scope:

For critical service and/or severe duty piping systems, directional changes are usually made by use of a manufactured elbow, such as ANSI B16.9 elbows for butt welding or ANSI B16.11 elbows for socket welding; or by shop or field bending of a length of straight pipe. Prior to the availability of butt welding elbows, mitred bends were often used. Because such joints require considerable time to cut and fit so that a more-or-less adequate juncture-weld can be made, mitred bends are seldom economical as compared to use of butt welding elbows; provided a butt welding elbow is available when needed. In large diameter piping, butt welding elbows may not be commercially available nor is it feasible to bend a length of large diameter straight pipe. Accordingly, mitre bends are being used in large diameter piping; for example in penstocks and, in England, for large diameter piping used in gas-cooled nuclear power reactors