Oversize Clamshell Bellows

The best way to replace a leaking expansion joint bellows is to replace the entire expansion joint assembly or at least a portion of the expansion joint that includes weld ends. In this way, the installers can avoid making the delicate bellows attachment weld in the fifi eld. However, this approach requires that the pipe be cut to replace the leaking bellows. Often it is not practical to cut the existing pipe or to remove the existing bellows element. The “oversize clamshell” is one approach to replacing these bellows.

The fifi rst step in the “oversize clamshell” is to verify the dimensions of the existing bellows and the available space around the existing bellows. An oversize bellows and attachment rings or standoff are then designed to fifi t the space available.

The new bellows will have a larger area than the original. It should be verifi ed that the increased pressure thrust caused by the larger bellows will not cause an unsafe load at the anchor points that restrain the expansion joint.

The oversize replacement bellow is then manufactured and cut in half longitudinally. The halves are match marked to insure a good fit. The new standoff rings are also cut in half and all weld surfaces are prepared for a weld design that is in accordance with the appropriate code. Threaded purge holes are put in the face of the standoff rings to allow for release of gas that may be leaking from the existing bellows element.

The fi rst step in installing the oversize clamshell is to fi t and tack the standoff rings. The “oversize clamshell” is then fi t to the standoff rings. If possible, the inside of the new clamshell bellows is purged with the proper welding gas to insure a good seam weld. When purging is not possible, a special fl ux is used to back the weld. Then the bellows is welded back together and the bellows are welded to the standoff rings. The purge holes in the standoff rings are capped to complete the assembly.

It is recommended where possible that all finished welds be dye penetrant inspected.

All field work should be performed by a crew that has been field qualified for clamshell work on the required bellows thickness. Senior Flexonics Pathway continuously trains and qualifi es welders to perform clamshell work under simulated job site conditions.

The “oversize clamshell” procedure may be performed while the system is on line, depending on the process media operating temperature and other conditions. It is recommended however, that the actual installation be inspected before an on line clamshell is executed.

A clamshell bellows must be of single ply construction. A clamshell bellows can be fi t with reinforcing rings for high pressure service. The bellows can also be certifi ed for use in ASME Section VIII code stamped pressure vessels.

For more information on oversize clamshells, contact Senior Flexonics Pathway On-Site Service.