Stub-in and Stub-on are methods for making a fabricated branch connection from the pipe. Both types are permitted by many of the international codes and standards including ASME B31. However, both of these are weak connections on piping systems and normally limited only for low pressure and temperature applications.
What is Stub-In?
In case of a Stub-In, A larger hole is drilled in the Header or Run Pipe and branch pipe whose end is contoured similar to the inside diameter (ID) of the Header is fitted inside the hole. Then both the stub-in branch pipe and the run pipe are welded together to form a connection similar to reducing tee. Stub-in is normally used when the Branch is more than one size smaller than the main pipe. For “stub in” connection the branch pipe extends to the inside of the main pipe.
What is Stub-On?
On the contrary, In case of a Stub-On branch connection, the hole that is cut in the Run pipe is the same as the inside diameter (ID) of the Branch Pipe (Not Header). The end of the Branch pipe is contoured the same as the outside diameter (OD) of the header pipe and is then fitted outside the hole on the Header pipe. It looks like the branch is seated “onto” the header pipe. Stub-on is generally used when the Branch is equal to or one size smaller than the main Pipe. For “stub on,” the stub extends only to the outside of the main pipe.
Additional Features for Stub-In and Stub-On
Both Stub in and Stub on branch connections can be made with or without a reinforcing Pad as per requirement. This requirement is normally governed by pressure and stress criteria. The reinforcement pad is basically a ring that is cut from the run pipe or from a plate with the same material as the run pipe. At the center of the pad, is a hole is made (the size the same as the branch pipe). When it is cut from a flat plate it is contoured to fit around the run pipe. The width of the reinforcement pad is normally one half the diameter of the branch pipe. The aim of this reinforcement is to substitute the material that was removed for making the branch connection from the header. A small-diameter hole, known as weep hole (1/4″ NPT) is normally drilled in the pad which acts as a vent during the welding process for the weld-generated gases to escape. Using full penetration welds, The ring or pad is then welded to the branch and the run pipe. Once, the work is completed, the small hole is fitted using a plug.
Both the stub-in and Stub-on connections, in a sense, reduces the cost of pipe fittings. It saves installation time as well because only one weld is required around the stub hole instead of three welds that is needed for joining welding tee connections.
Welding strength of Stub-in connection is as good as butt welding but welding steps are difficult in actual conditions. So Stub-in is comparatively stronger than stub on connections.
Stress Considerations for Stub-in and Stub on Connections
From Piping stress considerations calculated SIF of stub-ins and stub-on are much higher than weld-olets and ASME tees, which is significant when a detailed stress analysis is done on the system. While analysis extra caution needs to be considered as the stress generated will be higher. The use of both this type of branch connection is not preferred for severe cyclic applications, high-pressure temperature applications or category M fluid service applications.
Few more Resources for you..
The following video by Technical Piping will give you some more insight about Stub-in and Stub-on Connections: