Clam-shell construction  is the most commonly used method of fabricating jacketed or containment piping using insert flanges. In this method a section of the jacket pipe, closest to the flange, is split to allow an attachment weld to the back of the insert and the process pipe. An alternate method provides a means for the jacket pipe to be alternately slid back and forth during assembly to allow clearance for the welder to make the attachment weld between the back of the insert and the process pipe. They provide a very economical approach to jacketed pipe assembly when the specifications or codes require the back-of-the-insert hub to be welded to the inner pipe. This is also true when the specifications require a butt weld between the end of the inner pipe and the back of the insert. In addition, it allows inspection and non-destructive examination of the same weld prior to welding the two ends of the jacket pipe and avoids the necessity of clam-shelling the jacket. Insert flanges are offered with flanges matching B16.5 flange bolting for either the process pipe size or the jacket pipe size. The former are referred to as non-reducing or line size and the latter as reducing or jacket size.