One layer or multi-ply bellows

One layer or multi-ply bellows

Some expansion joint companies offer a multi-ply bellows for their metal expansion joint and some are interested in one-layer bellows

But which one is correct

According to the EJMA, both of them are correct, but ENETAF SANE COMPANY offers multi-ply bellows most of the times and in the following we will illustrate the reasons

Multi-ply Bellows has more fatigue life cycles

According to the bending stress theory, the amount of bending stress has a direct relation with distance from neutral axis. So, increasing the bellows thickness, increases the bending stress at the outer surface. Increasing the stress causes the decrease in fatigue life cycle. Therefore, decreasing of bellows thickness (Y) can increase the fatigue life cycle



But with decreasing of bellows thickness, the Hoop stress is increased more than the allowable stress which decreases the pressure tolerance of bellows. For solving this problem, we can use multi-ply bellows that create multiple neutral axes which decreases the max bending stress and increases the fatigue life cycle. Furthermore, with increasing the number of layers, the total thickness is increased that make the total stress less than the allowable stress.



Multi-ply bellows can absorb larger movements

Due to decreasing of bending stress with decreasing of bellows thickness, the bellows can absorb larger movements

Multi-ply bellows has low spring rates

Spring rate is the reaction force of the bellows when it is moved. Bellows with low spring rate is more suitable for high vibrating pipes

Multi-ply bellows has more safety

In the most piping lines, the service is flammable, poisonous, acidic and corrosive. Using the multi-ply bellows create a situation that only the inner layer has contact with fluid which can increase Expansion Joint life cycle. In the multi-ply expansion joint, the inner layer will be the first to fail and if there is a pressure gauge between layers, operators can replace the expansion joint as soon as possible and prevent the catastrophic failure