2015-04-09 16:07:39

Metallic vs Non Metallic(fabric) Expansion Joints

What are the reasons to use Fabric vs. Metallic Expansion Joints?

For many years (decades) non-metallic Expansion Joints have been the power industry's preferred type of joint for ducting applications.
The reasons to consider fabric as opposed to metallic are that fabric joints offer a lower initial and long run cost and they last for decades.
The following questions and answers provide a good overview of why this is true.

How does a Non-Metallic Expansion Joint compare to a Metallic Joint in price?

Metallic Expansion Joints represent a far higher initial cost. A full featured Non-Metallic Expansion Joint can cost as little as 1/3rd the price
of a Metallic Expansion Joint of the same size.
Due to the use of expensive alloys and the expensive process to form metallic bellows the cost can become prohibitive. Non-Metallic
Expansion Joints on the other hand do not require special forming and high-grade stainless steels to perform the same function.

What is the Typical Life-Span for a Non-Metallic Expansion?
A properly designed Non-Metallic Expansion Joint should not need any service for at least 15 years.
Because of how they are constructed, Metallic Expansion Joints are susceptible to fatigue crack failures. This does not apply to
Non-Metallic Expansion Joints since they are constructed using flexible belting which has an unlimited cycle life.
Long service life is obtained by using only proven Zero-Porosity Flexible Elements like PTFE Composites, FKM Fluoroelastomers,
and EPDM Elastomers, along with high grade reinforcing fabrics and incorporating full perimeter baffle liners and insulation pillows.

Does a Non-Metallic Expansion Joint allow for more movement than a Metallic Joint?
Yes, Non-Metallic Expansion Joints can handle much larger movements than a Metallic Expansion Joint. Metallic Expansion Joints
are not suitable for large movements and are vey susceptible to failures in cases where movements are greater than expected,
such as where seismic and wind/snow loads have not been factored. They cannot be used to help accommodate duct misalignment.
Fabric joint on the other hand can accommodate relatable large movements (both axially and laterally). A properly designed
Non-Metallic Joint can handle seismic, wind and snow load movements without problems. They can be used to correct duct misalignments.
The nature of the fabric joint is also much more forgiving. Fabric joint can allow for minor movement miscalculations and can even
be preset to accommodate lateral movements. Movements of 7" axial and 7" lateral are possible with Non-Metallic Expansion
Joints but impossible with Metallic Bellows.

Can Non-Metallic Expansion Joints accommodate System upsets?
Absolutely! Because of their ability to handle larger movements, Non-Metallic Expansion Joints can be designed to easily accommodate
significant thermal, wind and seismic excursions along with temperatures up to 2000°F. In addition all Expansion Joints can
accommodate an additional 1" of movement (or misalignment) in any direction. The flexible nature of Non-Metallic Expansion
Joints means you have a lot more room to accommodate sudden and unexpected movements.

Do Non-Metallic Expansion Joints Transmit Loads into the Ducting Systems
No, Non-Metallic Expansion Joints do not transmit any loads. This is also another reason they are less expensive.
With Metallic Expansion Joints you have to consider the cost to have your engineer calculate the loads. With Metal Joints you
may have to spend additional project dollars to add more supports and reinforce your ducting and flanges or other connection
points. Fabric Expansion Joints transmit no loads and are hassle free.

Are Non-Metallic Joints more or less costly to repair?
This is a really important question because Non-Metallic Expansion Joints are far less expensive to repair than Metallic.
When a Metallic Expansion Joint develops a crack or a dent, or it is damaged during installation, there are very significant repair
costs. In most cases the joint will have to be sent back to the manufacturer to be repaired. In other cases a highly trained technician
must visit the site to weld the defect. This field repair is costly and does not result in an "as new" condition.
Fabric joints, however, are easy to repair in the field. If a fabric joint is punctured, torn or otherwise damaged, a new section of belt
can be easily spliced in to replace the damaged area. The procedure for the repair is simple and can be done by plant personnel or by
a factory technician. Most importantly the repaired section is good as new and will not affect the performance of the joint.

How about Replacement Costs?
A fabric Expansion Joint can be completely overhauled and replaced for a fraction of its original purchase price. This is because,
normally, only the fabric portion of a Non-Metallic Expansion Joint needs to be replaced. Once the Flexible Element (belt) and
Pillow are replaced the expansion joint is as good as new!
Metallic Joints need to be replaced in whole at a far greater cost or be repaired by specially trained welders. This welding process results in a less than perfect part.
For many years (decades) non-metallic Expansion Joints have been the preferred type of joint for ducting applications.