joint for field handling and installation.
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 the dynamic forces generated by the anchor failure.
LIVE LENGTH – The total Active Length of Bellows.
MAXIMUM WORKING PRESSURE – Greatest pressure allowed on the expansion joint during operation.
MAXIMUM TEST PRESSURE – Highest permissible pressure which an expansion joint can be subjected without causing objectionable deformation of the bellows element. The test pressure is usually 1-1/2” times the maximum working pressure.
MOTION INDICATORS – Devices attached to an Expansion Joint for the purpose of indicating the movement of the Expansion Joint. These devices are useful in determining if the piping system is behaving as planned and if the actual movements being imposed upon the bellows are within the limits of the original design criteria. An example of motion indicators used on hinge or gimbal hardware is an indicator attached to the hinge pin with an angular scale attached to the hinge arm. This allows one to quickly determine the extent of angular offset.
MOVEMENT – The dimensional changes, which an expansion joint is required to absorb, such as those resulting from thermal expansion or contraction.
NECKS – See Tangents.
PANTOGRAPH LINKAGES – A scissor-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.
PIPE ALIGNMENT GUIDE – A pipe alignment guide is a form of framework fastened to some rigid part of the installation, which permits the pipeline to move freely along the axis of the pipe. Pipe alignment guides are designed primarily for use in applications involving lateral deflection and angular rotation.
PIPE SECTION – A pipe section is that portion of a pipeline between two anchors. All dimensional changes in a pipe section must be absorbed between these two anchors.
PRE-COMPRESSION – Compressing the expansion joint (shortening the F/F) so that in the cold position the joint has a given amount of compression set into the joint. The purpose of pre-compression is to allow for unexpected or additional axial extension.
PRESSURE BALANCED EXPANSION JOINT – A pressure balanced Expansion Joint is designed to absorb axial movement and/or lateral deflection while restraining the pressure thrust by means of tie devices interconnecting the flow bellows with an opposed bellows also subjected to line pressure. This type of expansion joint is normally used where a change of direction occurs in a run of piping but can be designed as an in-line device where no change of direction is necessary. The flow end of a pressure balanced expansion joint sometimes contains two bellows separated by a common connector, in which case it is called a universal pressure balanced expansion joint.
PRESSURE THRUST – Force reflected in lbs imposed on pipe system by bellows due to line pressure and effective area. Anchors must be able to accommodate this force.
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.
RATED MOVEMENT – Maximum amount of movement (axial compression, axial extension, lateral deflection, angular rotation, or any combination thereof) which an Expansion Joint is capable of absorbing. This rating may be different for each size, type, and make o Expansion Joint and is established by the manufacturer.
SHIPPING DEVICES – Rigid support devices installed on an expansion joint to maintain the overall length of the assembly for shipment. These devices may also be used to pre-compress, pre-extend or laterally offset the bellows. Sometimes called shipping rods or shipping bars.
SHIPPING RODS (BARS) – Temporary supporting members attached to an expansion joint to prevent movement of the joint and retain dimensional stability during shipping, handling and installation.
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.
SPRING RATE – Force required to compress, extend, laterally deflect, or angularly deflect an expansion joint one inch.
TANGENTS – The straight un-convoluted portions at the end of the bellows.
THRUST AREA – Area over which the effects of pressure in an expansion joint will produce a longitudinal force in the piping system.
TIED UNIVERSAL EXPANSION JOINT – See Universal Expansion Joint.
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 located 90 degrees opposed to the direction of rotation.
TORSIONAL ROTATION- The twisting of one end of the Expansion Joint with respect to the other and about its longitudinal axis. The twisting generally produces extremely high shear stress in the bellows . For this reason it is extremely important special hardware be used to limit the amount of torsional shear stress in the bellows.
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.
VANSTONE FLANGE – A loose, rotating type flange, sometimes called a lap joint flange.
WELD ENDS – The ends of an expansion joint equipped with pipe suitably beveled for welding to adjacent equipment or piping.