AN INTRODUCTION TO METAL BELLOWS EXPANSION JOINTS

AN INTRODUCTION TO METAL BELLOWS EXPANSION JOINTS

AN INTRODUCTION TO METAL BELLOWS EXPANSION JOINTS
WHAT ARE EXPANSION JOINTS
the definition of an expansion joint is “any device containing one or more metal bellows used to absorb dimensional changes such as those caused by thermal expansion or contraction of a pipe-line, duct, or vessel

Design considerations 
If an expansion joint is to fulfill its intended function safely and reliably, it must be kept in mind that
it is a highly specialized product. Interchangeability is rare in expansion joints, and in a real sense, each unit is custom-made for an intended application. It becomes necessary then, to supply the expansion joint manufacturer with accurate information regarding the conditions of design that the expansion joint will be subjected to in service. Because many of these design conditions interact with each other, designing a bellows becomes much like assembling a jig saw puzzle; one needs all of the pieces (design conditions) before a clear picture of an expansion joint design can emerge. The following is a listing of the basic design conditions that should be supplied to the manufacturer when specifying an expansion joint.
1-Size. Size refers to the diameter of the pipeline (or dimensions of the duct in the case of rectangular joints) into which the expansion joint is to be installed. The size of an expansion joint affects its pressure-retaining capabilities, as well as its ability to absorb certain types of movements.
2-Flowing Medium. The substances that will come in contact with the expansion joint
should be specified. In some cases, due to excessive erosion, or corrosion potential, or in cases of high viscosity, special materials and accessories should be specified. When piping systems containing expansion joints are cleaned periodically, the cleaning solution must be compatible with the bellows materials.
3-Pressure. Pressure is possibly the most important factor determining expansion joint design. Minimum and maximum anticipated pressure should be accurately determined. If a pressure test is to be performed, this pressure should be specified as well. While the determination of pressure requirements is important, care should be exercised to insure that these specified pressures are not increased by unreasonable safety factors as this could result in a design which may not adequately
satisfy other performance characteristics.
4-Temperature. The operating temperature of the expansion joint will affect its pressure
capacity, allowable stresses, cycle life, and material requirements. All possible temperature sources
should be investigated when determining minimum and maximum temperature requirements. In so
doing, however, it is important that only those temperatures occurring at the expansion joint location
itself be specified. Specifying temperatures remote from the expansion joint may unnecessarily result
in the need for special materials at additional expense.
5-Motion. Movements due to temperature changes or mechanical motion to which the expansion
joint will be subjected must be specified other extraneous movements, such as wind loading or installation misalignment must be considered. The various dimensional changes which an expansion joint is required to absorb, such as those resulting from thermal changes in a piping system

5-1 Axial Motion is motion occurring parallel to the center line of the bellows and can be either extension or compression.
5-2 Lateral Deflection, or offset, is motion which occurs perpendicular, or at right angles to the centerline of the bellows. Lateral deflection can occur along one or more axis simultaneously.
5-3 Angular Rotation is the bending of an expansion joint along its centerline.

Expansion Joint parts 

The basic unit of every expansion joint is the bellows. By adding additional components, expansion
joints of increasing complexity and capability are created which are suitable for a wide range of
applications.
1. BELLOWS – The flexible element of an expansion joint, consisting of one or more convolutions,
formed from thin material.
2. LINER – A device which minimizes the effect on the inner surface of the bellows by the fluid
flowing through it. Liners are primarily used in high velocity applications to prevent erosion of
the inner surface of the bellows and to minimize the likelihood of flow inducted vibration. Liners
come in single, tapered, or telescoping configurations according to the application requirements.
An expansion joint, if provided with liners, must be installed in the proper orientation with
respect to flow direction. Liners are sometimes referred to as internal sleeves.
3. COVER – A device used to provide external protection to the bellows from foreign objects,
mechanical damage, and/or external flow. The use of a cover is strongly recommended to all
applications. A cover is sometimes referred to as a shroud.
4. WELD END – The ends of an expansion joint equipped with pipe for weld attachment to
adjacent equipment or piping. Weld ends are commonly supplied beveled for butt welding.
5. FLANGED END – 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.
6. COLLAR – A ring of suitable thickness which is used to reinforce the bellows tangent, or cuff,
from bulging due to pressure.
7. HOLLOW 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. Hollow rings are usually formed from a suitable pipe or tubing section.
8. SOLID ROOT RINGS – Identical in function to hollow root rings, but formed from solid bar
stock for greater strength.
9. EQUALIZING RINGS – “T” Shaped in cross-section, these rings are made of cast iron, steel,
stainless steel or other suitable alloys. In addition to resisting internal pressure, equalizing rings
limit the amount of compression movement per convolution.
10. CONTROL RODS – Devices, usually in the form of rods or bars, attached to the expansion joint
assembly whose primary function is to distribute the applied movement between the two bellows
of a universal expansion joint. Control rods are NOT designed to restrain bellows pressure
thrust.
11. 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, limit rods are designed
to prevent bellows over-extension or over-compression while restraining the full pressure loading
and dynamic forces generated by the anchor failure.
12. 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◦ from the direction of rotation.
13. PANTO GRAPHIC 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. Panto-graphic linkages, like control rods, are NOT designed to restrain pressure thrust.

Types of Expansion Joints
There are several different types of expansion joints. Each is designed to operate under a specific set
of design conditions. The following is a listing of the most basic types of expansion joint designs, along with a brief description of their features and application requirements.

(1) SINGLE EXPANSION JOINT. The simplest form of expansion joint; of single bellows construction, it absorbs all of the movement of the pipe section into which it is installed.

(2) DOUBLE EXPANSION JOINT. A double expansion joint consists of two bellows jointed 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 time of manufacturer. Each bellows of a double expansion joint functions independently as
a single unit. Double bellows expansion joints should not be confused with universal expansion
joints.

(3) 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. A universal expansion joint is used in cases to accommodate greater amounts of lateral movement than can be absorbed by a single expansion joint.

(4) UNIVERSAL TIED EXPANSION JOINTS. Tied universal expansion joints are used when it
is necessary for the assembly to eliminate pressure thrust forces from the piping system. In this
case the expansion joint will absorb lateral movement and will not absorb any axial movement
external to the tied length.

(5) SWING EXPANSION JOINT. A swing expansion joint is designed to absorb lateral deflection
and/or angular rotation in one plane only by the use of swing bars, each of which is pinned at or near the ends of the unit.

(6) HINGED EXPANSION JOINT. Ahinged expansion joint contains one bellows and is designed
to permit angular rotation in one plane only by the use of a pair of pins running through plates attached to the expansion joint ends. Hinged expansion joints should be used in sets of 2 or 3 to
function properly.

(7) GIMBAL EXPANSION JOINT. A gimbal expansion joint is designed to permit angular rotation
in any plane by the use of 2 pairs of hinges affixed to a common floating gimbal ring.

(8) PRESSURE-BALANCED EXPANSION JOINT. A pressure-balanced expansion joint is designed to absorb axial movement and/or lateral deflection while restraining the bellows pressure thrust force by means of the devices interconnecting the flow bellows with an opposed bellows also subjected to line pressure. This type of joint is installed where a change of direction occurs in a run of pipe.

Installation
Metal bellows-type expansion joints have been designed to absorb a specified amount of movement
by flexing of the thin-gage bellows. If proper care is not exercised in the installation of the expansion
joint, cycle life and pressure capacity could be reduced, leading to premature failure and damage to
the piping system. It is important that the expansion joint be installed at the length specified by the manufacturer. They should never be extended or compressed in order to make up for deficiencies in length, nor should they be offset to accommodate misaligned pipe.
Remember that a bellows is designed to absorb motion by flexing. The bellows, therefore, must be
sufficiently thick to withstand the design pressure, while being thin enough to absorb the required
flexing. Optimum design will always require a bellows to be of thinner material than virtually any
other component in the piping system in which it is installed. The installer must recognize this relative fragility of the bellows and take every possible measure to protect it during installation. Avoid denting, weld spatter, arc strikes or the possibility of allowing foreign matter to interfere with the proper flexing of the bellows. It is highly recommended that a cover be specified for every expansion joint. The small cost of a cover is easily justified when compared to the cost of replacing a damaged bellows element. With reasonable care during storage, handling, and installation, the user will be assured of the reliability  designed and built into the expansion joint.
Expansion Joint Applications
The following examples demonstrate common types of expansion joint applications, some dealing with simple axial movement and others with more complex and specialized applications.