When is enough-enough but not too much?
Surveys of small-branch connections undertaken as a result of Technical Directive No. 12 have found a number of integrally reinforced branch fittings which were in service with insufficient attachment welds.
Based on the number of occurrences it is clear that a significant misunderstanding exists on how much welding is required for a complete installation of an integrally reinforced branch outlet fitting (O-Lets).
Investigation of the occurrences indicates that some personnel thought that as long as the weld profile exceeded the thickness of the main header, that the branch had sufficient reinforcement.
That is not true!
ASME B31 Series of Codes and the ASME Boiler and Pressure Vessel Code set the requirements for reinforcement of branch connections and nozzles.
The Codes require a full penetration groove weld to the outer edge of the manufacturer's weld bevel and that the weld be properly capped.
Research by the Pressure Vessel Research Council has determined that not following these requirements can result in a branch connection that is susceptible to cracking at the toe of the weld.
The adjacent detail describes the requirements for successful welding of an integrally reinforced branch fitting.
Note that the correct weld profile has the following characteristics:
- The fitting is welded fully, up to the outer edge of the weld bevel
- The groove weld has a transition weld cap that provides a smooth transition to the main pipe wall.
- The use of excessive weld metal is not allowed. A large increase in weld cross section is just as bad as not having enough weld cross section.
Following these simple "pretty is better" guidelines will ensure a branch connection weld that has the proper reinforcement for pressure retention and will provide sufficient fatigue endurance so as to not crack over the life of the installation.