Flanges · ASME B16.5
Reducing Flange — ASME B16.5
Also written RF
Reducing flange — a flange with a smaller bore than its outside dimensions, connecting to a smaller pipe. A reducing flange changes pipe size at a flanged joint without a separate reducer, saving space and a weld. Use it where the flow disturbance from a sudden change is acceptable.
- ASME B16.5
- 23 grades
- EN 10204 3.1 / 3.2
Reviewed by the Kanak Steel & Engineering Co. technical team · Updated October 2026
Reducing Flange: What to Know Before Ordering
- Outside dimensions of the larger size
- Bore / hub for the smaller pipe
- Weld neck, slip-on or threaded styles
Reducing Flange Materials & Grades
| Grade | Material standard | Best for |
|---|---|---|
| Inconel 825 reducing flange | ASTM B564 | Sulphuric and phosphoric acid service, chloride stress-corrosion cracking and sour gas |
| SS 904L reducing flange | ASTM A182 F904L | Dilute sulphuric acid, phosphoric acid and chloride-bearing cooling water |
| SS 304 reducing flange | ASTM A182 F304 | General corrosion resistance, food, water and architectural work |
| SS 304L reducing flange | ASTM A182 F304L | Welded tanks, piping and vessels in general corrosive service |
| SS 310 / 310S reducing flange | ASTM A182 F310 | Furnaces, burners and heat treatment at high temperature |
| SS 316 / 316L reducing flange | ASTM A182 F316L | Chloride-bearing process fluids, marine air and pharmaceutical service |
| SS 321 reducing flange | ASTM A182 F321 | High-temperature service between 425 and 815 °C |
| SS 347 reducing flange | ASTM A182 F347 | Welded high-temperature piping in refineries and power plants |
| Duplex 2205 reducing flange | ASTM A182 F51 / F60 | Strength with chloride resistance at a stainless-steel budget |
| Super Duplex 2507 reducing flange | ASTM A182 F53 | Seawater and high-chloride service where strength also matters |
| Inconel 600 reducing flange | ASTM B564 | High-temperature oxidation, caustic and nuclear service |
| Inconel 625 reducing flange | ASTM B564 | Seawater, high strength and high temperature in one alloy |
| Inconel 718 reducing flange | ASTM B637 | Very high strength at temperatures up to about 650 °C |
| Incoloy 800 / 800H / 800HT reducing flange | ASTM B564 | Furnaces, petrochemical heaters and high-temperature piping |
| Hastelloy C276 reducing flange | ASTM B564 | The most aggressive mixed acids, wet chlorine and hypochlorite |
| Hastelloy C22 reducing flange | ASTM B564 | Mixed oxidising and reducing acids, and weld overlays |
| Monel 400 reducing flange | ASTM B564 | Seawater, hydrofluoric acid and marine service |
| Monel K500 reducing flange | ASTM B865 | High-strength pump shafts, bolting and springs in seawater |
| Super Duplex S32760 reducing flange | ASTM A182 F55 | Seawater firewater, injection and offshore piping |
| ASTM A106 Gr B reducing flange | ASTM A105 | High-temperature process and steam piping in refineries and power plants |
| ASTM A333 Gr 6 reducing flange | ASTM A350 LF2 | Cold-climate and refrigerated piping down to −45 °C |
| ASTM A335 P11 reducing flange | ASTM A182 F11 | Steam and hot-oil piping up to about 550 °C |
| ASTM A335 P22 reducing flange | ASTM A182 F22 | High-pressure steam and hydrogen service up to about 580 °C |
Other flanges
Weld neck flangeA flange with a long tapered hub that is butt-welded to the pipe.Slip-on flangeA flange that slides over the pipe and is fillet-welded inside and outside.Blind flangeA solid disc that closes the end of a pipe, valve or vessel nozzle.Socket weld flangeA flange with a counterbore socket into which the pipe is inserted and fillet-welded.Threaded flangeA flange with a tapered internal thread that screws onto threaded pipe.Lap joint flangeA loose flange that sits behind a stub end welded to the pipe.Long weld neck flangeA weld neck flange with an extended straight neck, used as a vessel nozzle.Orifice flangeA pair of weld neck or slip-on flanges with pressure taps that hold an orifice plate.Spectacle blindA figure-8 plate with one solid end and one open ring, fitted between two flanges.Ring type joint flangeA flange with a machined groove for a metal ring gasket.Expander flangeA weld neck flange whose hub increases the pipe size.Anchor flangeA forged ring welded into a pipeline to fix it to a structure.Swivel ring flangeA flange with a rotating ring that allows bolt holes to be aligned after welding.
Reducing Flange dimensions — ASME B16.5 Class 150 & 300
Outside diameter, thickness and bolting are set by size and class. Values in mm; thickness excludes the 1.6 mm raised face.
| NPS | Cl 150 OD | Cl 150 thickness | Cl 150 bolt circle | Cl 150 bolts | Cl 300 OD | Cl 300 thickness | Cl 300 bolt circle | Cl 300 bolts |
|---|---|---|---|---|---|---|---|---|
| ½" | 90 | 9.6 | 60.3 | 4 × ½" | 95 | 12.7 | 66.7 | 4 × ½" |
| 1" | 110 | 12.7 | 79.4 | 4 × ½" | 125 | 15.9 | 88.9 | 4 × ⅝" |
| 1½" | 125 | 15.9 | 98.4 | 4 × ½" | 155 | 19.1 | 114.3 | 4 × ¾" |
| 2" | 150 | 17.5 | 120.7 | 4 × ⅝" | 165 | 20.7 | 127.0 | 8 × ⅝" |
| 3" | 190 | 22.3 | 152.4 | 4 × ⅝" | 210 | 27.0 | 168.3 | 8 × ¾" |
| 4" | 230 | 22.3 | 190.5 | 8 × ⅝" | 255 | 30.2 | 200.0 | 8 × ¾" |
| 6" | 280 | 23.9 | 241.3 | 8 × ¾" | 320 | 35.0 | 269.9 | 12 × ¾" |
| 8" | 345 | 27.0 | 298.5 | 8 × ¾" | 380 | 39.7 | 330.2 | 12 × ⅞" |
| 10" | 405 | 28.6 | 362.0 | 12 × ⅞" | 445 | 46.1 | 387.4 | 16 × 1" |
| 12" | 485 | 30.2 | 431.8 | 12 × ⅞" | 520 | 49.3 | 450.8 | 16 × 1⅛" |
Full chart: ASME B16.5 flange dimensions.
Reducing flange — questions buyers ask
Is a reducing flange better than a reducer plus flange?
It saves length and a weld, but a buttweld reducer gives smoother flow. Choose by space and process needs.
Which grades are reducing flanges available in?
Inconel 825, SS 904L, SS 304, SS 304L, SS 310 / 310S, SS 316 / 316L, SS 321, SS 347, Duplex 2205, Super Duplex 2507, Inconel 600, Inconel 625, Inconel 718, Incoloy 800 / 800H / 800HT, Hastelloy C276, Hastelloy C22, Monel 400, Monel K500, Super Duplex S32760, ASTM A106 Gr B, ASTM A333 Gr 6, ASTM A335 P11, ASTM A335 P22.
Which standard covers reducing flange dimensions?
ASME B16.5. The material standard depends on the grade.
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