Overview
ASME B16.1 covers the dimensional and pressure-temperature rating requirements for gray cast iron pipe flanges and flanged fittings. It is predominantly used in low-pressure, ambient-temperature services like municipal water, wastewater, and HVAC systems.
Scope & Applicability
Applies To Industries
Covers — cast iron flanges:
- Sizes from NPS 1 to NPS 96 (depending on class).
- Pressure classes: 25, 125, and 250.
- Mostly Flat Face (FF) designs.
Does NOT cover (exclusions) — the following are governed elsewhere:
- Steel flanges → ASME B16.5.
- High-pressure process service (cast iron is brittle and prohibited in hazardous hydrocarbon service by codes like B31.3).
Key Requirements
B16.1 deals with the brittle nature of cast iron:
- Facing — Class 125 flanges are strictly Flat Face (FF) and must be mated with full-face gaskets. If bolted to a Raised Face (RF) steel flange, the bending moment induced when tightening the bolts will fracture the cast iron flange.
- Bolting — restricts the yield strength of the bolting material (e.g., carbon steel A307 grade B) to prevent overtightening and cracking the flange.
Technical Details
The dimensions of B16.1 Class 125 cast iron flanges are identical to B16.5 Class 150 steel flanges, allowing them to bolt together. However, strict engineering precautions (Flat Face, soft gaskets, low-strength bolting) must be followed when mating cast iron to steel.
Engineering Notes
If a pump has a cast iron Class 125 flanged nozzle, you must ensure the connecting steel pipe flange is machined to a Flat Face (FF). Mating an RF steel flange to an FF cast iron pump nozzle is a classic cause of pump casing failure during installation.
Source, Edition & Status
- Publisher
- ASME
- Industries
- Waterworks, HVAC, General Industry
- Last Reviewed
- 2026-07-01
- Last Updated
- 2026-07-01
- Verified By
- Engineering content reviewer
- Content Status
- reviewed