What Is ASME B16.28?
Short-radius fittings exist for space-constrained installations where the larger sweep of a long-radius fitting cannot be accommodated. ASME B31.3 treats long radius as the default; short radius must be explicitly specified by the designer.
ASME B16.28 Scope
| Aspect | Coverage |
|---|---|
| Fitting types | 90° SR elbow, 180° SR return |
| Size range | NPS 1/2 to NPS 24 |
| Bend radius | 1D (center-to-end = nominal pipe diameter) |
| End preparation | 37.5° bevel, 1.6 mm root face (per ASME B16.25) |
| Wall thickness | Must match connecting pipe; min 87.5% of nominal at any point |
| Tolerances | OD at bevel: per ASME B16.9 tolerances |
| Marking | Manufacturer, material, size, schedule, SR designation |
| Materials | Per applicable ASTM specification (A234, A403, A815, B366) |
Key Dimensions
| NPS | Center-to-End A (90° SR) mm | Center-to-Center O (180° SR) mm |
|---|---|---|
| 2 | 50.8 | 101.6 |
| 4 | 101.6 | 203.2 |
| 6 | 152.4 | 304.8 |
| 8 | 203.2 | 406.4 |
| 10 | 254.0 | 508.0 |
| 12 | 304.8 | 609.6 |
For comparison, the equivalent long-radius dimensions per ASME B16.9 are 1.5 times larger. A 6-inch 90-degree LR elbow has A = 228.6 mm vs 152.4 mm for the SR version, saving 76.2 mm (3 inches) of space.
ASME B16.28 vs ASME B16.9
| Feature | ASME B16.28 (SR) | ASME B16.9 (LR) |
|---|---|---|
| Bend radius | 1D | 1.5D |
| Fittings covered | 90° elbows, 180° returns only | Full range (elbows, tees, reducers, caps, stub ends) |
| Size range | NPS 1/2 to NPS 24 | NPS 1/2 to NPS 48 |
| Default per B31.3 | No (must specify) | Yes |
| Pressure drop | Higher | Lower |
| Wall thinning at extrados | Greater (tighter bend) | Less |
| Pigging | Generally not piggable | Piggable (1.5D minimum) |
| Stress intensification | Higher SIF | Lower SIF |
Wall Thinning Considerations
The tighter bend radius of SR fittings causes more wall thinning at the extrados (outer curve) than LR fittings. ASME B16.28 still requires a minimum wall thickness of 87.5% of nominal at any measurement point. For high-pressure or erosive services, the starting wall must be thick enough to maintain this minimum after forming.
Some project specifications require SR elbows to be manufactured from one schedule heavier than the connecting pipe to compensate for the additional thinning.
When to Specify SR Fittings
Short-radius fittings are justified in limited circumstances:
- Tight spaces near vessels and equipment: Where the piping cannot extend 1.5D from the connection point
- Compact modular units: Skid-mounted packages with strict dimensional envelopes
- Return bends in heat exchangers: Where 180-degree SR returns reduce tube bundle spacing
- Low-pressure utility services: Where the higher pressure drop of SR fittings is acceptable
For all other applications, long-radius elbows per ASME B16.9 are the standard choice.
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