What Is an Expander Flange? Size Increase

An expander flange transitions from a smaller pipe to a larger flanged connection in a single component. The bore matches the smaller pipe, while the flange OD, bolt circle, and bolt holes match the larger NPS. It is functionally the reverse of a reducing flange—the pipe is smaller than the flange rather than larger.

Key Specifications

Property	Detail
Standard	ASME B16.5 (custom bore in standard flange body)
Size range	Combinations within NPS 1/2-24 per ASME B16.5
Pressure classes	150 through 2500
Common types	Weld neck expander (most common)
Materials	A105 (CS), A182 F304/F316 (SS), A350 LF2 (low-temp)
Face types	RF, FF, RTJ
Rating basis	Pressure class of the larger flange OD

Common Mistake

An expander flange uses the bolt pattern and gasket of the larger NPS. However, the pressure rating is governed by ASME B16.5 for the larger flange size—not the smaller pipe. Verify that the mating flange, gasket, and stud bolts all match the larger NPS.

Where Expander Flanges Are Used

Application	Example
Equipment nozzles	NPS 4 nozzle connecting to NPS 6 header flange
Pump connections	NPS 3 pump discharge to NPS 4 process flange
Instrument taps	NPS 2 instrument line to NPS 3 flanged connection
Valve-to-pipe transitions	Smaller valve bolted to larger pipe system
Future expansion	Install larger flange now for future upsizing

Expander Flange vs Reducing Flange

Feature	Expander Flange	Reducing Flange
Pipe size	Smaller than flange	Larger than flange bore
Bolt pattern	Matches larger NPS	Matches larger NPS
Flow direction	Small pipe to large flange	Large flange to small bore
Hub taper	Expands from bore to flange face	Contracts from flange OD to bore
Typical use	Upsizing at equipment connections	Downsizing at branch/equipment

Pro Tip

Expander flanges are less commonly stocked than reducing flanges. Lead times can reach 8-12 weeks for non-standard size combinations. Where project schedule is tight, a concentric reducer welded to a standard flange achieves the same result with shorter delivery.

Ordering Specification

Specify expander flanges with the smaller size first: “NPS 4 x 6 WN RF Class 300 A105 expander flange.” This means NPS 4 bore expanding to NPS 6 flange dimensions. The gasket, bolting, and mating flange all correspond to NPS 6.

Read the full guide to flanges