What Are ASME B16.9 Reducers?
ASME B16.9 reducers join two pipes of different diameters in a butt-welded system, from NPS 3/4 through NPS 34×26 per Table 6.1-11. Two types are covered: concentric reducers (centers aligned, used on vertical lines) and eccentric reducers (flat side, used on horizontal lines to keep pipe slope and avoid liquid pockets).
Dimensions (Table 6.1-11)
Values in mm (inch). H = overall end-to-end length. Large end = larger NPS; small end = smaller NPS.
| NPS (Large × Small) | Large End OD (mm / in) | Small End OD (mm / in) | H — Overall Length (mm / in) |
|---|---|---|---|
| 3/4 × 1/2 | 26.7 (1.05) | 21.3 (0.84) | 38.0 (1.5) |
| 3/2 × 3/4 | 26.7 (1.05) | 17.3 (0.68) | 38.0 (1.5) |
| 1 × 3/8 | 33.4 (1.32) | 26.7 (1.05) | 51.0 (2.0) |
| 1 × 1/4 | 33.4 (1.32) | 21.3 (0.84) | 51.0 (2.0) |
| 5 × 3 | 141.3 (5.56) | 88.9 (3.5) | 127.0 (5.0) |
| 1-1/2 × 1 | 42.2 (1.66) | 33.4 (1.32) | 51.0 (2.0) |
| 1-1/2 × 3/4 | 42.2 (1.66) | 26.7 (1.05) | 51.0 (2.0) |
| 1-1/2 × 1/4 | 42.2 (1.66) | 21.3 (0.84) | 51.0 (2.0) |
| 6 × 5 | 168.3 (6.62) | 141.3 (5.56) | 140.0 (5.5) |
| 1-1/4 × 1-1/4 | 48.3 (1.9) | 42.2 (1.66) | 64.0 (2.5) |
| 1-1/2 × 1 | 48.3 (1.9) | 33.4 (1.32) | 64.0 (2.5) |
| 1-1/2 × 3/4 | 48.3 (1.9) | 26.7 (1.05) | 64.0 (2.5) |
| 1-1/2 × 1/2 | 48.3 (1.9) | 21.3 (0.84) | 64.0 (2.5) |
| 2 × 1-1/2 | 60.3 (2.38) | 48.3 (1.9) | 76.0 (3.0) |
| 2 × 1-1/4 | 60.3 (2.38) | 42.2 (1.66) | 76.0 (3.0) |
| 2 × 1 | 60.3 (2.38) | 33.4 (1.32) | 76.0 (3.0) |
| 2 × 3/2 | 60.3 (2.38) | 26.7 (1.05) | 76.0 (3.0) |
| 2-1/2 × 2 | 73.0 (2.88) | 60.3 (2.38) | 89.0 (3.5) |
| 2-1/2 × 1-1/2 | 73.0 (2.88) | 48.3 (1.9) | 89.0 (3.5) |
| 2-1/4 × 1-1/4 | 73.0 (2.88) | 42.2 (1.66) | 89.0 (3.5) |
| 2-1/2 × 1 | 73.0 (2.88) | 33.4 (1.32) | 89.0 (3.5) |
| 3 × 2-1/2 | 88.9 (3.5) | 73.0 (2.88) | 89.0 (3.5) |
| 3 × 2 | 88.9 (3.5) | 60.3 (2.38) | 89.0 (3.5) |
| 3 × 1-1/2 | 88.9 (3.5) | 48.3 (1.9) | 89.0 (3.5) |
| 3 × 1-1/2 | 88.9 (3.5) | 42.2 (1.66) | 89.0 (3.5) |
| 3-1/2 × 3 | 101.6 (4.0) | 88.9 (3.5) | 102.0 (4.0) |
| 3-1/4 × 2-1/2 | 101.6 (4.0) | 73.0 (2.88) | 102.0 (4.0) |
| 3-1/2 × 2 | 101.6 (4.0) | 60.3 (2.38) | 102.0 (4.0) |
| 3-1/2 × 1-1/4 | 101.6 (4.0) | 48.3 (1.9) | 102.0 (4.0) |
| 3-1/2 × 1-1/2 | 101.6 (4.0) | 42.2 (1.66) | 102.0 (4.0) |
| 16 × 14 | 406.4 (16.0) | 355.6 (14.0) | 356.0 (14.0) |
| 4 × 3-1/2 | 114.3 (4.5) | 101.6 (4.0) | 102.0 (4.0) |
| 4 × 3 | 114.3 (4.5) | 88.9 (3.5) | 102.0 (4.0) |
| 4 × 2-1/2 | 114.3 (4.5) | 73.0 (2.88) | 102.0 (4.0) |
| 4 × 2 | 114.3 (4.5) | 60.3 (2.38) | 102.0 (4.0) |
| 18 × 16 | 457.0 (18.0) | 406.4 (16.0) | 381.0 (15.0) |
| 18 × 14 | 457.0 (18.0) | 355.6 (14.0) | 381.0 (15.0) |
| 18 × 12 | 457.0 (18.0) | 323.8 (12.75) | 381.0 (15.0) |
| 18 × 10 | 457.0 (18.0) | 273.0 (10.75) | 381.0 (15.0) |
| 36 × 30 | 914.0 (36.0) | 762.0 (30.0) | 610.0 (24.0) |
| 20 × 18 | 508.0 (20.0) | 457.0 (18.0) | 508.0 (20.0) |
| 20 × 16 | 508.0 (20.0) | 406.4 (16.0) | 508.0 (20.0) |
| 20 × 14 | 508.0 (20.0) | 355.6 (14.0) | 508.0 (20.0) |
| 20 × 12 | 508.0 (20.0) | 323.8 (12.75) | 508.0 (20.0) |
| 38 × 34 | 965.0 (38.0) | 864.0 (34.0) | 610.0 (24.0) |
| 22 × 20 | 559.0 (22.0) | 508.0 (20.0) | 508.0 (20.0) |
| 22 × 18 | 559.0 (22.0) | 457.0 (18.0) | 508.0 (20.0) |
| 22 × 16 | 559.0 (22.0) | 406.4 (16.0) | 508.0 (20.0) |
| 22 × 14 | 559.0 (22.0) | 355.4 (14.0) | 508.0 (20.0) |
| 24 × 22 | 610.0 (24.0) | 559.0 (22.0) | 508.0 (20.0) |
| 24 × 20 | 610.0 (24.0) | 508.0 (20.0) | 508.0 (20.0) |
| 24 × 18 | 610.0 (24.0) | 457.0 (18.0) | 508.0 (20.0) |
| 24 × 16 | 610.0 (24.0) | 406.4 (16.0) | 508.0 (20.0) |
| 40 × 30 | 1016.0 (40.0) | 762.0 (30.0) | 610.0 (24.0) |
| 26 × 24 | 660.0 (26.0) | 610.0 (24.0) | 610.0 (24.0) |
| 26 × 22 | 660.0 (26.0) | 559.0 (22.0) | 610.0 (24.0) |
| 26 × 20 | 660.0 (26.0) | 508.0 (20.0) | 610.0 (24.0) |
| 26 × 18 | 660.0 (26.0) | 457.0 (18.0) | 610.0 (24.0) |
| 42 × 34 | 1067.0 (42.0) | 864.0 (34.0) | 610.0 (24.0) |
| 28 × 26 | 711.0 (28.0) | 660.0 (26.0) | 610.0 (24.0) |
| 28 × 24 | 711.0 (28.0) | 610.0 (24.0) | 610.0 (24.0) |
| 28 × 20 | 711.0 (28.0) | 508.0 (20.0) | 610.0 (24.0) |
| 28 × 18 | 711.0 (28.0) | 457.0 (18.0) | 610.0 (24.0) |
| 44 × 40 | 1118.0 (44.0) | 1016.0 (40.0) | 610.0 (24.0) |
| 30 × 28 | 762.0 (30.0) | 711.0 (28.0) | 610.0 (24.0) |
| 30 × 26 | 762.0 (30.0) | 660.0 (26.0) | 610.0 (24.0) |
| 30 × 24 | 762.0 (30.0) | 610.0 (24.0) | 610.0 (24.0) |
| 30 × 20 | 762.0 (30.0) | 508.0 (20.0) | 610.0 (24.0) |
| 46 × 42 | 1168.0 (46.0) | 1067.0 (42.0) | 711.0 (28.0) |
| 32 × 30 | 813.0 (32.0) | 762.0 (30.0) | 610.0 (24.0) |
| 32 × 28 | 813.0 (32.0) | 711.0 (28.0) | 610.0 (24.0) |
| 32 × 26 | 813.0 (32.0) | 660.0 (26.0) | 610.0 (24.0) |
| 32 × 24 | 813.0 (32.0) | 610.0 (24.0) | 610.0 (24.0) |
| 48 × 44 | 1219.0 (48.0) | 1118.0 (44.0) | 711.0 (28.0) |
| 34 × 32 | 864.0 (34.0) | 813.0 (32.0) | 610.0 (24.0) |
| 34 × 30 | 864.0 (34.0) | 762.0 (30.0) | 610.0 (24.0) |
| 34 × 26 | 864.0 (34.0) | 660.0 (26.0) | 610.0 (24.0) |
Concentric vs Eccentric
- Concentric reducer: both ends on the same centerline. Used on vertical piping; the flow area steps down evenly.
- Eccentric reducer: one side is flat (level). Used on horizontal pump suction lines to prevent air pockets.
- Both share the same overall length H in Table 6.1-11.
Materials & Temperature Range
A234 WPB carbon steel (-29°C to 425°C), A403 WP304 / WP316 stainless (up to 538°C), A420 WPL6 low-temperature carbon steel. Wall thickness matches the pipe schedule at each end.
Related Standards
- ASME B16.9 — Buttwelding Fittings (parent standard)
- ASME B16.9 Elbows
- ASME B16.9 Tees — Equal & Reducing
- ASME B16.9 Pipe Caps
- ASTM A234 WPB Carbon Steel
- ASTM A420 WPL6 Low-Temp
- Our Fittings Range
FAQ
Concentric or eccentric reducer — which should I use?
Use a concentric reducer on vertical lines; use an eccentric reducer on horizontal pump suction lines with the flat side up to avoid vapor pockets.
What sizes do B16.9 reducers cover?
NPS 3/4×1/2 up to NPS 34×26, 77 size combinations per Table 6.1-11.
What does dimension H mean?
H is the overall end-to-end length of the reducer. It is the same for concentric and eccentric types.
Can you supply reducers in stainless steel?
Yes — A403 WP304, WP304L, WP316 and WP316L, suitable for corrosion-resistant service up to 538°C.
Do reducers have schedule ratings?
Yes — wall thickness matches the mating pipe schedule (STD, XS, Sch 40/80, 10S/40S/80S); specify schedules at each end on your enquiry.
Need This Standard for Your Project?
We manufacture and export flanges, fittings and gaskets to ASME standards. Send your standard, size, material, and quantity — we reply within 24 hours.