What Is ASME B16.5?

Quick Answer

ASME B16.5 is the standard for pipe flanges and flanged fittings in sizes NPS 1/2 through NPS 24, for pressure classes 150 through 2500. It defines flange dimensions, tolerances, drilling templates, facing requirements, pressure-temperature ratings, and material specifications. Nearly every flanged connection in North American piping relies on this standard.

What ASME B16.5 Covers

Section	Content
Scope	Flanges NPS 1/2-24, Classes 150-2500
Pressure-temperature ratings	Tables for 35 material groups (e.g., Group 1.1 for A105)
Flange types	WN, SO, blind, SW, threaded, lap joint
Facing types	RF, FF, RTJ, tongue & groove, male & female
Bolt patterns	Number, size, and bolt-circle diameter per NPS and class
Tolerances	OD, thickness, bore, bolt hole spacing
Materials	References ASTM specs (A105, A182, A350, etc.)
Marking	Required stamping: manufacturer, material, class, size

Pressure-Temperature Rating System

The core of ASME B16.5 is its pressure-temperature (P/T) rating tables. These tables list the maximum allowable working pressure at specific temperatures for each material group:

Material Group	Typical Material	Class 150 @ 100degF	Class 300 @ 100degF
1.1	A105, A216 WCB	285 psig	740 psig
1.5	A182 F304	275 psig	720 psig
1.7	A182 F316	275 psig	720 psig
1.9	A182 F304L	230 psig	600 psig
1.10	A182 F316L	230 psig	600 psig
3.2	A182 F11	290 psig	750 psig

Excerpt from ASME B16.5-2020 P/T tables. Always refer to the full table for your specific material and temperature.

How to Read the Standard

To find the dimensions of a specific flange:

1. Identify the flange type (WN, SO, blind, etc.)
2. Go to the relevant dimension table (e.g., Table 18 for WN flanges)
3. Find your NPS on the left column
4. Find your pressure class across the top
5. Read off: OD, thickness, bore, bolt circle, bolt size, number of bolts

Pro Tip

ASME B16.5 dimensions are in both metric and imperial. The inch dimensions are the master; metric values are soft conversions. If there is a discrepancy, the inch values govern.

Related Standards

Standard	Scope
ASME B16.5	NPS 1/2-24, Classes 150-2500
ASME B16.47	NPS 26-60, Classes 75-900
ASME B16.36	Orifice flanges
ASME B16.20	Metallic gaskets (spiral wound, ring joint)
ASME B16.34	Valves (references B16.5 P/T ratings)
EN 1092-1	European flange standard (not interchangeable)

ASME B16.5 is referenced by piping codes ASME B31.1 (power piping) and ASME B31.3 (process piping) for all flanged connections within its size range.

Read the full guide to flanges