What Is ASME B16.9? BW Fittings Standard

Quick Answer

ASME B16.9 is the standard that governs the dimensions, tolerances, testing, and marking of factory-made wrought buttweld fittings (elbows, tees, reducers, caps, stub ends) for NPS 1/2 through NPS 48. It is the primary dimensional reference for BW fittings in oil and gas, petrochemical, and power piping systems designed to ASME B31.1 or B31.3.

The standard title is “Factory-Made Wrought Buttwelding Fittings.” It does not specify materials; material requirements come from separate ASTM specifications such as A234 (carbon/alloy steel), A403 (stainless steel), A815 (duplex), and B366 (nickel alloys).

What ASME B16.9 Covers

Aspect	Requirement
Size range	NPS 1/2 to NPS 48
Fitting types	Elbows (LR/SR, 45°/90°/180°), tees, crosses, reducers, caps, lap joint stub ends
Dimensions	Center-to-end, end-to-end, back-to-face distances by size and schedule
Wall thickness	Must match connecting pipe schedule; minimum 87.5% of nominal at any point
Tolerances	OD at bevel end: +1/-1% (NPS 2.5-20), +/- 3.2 mm (NPS 22-48)
End preparation	37.5° bevel per ASME B16.25, 1.6 mm root face
Marking	Manufacturer name/trademark, material, size, schedule/wall, heat number
Testing	Per applicable ASTM material specification (hydro, NDE)

Key Dimensional Concepts

Every BW fitting has standardized dimensions tied to its nominal pipe size and type. For elbows, the critical dimension is the center-to-end distance (A). For a long-radius 90° elbow, this equals 1.5 times the nominal pipe size. A 10-inch LR 90° elbow has A = 15 inches (381 mm).

Reducers are defined by end-to-end length (H). Tees are defined by the center-to-end distance along the run (C) and the center-to-end distance of the outlet (M).

For a full table of ASME B16.9 fitting dimensions, refer to the detailed dimensional charts.

Relationship to Other Standards

ASME B16.9 works alongside several companion standards:

• ASME B16.25: Specifies the butt welding end preparation (bevel angle, root face, root gap)
• ASME B16.11: Covers forged fittings (socket weld and threaded) for small bore piping
• MSS SP-43: Covers wrought stainless steel BW fittings with lighter wall thicknesses (Schedule 5S/10S)
• MSS SP-75: High-yield BW fittings for pipeline service (matching API 5L grades)

Wall Thickness and Schedule

ASME B16.9 fittings are ordered to match the connecting pipe schedule. A fitting for SCH 40 pipe has a different wall thickness than one for SCH 80. The standard requires that wall thickness at any point must not be less than 87.5% of the nominal wall, matching the same minimum tolerance rule applied to seamless pipe under ASTM A106.

Ordering Tip

Always specify both the nominal pipe size and the schedule (or wall thickness) when ordering BW fittings. A “6-inch reducing tee” without a schedule callout is incomplete and will cause procurement delays.

Read the full guide to pipe fittings