What Is a Reducing Flange?

A reducing flange combines a size transition and a flanged connection into one component. The outer dimensions (OD, bolt circle, bolt holes) match the larger pipe size, while the bore matches the smaller pipe size. It eliminates the need for a separate pipe reducer welded to a standard flange, saving one weld joint, reducing face-to-face length, and lowering fabrication cost.

Key Specifications

Property	Detail
Standard	ASME B16.5 (not a separate standard—uses larger flange dimensions with smaller bore)
Size range	Any combination within ASME B16.5 NPS 1/2-24
Pressure classes	150 through 2500
Common types	Weld neck reducing, slip-on reducing, threaded reducing
Materials	A105 (CS), A182 F304/F316 (SS), A350 LF2 (low-temp)
Face types	RF, FF, RTJ
Mating flange	Standard flange of the larger NPS

Typical Size Combinations

Reducing Flange	Large End (Bolt Pattern)	Small End (Bore)
6 x 4	NPS 6	NPS 4
8 x 6	NPS 8	NPS 6
10 x 8	NPS 10	NPS 8
12 x 10	NPS 12	NPS 10
4 x 3	NPS 4	NPS 3

Common Mistake

Reducing flanges are practical for one or two size steps down (e.g., NPS 8 x 6 or NPS 6 x 4). For larger reductions (e.g., NPS 12 x 6), use a concentric or eccentric reducer welded to a standard flange. Excessive reduction in a single flange creates high turbulence and flow restriction.

Reducing Flange vs Reducer + Standard Flange

Feature	Reducing Flange	Reducer + Standard Flange
Weld joints	1 (butt weld to smaller pipe)	2 (reducer to pipe + flange to reducer)
Face-to-face length	Shorter	Longer (reducer adds length)
Fabrication cost	Lower	Higher
Flow transition	Abrupt (step change at bore)	Gradual (tapered reducer)
Availability	Less common in stock	Readily available separately
Stress analysis	Acceptable for most services	Better for high-cycle fatigue

Pro Tip

When ordering reducing flanges, specify both the large and small NPS: “NPS 8 x 6 WN RF Class 300 A105 reducing flange.” The bolt pattern, gasket, and stud bolts all correspond to the larger NPS. The mating flange is a standard NPS 8 flange.

Reducing flanges are commonly used at equipment nozzles, instrument connections, and branch headers where space is limited and eliminating a weld joint improves reliability.

Read the full guide to flanges