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 — with a centre-to-end dimension equal to the nominal pipe size — are used where a tight turn is required and space is constrained. SR elbows were originally standardised in ASME B16.28; that standard has been withdrawn and short radius elbows and returns are now covered by ASME B16.9. Understanding the hydraulic, structural and service limitations of SR elbows prevents their misapplication where the tighter bend radius would cause erosion, fatigue or excessive pressure drop.
ASME B16.28 Is Withdrawn: Where SR Elbows Sit Today
ASME B16.28 covered wrought steel buttwelding short radius elbows and returns. Its content was incorporated into ASME B16.9 and B16.28 was withdrawn, so a current order for an SR elbow should cite ASME B16.9. Many legacy piping classes, datasheets and isometrics still say "B16.28", and manufacturers still understand the term, but an inspector working to the current edition will check dimensions and marking against B16.9. Writing "ASME B16.9 short radius (formerly B16.28)" on the order removes any doubt.
One point the transfer did not remove is the pressure rating. B16.28 rated SR elbows at 80% of the rating calculated for straight seamless pipe of the same material and wall, because the tighter bend raises stress at the intrados. Before assuming an SR elbow carries the full rating of the matching pipe, the piping designer should confirm the rating basis given for short radius elbows in the edition of B16.9 named in the project specification.
LR vs SR: The Geometry
| Property | Long Radius (ASME B16.9) | Short Radius (ASME B16.9, formerly B16.28) |
|---|---|---|
| Centre-to-end (90°) | 1.5 × NPS | 1.0 × NPS |
| Bend radius | 1.5D | 1.0D |
| Resistance coefficient (K) | Lower | Higher — resistance rises as the bend radius falls |
| 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 intensification factor higher at 1.0D |
The centre-to-end values apply to 90° elbows; B16.9 tabulates the actual dimensions by size, and our article on ASME B16.9 centre-to-end dimensions shows how they are used in isometric take-off. SR 180° returns follow the same logic: their centre-to-centre spacing is tighter than LR returns, which is why they appear in compact heater coils and exchanger headers.
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 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 — the sharper bend increases particle impingement and erosion at the extrados (see erosion-corrosion in multiphase fittings)
- High-velocity gas or steam — the pressure drop penalty is significant and wall thinning risk is higher
- Cyclic service (steam hammer, pulsating flow from reciprocating compressors) — SR has a higher stress intensification factor and lower fatigue life
- Pigging service — SR elbows are not piggable; most cleaning pigs need at least LR elbows and intelligent inspection tools typically need 3D bends or larger (see pigging service fittings)
- Where the project piping class or owner specification excludes SR elbows — many do for hydrocarbon, steam and high-pressure classes
Design Checks Before Accepting an SR Elbow
Pressure. Confirm the rating basis described above. Where the SR elbow must carry the full design pressure of the line, a heavier wall than the connecting pipe may be needed, with the ends matched to the pipe wall at the weld bevel.
Flexibility and stress. In ASME B31.3 Appendix D the elbow flexibility characteristic h = TR₁/r₂² is proportional to the bend radius R₁. Halving the radius from 1.5D towards 1.0D lowers h, which raises both the stress intensification factor and the flexibility factor. A stress model that was built with LR elbows must be rerun if SR elbows are substituted — a direct swap is not stress-neutral. Our article on stress intensification factors for pipe fittings explains the calculation.
Hydraulics. Each SR elbow has a higher resistance coefficient than an LR elbow of the same size. One or two SR elbows rarely matter; a pump suction or compressor discharge with several of them can move the operating point, so the line should be re-checked with the actual fitting count.
Alternatives When Space Is Tight
Before settling on SR elbows, check whether the layout can be solved another way: rotate the elbow out of the congested plane, use two LR 45° elbows with a short spool, move the valve or flange that is forcing the tight turn, or use an LR elbow with a reducer placed downstream rather than upstream. For larger radii than LR, induction bends or 3D/5D bends are the usual route. Mitre bends are a further option in some low-pressure services, but ASME B31.3 sets its own limitations on their use.
EN 10253 Equivalents
On European projects, EN 10253 designates elbows by type rather than by LR/SR. Type 3D corresponds to long radius and type 2D (bend radius about 1.0 × D) is the short radius equivalent; type 5D is a larger-radius option. State the type, the steel name and number, and the part: EN 10253-2 for carbon and alloy steel, EN 10253-4 for austenitic and duplex stainless steel. Our overview of EN 10253-2 fitting types covers the full range.
Ordering SR Elbows: Purchase Order Checklist
"Elbow" alone defaults to LR in most manufacturers' interpretation, and an SR elbow supplied as LR (or vice versa) will not fit the isometric. Material specifications are the same for SR and LR: ASTM A234 for carbon and alloy steel, A403 for stainless, A815 for duplex, B366 for nickel alloys. Each line should state:
- Angle and radius in words: "90° short radius" or "180° short radius return"
- Dimensional standard: ASME B16.9 (state "formerly B16.28" if the piping class still cites it)
- NPS and wall thickness or schedule, matched to the connecting pipe
- Material specification and grade, e.g. ASTM A234 WPB or A403 WP316L
- End preparation and any bore matching required at the weld
- Inspection document: EN 10204 3.1, or 3.2 with the named inspection body
At goods receipt, check the centre-to-end dimension first — it is the quickest way to catch an LR elbow delivered against an SR line — and confirm the marking shows SR.
Frequently Asked Questions
Is ASME B16.28 still a current standard?
No. ASME B16.28 has been withdrawn and short radius elbows and returns are now covered by ASME B16.9. Orders can still reference B16.28 for clarity, but dimensions and marking are checked against the current B16.9 edition.
Does a short radius elbow have the same pressure rating as the pipe?
Not automatically. B16.28 rated short radius elbows at 80% of the rating calculated for matching straight seamless pipe. The piping designer should confirm the rating basis in the B16.9 edition used on the project and specify a heavier wall if full pipe rating is required.
Can I replace LR elbows with SR elbows without redoing the stress analysis?
No. The flexibility characteristic in ASME B31.3 Appendix D depends on the bend radius, so an SR elbow has a higher stress intensification factor and a different flexibility factor. The stress model must be rerun with the SR elbows included.
What is the EN 10253 equivalent of a short radius elbow?
EN 10253 type 2D elbows, with a bend radius of about 1.0 × D, are the short radius equivalent. Type 3D corresponds to long radius.