Stub Ends and Lap Joint Flanges: When to Use This Assembly and How to Specify It
The stub end / lap joint flange assembly is a two-piece alternative to a conventional weld-neck or slip-on flange. The stub end is welded to the pipe; the lap joint flange (also called a backing flange) rotates freely around it. This decouples the high-cost corrosion-resistant alloy stub end from the flange, which can be a standard carbon steel backing flange — a significant cost saving in alloy-heavy piping systems.
Assembly Components
Stub end: A short pipe section with a formed lap (flange face) at one end. The lap contacts the gasket and provides the sealing face. The stub end is welded to the process pipe by a standard butt weld — it is made from the same corrosion-resistant alloy as the pipe (316L, duplex, Inconel 625, etc.). ASME B16.9 covers stub ends; MSS SP-43 covers stub ends in lighter schedules for low-pressure service.
Lap joint (backing) flange: A flange with a flat face and a bore large enough to slide over the pipe OD. It is not welded — it rotates freely, which allows bolt hole alignment during assembly. The backing flange carries the bolt loads and can be standard ASME B16.5 carbon steel (even on 316L or duplex piping) because it never contacts the process fluid. This is the cost advantage: the flange bulk is cheap CS; only the stub end (a short, light fitting) is the expensive CRA alloy.
When Stub End / Lap Joint Is the Right Choice
- Alloy piping with frequent dismantling: The rotatable flange means bolt holes always align without rotating the pipe — ideal for equipment that is frequently removed for inspection or cleaning (strainers, instruments, heat exchanger covers)
- High-alloy systems where flanges are the major cost: In Inconel 625 or Hastelloy C-276 piping, stub end + CS backing flange saves 60–70% of flange cost vs solid alloy weld-neck flanges
- Lined or PTFE-sleeved piping: Stub end assemblies are standard in PTFE-lined pipe where the lining terminates at the lap face
- Low-pressure service (PN 10–PN 25 / Class 150): The lap joint connection is less rigid than weld-neck; adequate for low-pressure service but not the preferred choice for high-pressure cyclic or high-temperature creep service
Limitations and Where Not to Use Stub Ends
- High-pressure service (Class 600+): the lap-to-backing-flange interface is not as rigid as a weld-neck — use weld-neck flanges in high-pressure or high-cycle fatigue applications
- Piping subject to large bending moments: the lap joint cannot transfer large external moments efficiently
- Where crevice-free design is required (pharmaceutical, ultra-high purity): the annular gap between the stub end and backing flange is a potential crevice — use solid flanges instead
- Do not use short-pattern stub ends (MSS SP-43) interchangeably with long-pattern (ASME B16.9) — they have different face-to-end dimensions and will mis-align the isometric
Specifying on the Purchase Order
Stub end: ASME B16.9 or MSS SP-43, material grade (e.g. ASTM A403 WP316L), NPS, schedule, long-pattern or short-pattern, EN 10204 3.1 certificate. Backing flange: ASME B16.5 Class 150 (or relevant class), ASTM A105 (CS), bore to match pipe OD, lap joint face. Always order backing flanges in the same pressure class as the stub ends — a Class 150 stub end with a Class 300 backing flange will not align correctly. State on PO that the assembly must be checked for correct lap engagement before shipping.
Cost Comparison: Weld-Neck vs Stub End Assembly in Inconel 625
| Component | Solid WN Flange (Inconel 625) | Stub End + CS Backing Flange |
|---|---|---|
| Alloy content | 100% Inconel 625 throughout | Only stub end is Inconel 625; flange is CS |
| Relative cost | High | 30–40% of WN flange cost |
| Bolt hole alignment | Fixed — pipe must rotate for alignment | Backing flange rotates freely — no pipe rotation needed |
| Pressure suitability | All pressure classes | Class 150–300 preferred; Class 600+ use WN |