The nominal size of an ASME flange = Nominal Pipe Size (NPS) of the matching pipe.

An NPS 6 flange is designed to connect to an NPS 6 pipe.

NPS = Nominal Pipe Size (inch designation); DN = Diameter Nominal (metric equivalent).

Important note: NPS is a nominal designation, NOT the actual outer diameter or inner diameter of pipe/flange.

1. Basic Matching for Common Flange Types (WN, SO, SW, Threaded)

Weld Neck Flange (WN)

NPS of flange = NPS of pipe.

The hub bore is normally furnished for Schedule 40 pipe as standard.

If you use Schedule 80 / 160 heavy-wall pipe, you must order a special machined bore to match pipe ID, otherwise an internal step will form.

Slip-On Flange (SO)

Flange NPS = Pipe NPS.

Flange inner clearance fits the pipe outer diameter (OD) defined in ASME B36.10M / B36.19M.

Socket Weld Flange (SW)

Flange NPS = Pipe NPS.

Socket cavity matches pipe OD. Normally used ≤ NPS 4.

Threaded Flange (TH)

Flange NPS = Pipe NPS; thread size is NPT matching the same NPS.

Blind Flange (BL)

Blind flange NPS corresponds to the NPS of the piping opening it seals off. No pipe is welded to it.

2. Special Case: Lap Joint Flange (LJ Flange + Stub End)

This is the most frequently misunderstood combination:

LJ flange NPS = Stub End NPS = Pipe NPS

The LJ flange itself has a large inner hole; it does not grip the pipe.

All sealing load transfers to the flared face of the stub end (ASME B16.9 / MSS SP-43), which is butt-welded directly to the pipe.

Example: NPS 4 LJ flange works only with NPS 4 stub end + NPS 4 pipe.

3. Standard Scope Boundary

NPS ½ ~ NPS 24: ASME B16.5 flanges

NPS 26 ~ NPS 60: ASME B16.47 large diameter flanges (Series A / B)

You cannot mix B16.5 NPS24 and B16.47 NPS26 — bolt patterns are completely different.

4. Reducing Flange (Exception to 1:1 matching)

A reducing flange has two different nominal sizes:

Example: NPS 6 × NPS 4 reducing weld neck flange

Large side: NPS6 (matches NPS6 piping / equipment nozzle)

Small side: NPS4 (welds to NPS4 pipe)

Marking always lists both sizes.

5. Critical Distinction: NPS vs Actual Pipe OD

From ASME B36.10M seamless/welded steel pipe:

NPS ≤ 12: NPS number ≠ pipe OD

NPS 2 pipe OD = 2.375 inch

NPS ≥14: Pipe OD equals the NPS value

NPS14 pipe OD =14.000 inch

Flange sizing is always governed by NPS, not measured OD. Do not select flange size by caliper measurement alone; confirm NPS first.

Practical Ordering Examples

NPS 6 Class300 WN RF Flange ASME B16.5 → For NPS6 pipe

NPS 4 Class150 LJ Flange + NPS4 Stub End → For NPS4 pipe

NPS12 × NPS8 Reducing WN Flange → Connect NPS12 nozzle to NPS8 pipe

Quick Summary

✅ Normal connection: Flange NPS = Pipe NPS

⚠️ Heavy-wall pipe: Specify custom bore on WN flanges

⚠️ Lap joint system: Match flange NPS to stub end & pipe NPS

⚠️ Reducing flange: Two different NPS values on one flange

❌ Never select flange size based only on measured pipe outer diameter — always use NPS.