ASME B16.28 Short Radius Elbows: When to Use SR and When to Use LR
Long radius (LR) elbows to ASME B16.9 — with a centre-to-end dimension equal to 1.5 times the nominal pipe size — are the default for the vast majority of process piping. Short radius (SR) elbows to ASME B16.28 — with a centre-to-end dimension equal to the nominal pipe size — are used where tight-turn geometry is required and space is constrained. Understanding the hydraulic, structural, and service limitations of SR elbows prevents their misapplication in situations where the tighter bend radius would cause erosion, fatigue, or excessive pressure drop.
LR vs SR: The Geometry
| Property | Long Radius (ASME B16.9) | Short Radius (ASME B16.28) |
|---|---|---|
| Centre-to-end (90°) | 1.5 × NPS | 1.0 × NPS |
| Bend radius | 1.5D | 1.0D |
| Pressure drop coefficient (K) | ~0.25–0.35 | ~0.50–0.75 (approx 2× LR) |
| Wall thinning at extrados | Less severe | More severe — tighter bend → more thinning |
| Erosion susceptibility | Lower | Higher — sharper centrifugal particle impingement |
| Fatigue performance | Better | Lower — stress concentration factor higher at 1.0D |
When SR Elbows Are Acceptable
- Clean, non-erosive liquids at moderate velocity — cooling water, clean utility services, instrument air distribution
- Space-constrained equipment rooms, valve manifold areas, and platform piping where layout physically cannot accommodate LR geometry
- Low-pressure, low-velocity service where the higher pressure drop K-factor is acceptable in the hydraulic design
- Gravity drain lines where pressure drop is not the design driver
When SR Elbows Are Not Acceptable
- Multiphase or slurry flow — sharper bend dramatically increases particle impingement and erosion at the extrados
- High-velocity gas or steam — pressure drop penalty is significant and wall thinning risk is higher
- Cyclic service (steam hammer, pulsating flow from reciprocating compressors) — SR has higher stress concentration factor and lower fatigue life
- Pigging service — pigging pigs cannot navigate SR elbows (minimum piggable bend is typically 3D, sometimes 5D)
- Where ASME B31.3 requires long radius for the specific fluid service category and design pressure
Ordering SR Elbows
SR elbows are not covered by ASME B16.9 — they fall under ASME B16.28. Both standards use the same ASTM material specifications (A234 for carbon and alloy steel, A403 for stainless). The critical point on the purchase order: always state "Short Radius per ASME B16.28" explicitly — "elbow" alone defaults to LR per B16.9 in most manufacturers' interpretation, and an SR supplied as LR (or vice versa) will not fit the isometric. For EN 10253 projects: SR elbows fall under EN 10253-4 (nickel alloys) rather than EN 10253-2 — the standard must be stated on the PO.