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ASME B16.28-1994 scan — Wrought Steel Buttwelding Short Radius Elbows and Returns
📐 ASME B16.28-1994 is the American National Standard that governs wrought carbon and alloy steel buttwelding short radius elbows and returns. It establishes ratings, overall dimensions, testing, tolerances, and markings for this family of compact pipe fittings, complementing the long radius fittings covered by ASME B16.9. The Standard was developed by Subcommittee F (Steel Threaded and Welding Fittings) of the ASME B16 Committee, approved by ANSI on October 10, 1994, and issued on November 30, 1994, with an automatic addenda subscription service.
💡 Short radius elbows carry unique value in space-constrained piping systems: the center-to-end dimension is only 1.0 times the nominal size, roughly one-third less than a long radius elbow (1.5 times NPS), which is decisive when equipment and pipe centers are tightly packed.
1. Scope and General Framework 📌
Section 1 of the Standard states that it covers ratings, overall dimensions, testing, tolerances, and markings for wrought carbon and alloy steel buttwelding short radius elbows and returns. The term “wrought” denotes fittings made from pipe, tubing, plate, or forgings, as distinct from cast products.
The Standard explicitly excludes fabricated fittings that employ intersection welds; the design of such fittings is governed by the ASME B31 Code for Pressure Piping or the ASME Boiler and Pressure Vessel Code. Installation welding is likewise outside the scope — it must be performed in accordance with the piping code or regulation that governs the system into which the fittings are installed. Products manufactured to a prior edition of a referenced standard, while otherwise conforming to this Standard, are considered to be in conformance.
Historically, B16.28 grew out of MSS Standard Practice SP-59 (1959), which addressed short radius buttwelding fittings not covered by ASA B16.9-1958. It was approved as ASA B16.28-1964, reaffirmed in 1972, revised in 1978 with metric equivalents, and revised again in 1986 with inch dimensions established as standard. The 1994 edition introduced the change of the computed proof test pressure factor from 80% to 100%, along with various editorial and technical revisions.
⚠️ Fabricated fittings with intersection welds are outside the scope of B16.28. Their design, strength verification, and welding must revert to the ASME B31 piping codes or the Boiler and Pressure Vessel Code — never apply B16.28 dimension tables to them directly.
2. Design and Pressure Ratings 🔧
Per Section 2, fitting design shall be established by mathematical analysis in accordance with nationally recognized pressure vessel or piping codes, or, at the manufacturer’s option, by proof testing in accordance with Section 9 of the Standard. To meet design requirements, some portions of formed fittings may need to be thicker than the pipe wall. A notable technical change in the 1994 edition is that the factor used in computing proof test pressure was increased from 80% to 100%, meaning the theoretical calculated pressure is now used in full as the basis of the proof test — a tightening of the strength margin verification.
The principal product and geometry parameters are summarized below (exact values per Tables 2, 3 and Annex A):
| Product Type | Nominal Size Range | Key Dimension Relationship | End Type |
|---|---|---|---|
| Short radius 90° elbow (SR) | NPS 1/2 – NPS 24 | Center-to-end A ≈ 1.0 × NPS | Buttwelding beveled end |
| Short radius 180° return (SR) | NPS 1/2 – NPS 24 | Center-to-center B ≈ 2.0 × NPS | Buttwelding beveled end |
| Fitting wall thickness | Matching the connected pipe | Formed areas may be thickened to satisfy design | Weld bevel and root face per Fig. 1 |
💡 Quick memory aid: a short radius elbow bends “one diameter over” — 90° elbow center-to-end equals 1×NPS and a 180° return has 2×NPS center-to-center, versus 1.5×NPS and 3×NPS for long radius fittings (B16.9).
3. Dimensions, Tolerances, and End Preparation 📏
Fitting dimensions are given in Section 6 and tolerances in Section 10. Table 1 lists tolerances, Table 2 covers short radius elbows, Table 3 covers short radius 180° returns, and Annex A provides metric (SI) equivalents. End preparation per Section 7 requires a welding bevel and root face (Fig. 1) so the fitting can be butt-welded to a matching pipe end.
| Tolerance Item | Typical Range (inch, per Table 1) | Notes |
|---|---|---|
| Center-to-end / center-to-center | Approx. ±1/16 small sizes, ±1/8 large sizes | Affects alignment and cold pull |
| End outside diameter | Per buttwelding end requirements | Limits weld root gap and misalignment |
| Out-of-roundness (ovality) | Within specified outer diameter deviation | Control of port cross-section after forming |
| Wall thickness | Generally not less than 87.5% of nominal | Guards minimum thickness at thinned areas |
🚨 Short radius elbows thin noticeably on the extrados (outer bend) during forming. Beyond checking center distances and envelope space, always verify that the remaining thickness at the extrados satisfies the design pressure — never size a high-class or heavy-wall application on nominal wall thickness alone.
4. Testing and Inspection 🧪
Section 8 sets out production tests, which normally include pressure testing of the fittings to verify overall strength and integrity. Where the design proof test route is used, Section 9 prescribes the complete procedure: calculation of the proof pressure, loading method, and acceptance criteria. With the 1994 change to a 100% computed-pressure factor, fittings certified through the proof test route now demonstrate their pressure capability against the full theoretical calculated pressure.
5. Engineering Practice Essentials 🛠️
- 🔍 Fitting selection: choose short radius elbows when space is limited and some pressure drop is acceptable; prefer long radius (B16.9) elbows where pressure loss or turbulence matters.
- 🔧 Bevel matching: the fitting bevel angle and root face must match the field pipe preparation, and installation welding must comply with the B31 code requirements on welder qualification and procedure qualification.
- 📊 Thickness verification: forming thinning is inherent to short radius elbows; verify strength against the thinnest section (extrados) in the design documents.
- 📋 Marking and traceability: per Section 4, mark manufacturer’s identification and material as required to ensure traceability.
6. Frequently Asked Questions ❓
❓ Q: What is the difference between B16.28 and the elbows in B16.9?
A: B16.9 covers long radius (1.5×NPS) and special elbows; B16.28 specifically covers short radius (1.0×NPS) elbows and returns. Their dimensional bases differ and they are not interchangeable.
A: B16.9 covers long radius (1.5×NPS) and special elbows; B16.28 specifically covers short radius (1.0×NPS) elbows and returns. Their dimensional bases differ and they are not interchangeable.
❓ Q: Does the 1994 change of proof test factor from 80% to 100% affect users?
A: Fittings certified to the new edition have a more fully verified strength margin. In practice, still confirm the design route (mathematical analysis or proof test) and the consistency of material lots, and request proof test reports where needed.
A: Fittings certified to the new edition have a more fully verified strength margin. In practice, still confirm the design route (mathematical analysis or proof test) and the consistency of material lots, and request proof test reports where needed.
❓ Q: How should the term “wrought” be understood?
A: It means the fitting is made from pipe, tubing, plate, or forgings (hot or cold formed), in contrast to cast products. Fabricated fittings with intersection welds are not covered by this Standard.
A: It means the fitting is made from pipe, tubing, plate, or forgings (hot or cold formed), in contrast to cast products. Fabricated fittings with intersection welds are not covered by this Standard.
🔍 Q: What services are suitable for short radius elbows?
A: They suit low-to-medium pressure lines where space is limited and pressure drop is acceptable. In high-pressure, high-flow systems, additional local turbulence and resistance should be checked hydraulically before adoption.
A: They suit low-to-medium pressure lines where space is limited and pressure drop is acceptable. In high-pressure, high-flow systems, additional local turbulence and resistance should be checked hydraulically before adoption.
