Choosing a Flange

Quick pick (TL;DR)

Pick the class from design P AND T (B16.5 rating tables).
Weld-neck + RF + spiral-wound gasket covers most process service up to Class 600.
Move to RTJ at Class 900+ or for hydrogen/high-temperature.
Use FF only when mating cast-iron / FRP equipment.

Class comes from the P–T rating

A class's allowable pressure falls with temperature — pick from P and T together (B16.5).

Example — why temperature matters

CS flange (illustrative) Class 150 @ 38°C ≈ 19.6 barg allowable Class 150 @ 200°C ≈ 13.8 barg allowable Class 150 @ 400°C ≈ 6.5 barg allowable Lesson 16 barg @ 200°C → Class 150 not enough → select Class 300

Facing & type

Facing	Use
Raised Face (RF)	Default to Class 600.
Ring-Type Joint (RTJ)	Class 900+ / critical.
Flat Face (FF)	Cast-iron / FRP.

Type	Use
Weld-neck	High/critical service.
Slip-on	Lower pressure, easy fit-up.
Blind	Close a line/nozzle.

How to choose

Find the class from P and T.
Match connected equipment.
RF to Cl600; RTJ Cl900+.
WN for severe; SO for utility.
Match gasket and bolting.

Common mistakes

Ignoring temperature derating.
RF steel to FF cast iron → cracking.
Mismatched facing/gasket or bore.

Requisition & datasheet

Size, class, type, facing, bore, material.
Gasket/bolting; standard.

Frequently asked questions

What should I know about Choosing a Flange?

Pick the class from design P AND T (B16.5 rating tables). Weld-neck + RF + spiral-wound gasket covers most process service up to Class 600. Move to RTJ at Class 900+ or for hydrogen/high-temperature. Use FF only when mating cast-iron / FRP equipment.

Which standards or codes apply?

ASME B16.5 (NPS ≤ 24) and B16.47 (larger); pressure–temperature rating tables by material group; B16.20/B16.21 gaskets.

Standards — ASME B16.5 / B16.47; P–T tables; B16.20/21.