Long Radius vs Short Radius Elbow
A long radius (LR) elbow has a center-to-end dimension equal to 1.5 times the nominal pipe size (1.5D). A short radius (SR) elbow has a center-to-end dimension equal to 1 times the nominal pipe size (1D). Both types are defined in ASME B16.9 and available in 45°, 90°, and 180° configurations.
When to Use Each Type
LR elbows are the default in process piping. ASME B31.3 (Process Piping) assumes long radius unless the designer specifies otherwise. SR elbows serve a narrower role: tight spaces where the piping layout cannot accommodate the longer sweep of a 1.5D fitting.
| Feature | Long Radius (LR) | Short Radius (SR) |
|---|---|---|
| Center-to-end | 1.5D | 1D |
| Pressure drop | Lower (~60% of SR) | Higher |
| Erosion/corrosion | Less (gentler flow path) | More (sharper turn) |
| Space required | More | Less |
| Pigging capability | Yes (standard) | Difficult |
| Wall thinning | Less at extrados | More at extrados |
| Cost | Standard pricing | Similar or slightly less |
| Default per B31.3 | Yes | Must be specified |
| Available angles | 45°, 90°, 180° | 45°, 90°, 180° |
Dimensional Example
For a 6-inch 90° elbow:
- LR: Center-to-end (A) = 1.5 x 6 = 9 inches (228.6 mm)
- SR: Center-to-end (A) = 1 x 6 = 6 inches (152.4 mm)
The SR elbow saves 3 inches of space per turn. On a pipe rack with hundreds of direction changes, this can matter. But the tradeoff in increased pressure drop and erosion usually makes LR the better engineering choice.
Pressure Drop Comparison
The resistance coefficient (K-factor) for a 90° LR elbow is approximately 0.25-0.30. For a 90° SR elbow, it rises to approximately 0.35-0.45. In systems with many elbows, such as offshore platforms or compact modular plants, these differences accumulate and affect pump sizing and energy consumption.
Selection Criteria
Use long radius when:
- Standard process piping per ASME B31.3 or B31.1
- Pipeline pigging is required (the 1D turn in SR elbows can jam pigs)
- Erosive or corrosive service where wall thinning at the extrados is a concern
- No space constraints dictate otherwise
Use short radius when:
- Physical space is limited (e.g., near vessel nozzles, within skid-mounted packages)
- The piping specification explicitly calls for SR
- Low-pressure, non-critical utility services where pressure drop is irrelevant
For branch connections at non-standard angles, consider lateral tees or weldolets instead of elbows combined with tees.
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