Correct matching between flange dimensions and pipes is the foundation of tight connection and safe operation of pipeline systems. Mismatch will lead to assembly difficulty, poor sealing and pipeline leakage. Below is the step-by-step matching guide.

1. Confirm the same standard first

Flange and pipe must follow a unified standard system.

ASME system: ASME B16.5 flanges match ASME B36.10 / B36.19 pipes

EN system: EN 1092-1 flanges match EN 10216 / EN 10217 pipes

GB system: GB/T 9112~9124 flanges match GB/T 8163, GB/T 9711 pipes

JIS system: JIS B2220 flanges match JIS G3454, JIS G3455 pipes

Important note: Flanges from different standards cannot be matched randomly, even if DN/NPS and pressure grade look the same.

2. Match nominal size (DN / NPS)

Nominal size is the core code for flange-pipe matching.

NPS (ASME): NPS 2 flange fits NPS 2 pipe

DN (EN / GB / JIS): DN50 flange fits DN50 pipe

 DN/NPS is a nominal code, not the actual outer diameter. The pipe outer diameter is fixed under each standard. The flange bore must adapt to the pipe’s outer diameter.

For example: DN50 pipe in EN system and NPS2 pipe in ASME system have different outer diameters. A DN50 EN flange cannot fit an NPS2 ASME pipe directly.

3. Check pipe outer diameter & flange bore

Weld Neck (WN) / Slip On (SO) flanges: Flange inner bore shall match pipe outer diameter.

Plate flat flanges (PL): Bore size must suit pipe OD for welding.

Socket weld (SW) flange: Socket inside diameter matches pipe outer diameter.

When ordering, clearly specify pipe schedule (wall thickness):

ASME pipe: Sch40, Sch80, Sch160

EN/GB pipe: Series 1 / Series 2

Schedule affects pipe wall thickness. It will influence welding fit-up and insertion depth for slip-on flanges.

4. Verify pressure class consistency

Flange pressure rating must meet or exceed the pipe design pressure at operating temperature.

The pipe and flange should adopt matching pressure class in the same standard.

Example: PN16 flange with PN16 rated pipe; Class150 flange with Class150 piping components.

5. Check flange type & welding form

WN flange: Butt-welded to pipe. Pipe end bevel needs to be processed for butt welding.

SO slip-on flange: Pipe passes through the flange bore, fillet weld on both sides. Pipe outer diameter must fit the flange bore with small clearance.

Threaded flange: Matches threaded pipe, no welding required.

6. Sealing face and gasket matching

After matching flange and pipe dimensions, the sealing face must match the gasket type.

RF (Raised Face) matches standard RF gasket; RTJ flange uses ring joint gasket for high-pressure service.