In flange technical communication, “diameter” is ambiguous — there are multiple diameters on one flange. You must distinguish them clearly when quoting or drawing.

Take ASME B16.5 flange as standard.

1. Nominal Diameter (the most important one)

Name: NPS (Nominal Pipe Size) / DN

✅ This is what people usually mean when simply saying “flange size / flange diameter”.

Not the actual measured dimension of the flange plate.

It defines which pipe, fittings, valve nozzles the flange matches.

Example: NPS 6 flange = flange for NPS 6 pipeline.

Customer sentence example: I need a 6-inch flange

→ Normally refers to NPS 6, not outer diameter.

2. Outer Diameter of Flange (OD / Outside Diameter)

Symbol: OD

Overall diameter of the circular flange plate.

Same NPS but different Class will have different OD.

Example:

NPS6 Class150 WN RF OD ≠ NPS6 Class300 WN RF OD.

Used for installation layout, space checking.

3. Bolt Circle Diameter (BCD / PCD)

Pitch Circle Diameter

The circle passing through the center of all bolt holes.

Critical for flange bolting interchangeability.

Two flanges can connect only if their PCD, bolt hole quantity & bolt hole diameter match.

4. Bore Diameter

WN flange: Hub bore ID (matches pipe inner diameter)

SO flange: Inner hole diameter (pipe passes through here)

Blind flange: No bore.

5. Raised Face Diameter (RF Diameter)

For RF flange: The diameter of the raised sealing face.

Relevant for gasket selection.

Quick Summary for business communication

When a client asks for flange diameter without extra description:

👉 First confirm: Do you mean NPS nominal size or flange outer diameter?

Standard expression habit:

Flange specification: described by NPS + Class

Outer diameter, PCD, bore belong to dimensional drawing data.

Simple English version for sending to customers

There are several diameters on a flange:

NPS (Nominal Pipe Size): Nominal size of the flange, determines matching pipe size. This is the main size we use for ordering.

Flange Outer Diameter (OD): Overall diameter of the flange disc.

PCD (Bolt Circle Diameter): The diameter of the circle where bolt holes locate.

Bore diameter: Inner hole size of the flange.