Standards Reference
ANSI B16.1: Cast Iron Pipe Flanges
ANSI B16.1 covers cast iron pipe flanges and flanged fittings in Class 25, 125, and 250. It might look like a niche legacy standard until you realize the entire lightweight steel flange family (Class 125LW) was designed to bolt up to it. This is the spec that keeps decades of municipal and HVAC piping serviceable.
What ANSI B16.1 Covers
B16.1 sets dimensions, tolerance, drilling, and pressure-temperature ratings for gray iron flanges and flanged fittings. The standard is organized into three classes that reflect the pressure regime the flange is designed for.
CLASS 25
Light-Duty Cast Iron
25 psi maximum non-shock working pressure. Largely obsolete on new installations, but you will run into it on older municipal and industrial water systems.
CLASS 125
Standard Cast Iron
The most common cast iron class. 175 psi cold working pressure in smaller sizes, derating with diameter. This is the bolt pattern that Class 125LW steel flanges were designed to match.
CLASS 250
Heavy Cast Iron
400 psi cold working pressure in smaller sizes. Heavier cross-section than Class 125, used where higher pressure or shock loading is expected.
Why ANSI B16.1 Still Matters
Cast iron flanges are not going to win any modern process piping competitions. So why does this standard show up so often on quotes? Three reasons.
Retrofit Compatibility
Municipal and industrial water systems built decades ago used Class 125 cast iron flanges as their default. Replacing a fitting, valve, or section of pipe means matching that bolt pattern. Class 125LW steel flanges exist specifically for this.
HVAC Service
Chilled water, condenser water, and low-pressure steam systems in commercial and institutional buildings still spec to B16.1 drilling. Pumps, valves, and strainers often ship with B16.1 flanged ports.
Low-Pressure Distribution
Plant water mains, fire protection headers below 175 psi, and older process water loops continue to be specified in B16.1 drilling so the parts ecosystem stays interchangeable.
The Class 125LW Connection
Here is the key piece of the puzzle. Class 125LW (Lightweight) steel flanges are forged or plate-cut steel flanges built with the same outside diameter, bolt circle, and bolt hole count as ANSI B16.1 Class 125 cast iron flanges. They are lighter in cross-section than a comparable B16.5 Class 150 steel flange, which means they cost less and weigh less, and they bolt directly to existing B16.1 Class 125 cast iron fittings without an adapter.
Class 125LW flanges are not B16.5 flanges. They are not B16.1 flanges either. They are typically produced to AWWA C207 material specifications and used heavily in waterworks, fire protection, and HVAC service where the pressure rating fits and the cost savings make sense.
When a spec calls for a steel flange that bolts to a cast iron valve or pump, the answer is almost always a Class 125LW slip-on, blind, or weld neck flange.
Materials and Typical Applications
Cast Iron Grades
Gray iron per ASTM A126 Class A, B, and C is the historical material for B16.1 flanges. Class B is the most common. Ductile iron (ASTM A536) flanges exist for higher impact resistance but fall under separate dimensional standards in most cases.
Cast iron is brittle. This is why flat-face flanges (FF) are used when mating steel to cast iron - a raised face concentrates load and can crack the cast iron flange.
Where You See B16.1
- Municipal water distribution (low pressure)
- Wastewater treatment plant headers
- HVAC chilled and condenser water
- Low-pressure steam and condensate
- Fire protection mains (older systems)
- Plant utility water and any application using cast iron pumps, valves, or strainers
Need Class 125 or 125LW Flanges?
We stock Class 125LW steel flanges 4 inch through 144 inch in slip-on, weld neck, and blind configurations, and we source cast iron flanges when the spec calls for the real thing. Tell us the mating part and we will match the bolt pattern.