Bellows Assets Recently Acquired by U.S. Bellows, Inc.
Located in Houston, Texas, U.S.A.

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Lortz metal expansion joints utilize one or more bellows that retain system pressure while deforming to accommodate system movements. Metal bellows are produced beginning with a welded tube (seamless tube use is very rare), and mechanically or hydraulically forming “convolutions” in the number and shape to meet piping or ducting system requirements. Metal bellows, as a detail part, are rarely provided to customers due to the “thin” material thickness which requires specialized welding processes to attach the bellows to pipe or flanges.

What is a Metal Bellow?


Metal bellows are designed to absorb thermal and/or mechanical movements in piping or ducting systems while retaining system operating pressure at system temperature. Bellows can absorb the following movements:

Whereas metal bellows can be designed to resist torsional loads, metal bellows cannot tolerate torsional movement. Metal bellows must be designed to avoid system resonant vibration frequency (if vibration exists) in order to prevent immediate mechanical bellows failure. Failure to specify torsion or vibration (if either exists) can result in immediate bellows failure.


Metal bellows cannot restrain longitudinal pressure loads without integral restraining hardware such as tie rods, hinges, gimbals or external pipe anchors. Longitudinal pressure loads on a bellows results in "pressure thrust". Pressure thrust force is created by the system and/or test pressure acting on the area of the "mean diameter" of the bellows. (Bellows mean diameter = bellows convolution I.D. + ((O.D - I.D)/2 ). A pressurized, unrestrained metal bellows expansionjoint in a piping system without anchors will elongate (extend) due to pressure thrust which can result in immediate bellows "squirm" and failure. Pressure thrust forces are typically higher than all other system forces combined.


Pressure applied to a bellows is limited by "Hoop Stress" (EJMA S2) and "Bulge Stress" (EJMA S4). Hoop Stress runs circumferentially around the bellows resulting from pressure differential between the inside and outside diameter of the bellows. Hoop stress is what holds a bellows together similar to hoop rings on a wood barrel. Hoop stress must be held to code allowable levels. Bulge Stress runs longitudinal to the bellows centerline acting on the sidewall of the bellows convolutions. Bulge Stress is also calculated to code.

Understanding Metal Bellows Pressure Thrust is Extremely Important

With rigid pipe installed between two flanges - pressure thrust is restrained by the strength of the pipe.

With a thin wall convoluted bellows welded to two flanges, the bellows reaction to pressure thrust results in the bellows growing in length until the bellows "squirms" and/or the convolutions stretch out to become the tube from which they were formed.


Metal bellows are designed to retain loads imposed by internal and/or external system pressure and/or test pressure. Bellows convolution geometry, number of convolutions, material type and material thickness all affect bellows pressure retaining capability.

Over pressurization and/or improper guiding of a metal bellows expansion joint can cause the bellows to "squirm". Squirm can lead to permenant deformation and/or immediate failure of the bellows.


In addition to longitudinal pressure thrust loads, movement within a bellows requires a "force" to cause the bellows to compress, extend or angulate. Bellows "spring rate" is a design consideration. To calculate the load (force) imposed on equipment adjacent to the expansion joint, use the equation below.

F = K x X
F - The load (force) imposed on equipment on either side of the bellows.
K - The bellows spring rate (expressed as pounds/inch of movement for axial and lateral movements, and inch/pound per degree for angular movement)
X - The anticipated or specified movement

The result is referred to as "spring force". For a bellows expansion joint without integral longitudinal pressure restraining hardware, one must add the bellows spring force to the pressure thrust force to determine the total force imposed on adjacent equipment or pipe anchors. Other loads that must be considered are dead weight, frictional, wind, etc.


When a bellows compresses, extends or angulates, the movement is absorbed by deformation of the side walls of the bellows convolutions. The stress caused by the movement is referred to as the bellows deflection bending stress (EJMA S6). This stress is highest at the "crest" and "root" of the bellows convolution. Metal bellows are designed to function with a deflection bending stress value that far exceeds the yield strength of the bellows material. Therefore, most metalexpansion joints are designed to deflect in the "plastic" range of materials and the bellows will take a permanent "set" at the rated bellows movements. Bellows are rarely designed to operate in the elastic range of materials. Bellows operating in the plastic material range will eventually fail due to fatigue after a finite number of movement cycles. Realistic cycle life should be specified for bellows design. As the chart on page 26 shows, the higher the cycle life, the "weaker" the bellows design pressure capability. The "safest" bellows design results from real-world cycle life, pressure, movement and temperature data.


The chart above shows the complexity of bellows design with the relationship of bellows geometry, material thickness, pressure and movement. Optimum bellows design requires actual pressure and temperature to be specified along with actual calculated thermal movement to be absorbed by the bellows. Overstating system data will result in a less safe bellows design. Most system designers think that specifying an extended bellows cycle life increases system reliability, whereas a longer than necessary specified bellows cycle life in most cases has the opposite result. As the chart above shows, the relationship between cycle life and pressure stability is a "balancing act". The higher the cycle life, the lower the pressure retaining capability of a given bellows design. Note the red values in the chart, when cycle life is higher, squirm pressure is lower.

The Standards of the Expansion Joint Manufacturers Association (EJMA) covers the subject of bellows cycle life very well and Lortz recommends that system designers refer to the latest edition of the EJMA Standards.


Bellows material selection is determined through knowledge of the system process and media. Responsibility for the selection of bellows materials is that of the system process designer or end user.


Typical Expansion Joint Applications

Axial Movement Only


Combined Movements


Angular Movements
Hinge & Gimbal


Main anchors must be designed to withstand all of the forces and movements imposed on them in the piping system section in which they are installed. This includes bellows pressure thrust, media flow, bellows spring force and frictional forces of pipe guides, pipe supports, and directional anchors. The weight of the pipe, including contents and forces and / or movements resulting from wind loads may also have to be considered in the main anchor design.


Intermediate anchors are not designed to withstand bellows pressure thrust force. When unrestrained metal bellowsexpansion joints are installed in a pipe section, intermediate anchors must be designed to withstand all of the non-pressure forces acting upon it which consists of bellows spring force and other frictional forces such as pipe guides.


Pipe rings, U-bolts, roller supports and spring hangars are typical pipe support devices. A properly designed pipe support permits free movement of piping while supporting the dead and live weight of piping, valves and other components of a piping system.

Piping or ducting systems in which metal bellows expansion joints are installed must be properly guided and supported in order for the expansion joint to function properly. It is generally recommended that the expansion joint be installed near a pipe anchor and that the first guide be installed a maximum of four (4) pipe diameters away from the expansionjoint. The distance between the first and second guide should not be greater than 14 pipe diameters.

Proper guiding and supporting of piping systems containing expansion joints is critical.


The first guide must be located a maximum of 4 pipe diameters from the end of the bellows; the second guide
a maximum of 14 pipe diameters. The chart below is for all bellows with inside diameter the same as piping.

This chart is for general reference only. Piping and ducting systems should be designed by qualified engineers and consider all system requirements.