Why an Expansion Joint Works

                                                                                                                                                                A publication of Oakridge Bellows

On the surface it doesn’t seem to make sense. If the code requires a pipe in a system to have a 3/8” thick wall to contain the internal pressure, how can an expansion joint bellows get by with just a 1/16” thick wall?

Balance the Forces

It’s how the material is arranged. For a pipe, there needs to be a certain amount of metal (in the cross section) per unit length ‘L’ in order to resist the internal pressure force that wants to pull it apart.

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More Metal, Please

A single convolute of a bellows is made up of a thin wall of material that is ‘squeezed’ into that same unit length ‘L’. It is the shape of the convolute that causes it to have a cross sectional area that comes close to that of a pipe.

A pipe is typically thicker by design to account for future thinning due to corrosion. A bellows, instead, is always made with a corrosion resistant material.

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Form and Function

By a great stroke of fortune, the convolute shape that helps to increase its pressure capacity ALSO makes it flexible in the longitudinal direction.

That’s really handy because with several convolutions in a row, the bellows can also angulate and move laterally.

The shape of the convolutions increases its pressure capacity along with making it flexible.

There are other pressure limitations a bellows has compared to a pipe, but an expansion joint is a useful device for extending the life of piping, nozzle connections, and equipment.

The bellows exposed

There’s no mystery as to why a bellows can flex – it’s convoluted for crying out loud. This could be one short article. Except here’s the tantalizing part; when the bellows material flexes, the stresses shoot well into the plastic range.

At this point the collective mechanical engineering community gasps, women faint, dogs bark throughout the neighborhood.

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Deflection stresses

The engineering schools taught us that if material yielded, it failed. Sure, exceptions were probably noted but it wasn’t covered on the test and ergo it didn’t exist.

Take your typical metal beam. Bend it a little, let go, and it springs back. When it springs completely back its stress levels are said to be elastic. If bent too far it takes a permanent set; its stresses are inelastic or ‘plastic’. Static structures such as buildings, bridges, and pressure vessels are designed with stresses in the elastic range. Elastic is good. But inelastic is not necessarily bad, it can be our friend.

When does the code say inelastic is OK? Consider our loaded beam again but modify the conditions to include a limit stop. Now when the beam permanently yields the load is not allowed to continue to deflect the beam. The beam can be pulled back and forth; each time the material exceeds its yield strength but does not break.

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There is a limit to this thumbing-its-nose-at-the-sacred-yield-strength-rule behavior. After a certain number of cycles the part will develop fatigue cracks – now it is considered failed.

Equations have been developed to predict when this will happen. And testing, LOTS of testing, testing-until-you-want-to-puke testing has been performed to validate the equations.

When expansion joints do finally come to the sunset of their lives it is rarely fatigue that finishes them off, but rather the steady and inevitable plodding of corrosion that makes all metal mortal.

Sharing good company

Metal bellows expansion joints do not stand alone in this special category. Many interested parties overlook that piping systems designed without expansion joints often operate with deflection stresses in the plastic range (more gasping, etc., etc.). That’s why they too, are designed by the code to have a finite cycle life.

So most process piping systems have inelastic deflection stresses after all. Redesign the piping system with an expansion joint and the inelastic piping stresses fall into the elastic range.

Sometimes its better to have the bellows flex than the piping.

The force is always with us

A bellows exerts a longitudinal force when internally pressurized. Big deal – so does a pipe. Ahhh, yes but that force is quietly restrained by the rigidity of the pipe.

When a longitudinally flexible element (AKA a bellows) is inserted, there is no longer a natural restraint. If that force is overlooked in a piping design it can bend pipe, structures, move equipment, and generally ruin an otherwise uneventful day.

The source of the force

Consider a pressurized pipe blanked off with blind flanges. There is a force on the blind flanges that the bolts have to restrain. That load is transferred through the bolts and on to the cross-sectional area of the pipe. That force is equal to the pipe inside diameter cross-sectional area (in2) multiplied by the internal pressure (lb/in2).

Now, slip a bellows into that assembly and there is nothing holding that force back; without a restraint somewhere, it will elongate.

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Becoming one with the force

In many piping systems this load is restrained by anchors located at each change of pipe direction. The piping still has a portion of that pressure thrust acting on the end of the pipe so guides are included to keep the pipe straight and prevent buckling.

Other piping system types have load bearing hardware in the form of rods, or pinned linkages that restrain the pressure thrust within the expansion joint assembly. These devices, by design, usually limit the bellows movement to just lateral or angular directions.

Don’t underestimate the force

Designers need to determine the right expansion joint system which will either have load bearing anchors on the piping or load bearing hardware on the expansion joint assembly.

Maintenance needs to understand the role of these load bearing parts and not remove any rods, plates, or pins that are critical for operating conditions.

Contact us

This article, with original graphics, can be found on our website.

For more helpful articles about expansion joints and metal bellows, Oakridge Bellows is located in New Braunfels, Texas – You can call us at (830) 626-7773 or find us on the web at www.oakridgebellows.com.

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