ASME B16.5

Covers — dimensional and rating requirements for steel/alloy flanges:

• Sizes from NPS 1/2 through NPS 24.
• Pressure classes: 150, 300, 400, 600, 900, 1500, and 2500.
• Flange types: Weld Neck (WN), Slip-On (SO), Blind (BL), Threaded (THD), Socket Weld (SW), and Lap Joint (LJ).
• Flange faces: Raised Face (RF), Flat Face (FF), Ring-Type Joint (RTJ).

Does NOT cover (exclusions) — the following are governed elsewhere:

• Flanges larger than NPS 24 → ASME B16.47.
• Cast iron flanges → ASME B16.1.
• Copper alloy flanges → ASME B16.24.
• Orifice flanges for flow measurement → ASME B16.36.

B16.5 sets the rigid geometric framework for bolted joints:

• Dimensions — standardizes bolt circle diameter, number of bolts, bolt hole diameter, flange thickness, and hub dimensions, locking in the geometry for each NPS and Class combination.
• Pressure-Temperature Ratings — assigns maximum allowable working pressures at specific temperatures based on the material group.
• Materials — restricts flange materials to specific ASTM specifications (e.g., A105, A182) grouped into tables.

The heart of B16.5 is its Pressure-Temperature (P-T) rating tables.

P-T Ratings Explained

A "Class 150" flange does not mean it is rated for 150 psi. For a standard carbon steel flange (Material Group 1.1, e.g., ASTM A105), a Class 150 flange can withstand 285 psi at ambient temperature. As temperature increases, the allowable pressure drops. At 400°F (204°C), the rating drops to 200 psi.

Flange Types & Application

• Weld Neck: Tapered hub transitions stress to the pipe. Best for severe service, high pressure, and cyclic loading.
• Slip-On: Slides over the pipe, welded inside and out. Cheaper and easier to align, but weaker in fatigue than Weld Neck. Not recommended for severe service.
• Lap Joint: Used with a stub end; the flange spins freely. Ideal for systems needing frequent dismantling or where the pipe is an expensive alloy but the flange can be cheap carbon steel.

Where to find it in the code: P-T Ratings → Table 2 · Dimensions → Tables 8 through 22.