Expansion Joint Design Basics

Piping Flexibility and Equipment

All materials expand and contract with thermal change. In the case of piping systems, this dimensional change can produce excessive stresses throughout the piping system and at fixed points such as vessels and rotating equipment, as well as within the piping itself.

Pipe loops may add the required flexibility to a piping system if space permits, however the initial cost of the additional pipe, elbows and supports must be considered. In addition, increased continuous operating costs due to pressure drop may result from the frictional resistance of the flowing media through additional elbows and pipe. In some cases, pipe diameter must be increased to compensate for losses due to pressure drop.

A practical and cost effective means of achieving piping system flexibility in a compact design is through the application of expansion joints. The most efficient piping system is the shortest and most directly routed system and expansion joints make this possible.

Expansion joints provide an excellent solution for isolation of settlement, seismic deflection, mechanical vibration and sound attenuation transmission produced by rotating equipment.

Design Basics for Metal Bellows Expansion Joints

Metal bellows expansion joints consist of a flexible bellows element, appropriate end fittings such as flanges or butt-weld ends to allow connection to the adjacent piping or equipment, and other accessory items that may be required for a particular service application.

Bellows Design Bellows are manufactured from relatively thin-walled tubing to form a corrugated cylinder. The corrugations, commonly referred to as convolutions, add the structural reinforcement necessary for the thin-wall material to contain system pressure. The bellows designer selects the thickness and convolution geometry to produce a bellows design that approaches, and often exceeds the capacity of the adjoining pipe to contain system pressure at the specified design temperature. Flexibility of the bellows is achieved through bending of the convolution sidewalls, as well as flexing within their crest and root radii. In most cases, multiple convolutions are required to provide sufficient flexibility to accommodate the expected expansion and contraction of the piping system.

Movement Capabilities

Axial Compression: Reduction of the bellows length due to piping expansion.	Lateral Offset: Transverse motion which is perpendicular to the plane of the pipe with the expansion joint ends remaining parallel.
Axial Extension: Increase of the bellows length due to pipe contraction.	Angular Rotation: Bending about the longitudinal centerline of the expansion joint.

Torsion: Twisting about the longitudinal axis of the expansion joint can reduce bellows life or cause expansion joint failure and should be avoided. Expansion joints should not be located at any point in a piping system that would impose torque to the expansion joint as a result of thermal change or settlement.

Cycle Life for Expansion Joints

In most applications, design movements cause the individual convolutions to deflect beyond their elastic limits, producing fatigue due to plastic deformation, or yielding. One movement cycle occurs each time the expansion joint deflects from the installed length, to the operating temperature length, and then back again to the original installation length.

In the majority of applications, total shutdowns are infrequent, therefore a bellows with a predicted cycle life of one or two thousand cycles is usually sufficient to provide reliable fatigue life for decades of normal service. High cycle life designs may be desirable for service applications that include frequent start up/shut down cycles. The bellows designer considers such design variables as material type, wall thickness, the number of convolutions and their geometry to produce a reliable design for the intended service with a suitable cycle life expectancy.

Squirm (Bellows Giving in to Pressure)

An internally pressurized bellows behaves in a manner similar to that of a slender column under compressive load. At some critical end load, the column will buckle, and in a similar manner, at a sufficient pressure, an internally pressurized bellows that is installed between fixed points will also buckle, or squirm.

Bellows squirm is characterized by a gross lateral shift of the convolutions off of the longitudinal centerline. Bellows squirm can reduce cycle life, or in extreme cases, produce a catastrophic failure. To avoid squirm, the bellows designer must limit movement capacity and flexibility to a level that insures that the bellows retains a conservative margin of column stability beyond the required design pressure. End Fittings Expansion joints will include appropriate end fittings such as flanges or butt-weld ends that should match the dimensional requirements and materials of the adjoining pipe, or equipment. Small diameter compensators are available with threaded male ends, butt weld ends or copper sweat ends. Threaded flanges may be added to the threaded end compensators if a flanged connection is preferred. Accessories Flow liners are installed in the inlet bore of the expansion joint to protect the bellows from erosion damage due to an abrasive media or resonant vibration due to turbulent flow or velocities which exceed: For air, steam and other gases Up to 6" dia. - 4 ft./sec./inch of diameter Above 6" dia. -25 ft/sec For water and other liquids Up to 6" dia. - 2 ft./sec./inch of diameter Above 6" dia. -10 ft./sec.

Expansion joints that are installed within ten pipe diameters downstream of elbows, tees, valves or cyclonic devices should be considered to be subject to flow turbulence. The actual flow velocity should be multiplied by 4 to determine if a liner is required per the above guidelines. Actual or factored flow velocities should always be include with design data, particularly flow that exceeds 100 ft./sec. which require heavy gauge liners.

External Covers are mounted at one end of the expansion joint, providing a protective shield that spans the length of the bellows. Covers prevent direct contact with the bellows, offering personnel protection, as well as protection to the bellows from physical damage such as falling objects, weld splatter or arc strikes. Covers also provide a suitable base for external insulation to be added over an expansion joint. Some insulating materials, if wet, can leech chlorides or other substances which will could damage a bellows.

Tie rods eliminate pressure thrust and the need for main anchors required in unrestrained piping system. Axial movement is prevented with the use of tie rods. Designs that have only two tie rods have the additional ability to accommodate angular rotation. Limit rods are similar, however they accommodate a specified axial capability.

Expansion Joint and Applications Engineering

Design Considerations

The addition of expansion joints in a piping system introduces reaction forces produced by the expansion joint that must be accommodated in the design of the piping system.

Spring Force Expansion joints behave in a manner that is similar to a spring; as movement occurs, expansion joints produce a resistive force. This resistance is stated as spring rate and measured as the force required to deflect the bellows 1" in the axial or lateral direction; or inch-lbs./degree for angular rotation. Spring force is the spring rate times the deflection.

Pressure Thrust If we consider a pipe section with blind flanges attached at each end, it is obvious that internal pressure produces a thrust force against the flange surfaces in opposing directions, however the longitudinal rigidity of the pipe prevents elongation.

If we add an expansion joint in the center of the pipe, this rigidity is lost and the thrust force may overcome the spring resistance of the bellows, producing elongation and possibly uncorrugating the bellows.

A pressurized bellows behaves like a hydraulic cylinder. Internal pressure bears against the walls of the convolutions, just as pressure bears against the face of a piston. This pressure produces a force that is equal to the internal pressure multiplied by the effective area of the bellows mean diameter ([ID + OD]/2) and will cause the flexible bellows to extend outward unless it is restrained from doing so. In most pressure piping applications, pressure thrust is usually much greater than spring force.

Pipe Anchors

By adding fixed points in the piping system, referred to as main anchors, the expansion joint is prevented from extending. Pressure thrust force is directed into the immovable main anchor. Now the joint is forced to compress or extend axially solely in response to dimensional changes in the pipe segment located between these main anchors.

Anchor design requires the consideration of forces due to pressure thrust at system test pressure, which is customarily 1 1/2 times the design pressure. In addition, bellows spring forces produced by deflection, friction force due to pipe movement across contact surfaces, forces and moments resulting from wind loading, bending and other influences must be considered in the design of anchors.

Main anchors are intended to anchor the pipe from motion in any direction.

Directional main anchors are, as the name implies, intended to anchor the piping system in one direction, while allowing movement to occur from a transverse direction.

Intermediate anchors can isolate multiple expansion joints that are installed in series to accommodate large motions beyond the capability of a single joint. This separation is required to insure that each expansion is able to function as intended and not be affected by the flexibility characteristics of adjoining expansion joints. Intermediate anchors react only differences in spring force and are not exposed to pressure thrust.

Pipe Guides for Joints

With the addition of an expansion joints and anchors, each pipe segment now behaves like a slender column under the compressive load of expansion joint pressure thrust and/or spring force bearing against the anchors. Bowing or buckling at the expansion joint may occur unless the pipe is properly guided.

Pipe guides are required to stabilize this slender column, preventing buckling and insuring that pipe growth is directed into the expansion joint as axial movement.

The first pipe guide must be located within four pipe diameters of each side of the expansion joint and a second guide placed within 10-14 pipe diameters of the first guide. Additional guides may be required based on guide spacing tables that consider diameter and system pressure. A convenient intermediate guide spacing chart is provided on page 35.

The recommendations given for pipe guides represent the minimum requirements for controlling pipelines which contain expansion joints and are intended to protect the expansion joint and pipe system from undefined external forces which could cause system failure.

Installation Misalignment

Installation misalignment reduces the total movement capacity of the expansion joint. Correction of misalignment should be completed prior to installation of the expansion joint. If misalignment can not be avoided, contact one of our engineers for guidance.

Concurrent Movements

Expansion joint movement capacity is listed in this catalog as the non-concurrent capacity for each type of movement. Axial, lateral and angular movements usually occur simultaneously, therefore it is essential that the concurrent movement capacity of the expansion joint be determined. This may be calculated by determining the required percentage of non-concurrent capacity required to meet each type of specified motion. The sum of these percentage values may not exceed 100.

Joint Product Selection Guide

Mid-Corr expansion joints employ a standardized bellows design ideally suited for general industrial applications. Offered with flanges or butt weld ends from 2" to 24" nominal diameter for design pressures to 300 psig at 800° F. Large diameter available in our Metal Catalog.

High-Corr bellows are hydraulically formed to produce superior fatigue life and maximum strength for severe service applications. This product provides an excellent means of absorbing large pipe motions (up to 7 1/2"). High-corr bellows are available in two styles: Free- Flexing and Controlled-Flexing.

Free-Flexing expansion joints are widely used in process and steam piping applications to 50 psig. In addition, the Free-Flexing expansion joint is recommended for compressor connections, engine intake and exhaust piping, ventilation and pump suction or discharge lines.

Controlled-Flexing expansion joints combine the Free-Flexing bellows design with mated neck rings and control rings between each convolution. This rugged construction reinforces the bellows for higher pressure applications. With an external cover this expansion joint provides a high degree of safety for the most severe operating conditions.

Externally Pressurized expansion joints have a heavy duty packless design that enables this product to accommodate large amounts of axial motion at high pressure without the risk of bellows squirm. Limited to axial movement only, the bellows is fully enclosed within an outer shell which is constructed of standard weight pipe, offering the highest degree of protection for the bellows and personnel. External insulation may be added directly over the outer case and/or direct bured.

Expansion Compensators provide the inherent performance benefits and safety features of the externally pressurized expansion joint design in a compact package. Intended primarily for steam supply and condensate return lines, as well as hot and chill water piping, this product is suitable for any small diameter axial expansion application.

Pressure Relief/Safety Valve Connectors combine the design principles of the rugged externally pressurized expansion joint with a unique capability to also accommodate lateral and angular movements. Intended to replace devices such as drip pan elbows, this product enables pressure relief and safety relief valve discharge piping to be fully sealed.

Flexible Metal Pump Connectors reduce stresses at piping connections to sensitive rotating equipment such as pumps and compressors. Capable of absorbing thermal growth, piping misalignment, vibration and noise, pump connectors offer extended service life for all rotating equipment.

Non-Metallic Expansion Joints can also be used for similar applications as the Flexible Metal Pump Connector in a non-metallic construction.

Exhaust Flexible Connectors are designed for low pressure applications such as stationary and marine gas turbine and diesel engine exhaust and low pressure ducting. Large motion capability, low spring forces and reduced weight make this product ideally suited to thin-wall duct systems.

Pipe Alignment Guides are an essential component of any properly designed piping system that employs expansion joints. These guides permit axial motion, while restricting lateral, angular and bowing movements.