Stub-in vs Stub-on: Differences between Stub-in and Stub-on Piping Connection

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. In this article, let’s explore the differences between Stub-in and Stub-on branch connections.

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 connection 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.

Stub-in and Stub-on Connection
Fabricated Piping Branch Connections

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, 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 the 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.

Stub-in vs Stub-on Piping Connection

The main differences between a stub-in and stub-on branch piping connection is tabulated below:

Stub-in pipe ConnectionStub-on branch connection
As explained above, Stub-in is used when the run pipe and branch pipe has a difference of more than one size.On the contrary, Stub-on pipe branch connection is applicable when the branch pipe isequal to run pipe only only one size lower than the header pipe. 
In case of Stub-in branch connection,  the pipe welding between the branch and header is of butt-weld type.
This welding is quite difficult.
Stub-on branch connections are made using fillet welds. 

Welding is easier as compared to stub-in welding method.
Able to withstand more pressure.Comparatively weaker than stub-in branch connection and hence, handles less pressure.
The branch edge of a stub-in connection matches with internal pipe diameter of the header.For stub-on branch connection, the branch edge lies on the outside diameter of the header.
Usually have more weld strength value than sub-on branch connections.Possess comparatively less weld strength value
Table 1: Stub-in vs Stub-on

Few more Resources for you..

Briefing about Reinforcement Pad
Piping Design and Layout
Piping Materials
Piping Stress Analysis
Piping Interface

The following video by Technical Piping will give you some more insight about Stub-in and Stub-on Connections:

Stub-In & Stub-on Pipe Connection Video by Technical Piping
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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

5 thoughts on “Stub-in vs Stub-on: Differences between Stub-in and Stub-on Piping Connection

  1. Hellow Mr DEY I am regularly studying your post needless to say it’s very important for piping engineer. Thanks DIBYENDU CHATTERJEE mail

  2. In the video “stub-on” sketch the landing of 1.6±0.6 is wrongly marked, on the chamfer-side

  3. Dear Anup Kumar Dey,

    I am familiar with stub-in en stub-on connections. I can read and make isometrics. My question is how do you dimension on an isometric drawing the length of a stubpipe?

    1) Do you dimension to the centre of the runpipe?
    2) Do you dimension to the outside diameter of the runpipe?
    3) or different?

    Thanx in advance, from the Netherlands

    1. Definitely piping dimensions usually takes it’s coordinate from the centre of pipe to the centre or end of the fitting at the other end of the stub. E.g. An Elbow, a Flange, Tee or Reducer, etc.

      Therefore pipe stubs dimension is taken from CL of Header to the CL or the end of the opposite fitting.

      Thanks. This is from a pipe fitter’s experience.

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