What Is Stub-In Connection?
A stub-in connection (also called a stub-on or set-on connection) is a branch pipe welded directly to the run (header) pipe through a hole cut in the header wall, without using a manufactured fitting such as a tee, weldolet, or saddle. The branch pipe end is contoured (fish-mouthed or saddle-cut) to match the curvature of the header OD and then fillet or full-penetration welded to the header.
Stub-in connections are the simplest and most economical branch method but have the highest stress intensification factor (SIF) among standard branch types, making them a critical consideration in piping stress analysis.
When Stub-In Connections Are Used
| Condition | Stub-In Permitted | Alternative |
|---|---|---|
| Branch/header ratio <= 0.5 | Yes, per most project specs | Weldolet or reinforced stub-in |
| Branch/header ratio > 0.5 | Generally not permitted | Welding tee (ASME B16.9) |
| Non-cyclic service (low SIF concern) | Yes | Stub-in is acceptable where fatigue is not governing |
| Severe cyclic service (>7,000 cycles) | Not recommended | Welding tee or integrally reinforced fitting |
| High-pressure service (Class 600+) | Reinforced stub-in or fitting required | Reinforcing pad, weldolet, or forged tee |
| Small bore piping branches | Limited; use branch fittings instead | Sockolet, threadolet per pipe class |
Reinforcement Requirements (ASME B31.3)
Every branch opening in a pressure-containing header removes material from the pressure boundary. ASME B31.3 para. 304.3 requires the area replacement method to verify adequate reinforcement:
| Parameter | Symbol | Definition |
|---|---|---|
| Required reinforcement area | A_req | t_min x d_branch x (2 - sin(beta)), where beta = branch angle (90 deg for perpendicular branch) |
| Available area in header wall | A_1 | Excess thickness in header wall above minimum required |
| Available area in branch wall | A_2 | Excess thickness in branch pipe above minimum required |
| Reinforcement pad area | A_3 | Area of welded pad (if used) |
| Acceptance criterion | - | A_1 + A_2 + A_3 >= A_req |
If the combined excess area in the header and branch walls meets or exceeds the required reinforcement area, no additional reinforcement (pad or fitting) is needed. If not, a reinforcing pad (saddle plate) must be added, or the connection must be changed to a manufactured fitting (weldolet, tee).
Stub-In vs Other Branch Connections
| Connection Type | SIF (typical) | Cost | Reinforcement | Fatigue Life |
|---|---|---|---|---|
| Unreinforced stub-in | 2.5-6.0+ | Lowest | Header/branch excess wall only | Poorest |
| Reinforced stub-in (with pad) | 1.8-4.0 | Low | Saddle reinforcing pad added | Improved |
| Weldolet | 1.5-3.0 | Moderate | Self-reinforcing forged fitting | Good |
| Welding tee (ASME B16.9) | 1.5-3.5 | Highest | Integrally reinforced by design | Best |
| Sweepolet | 1.2-2.0 | Moderate-high | Contoured geometry reduces SIF | Best for cyclic service |
Fabrication Requirements
| Aspect | Requirement |
|---|---|
| Hole cutting | Contoured hole in header; no flame-cut notches; edges ground smooth |
| Branch preparation | Saddle-cut (fish-mouth) to match header OD curvature |
| Weld type | Full-penetration groove weld with fillet cap per ASME B31.3 Fig. 328.5.4E |
| Weld examination | RT or UT per NDE plan; MT or PT on root and cap passes |
| Fit-up gap | Maximum 1.6 mm (1/16 in.) per ASME B31.3 |
| Reinforcing pad | If required, welded to header with continuous fillet weld; tell-tale hole drilled for leak testing |
Stub-in connection allowances, reinforcement requirements, and branch fitting alternatives are governed by the pipe class specification.
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