Resistance Welded Plies

The Only Way to Insure Performance of Ply Testable Expansion Joints

Ply testable expansion joints have become the industry norm for critical service applications. The ply testable bellows design provides 100% safety factor on bellows integrity while providing a means for end users to diagnose indications of leakage. However, the ply testable concept is only as good as the integrity of the bellows attachment welds.

There are two common ways to fuse the bellows plies together before making the attachment weld. The fi rst method employs a manual edge weld that fuses the plies together after the bellows have been trimmed to length. The second method uses an automatic resistance weld to fuse the plies together prior to fi nal trimming.

The edge-welding concept results in a weld of varying thickness and penetration around the perimeter of the bellows. A portion of the edge weld is remelted as it becomes part of the attachment weld. During attachment welding it is common for a leak path to develop in the edge weld allowing media to pass around the attachment weld and into the space between the plies. If this breech of weld integrity is discovered during fi nal inspection, repairs can be made. However, the diffi culty of discovering the location of the leak can compromise the repair process. Leaks often remain undetected during manufacturing.

It is also quite common for leak paths to develop later in manual edge welds during and after start up of the system. Final pressure testing prior shipping does not always insure success of ply testable bellows with manual edge welds.

The only proven way to produce reliable ply testable bellows is to resistance weld the plies together prior to trimming. The resistance weld results in a wide area of fusion. The bellows is fi nal trimmed through the resistance weld and the attachment weld is made to a single integral layer of bellows material.

Specify “Bellows plies must be resistance welded prior to trimming” to insure that your ply testable bellows perform as intended in service.