ANSI B16.1 Cast Iron Flanges
Class 125 Lightweight Flanges
Flat face cast iron flanges for low-pressure water and steam
ANSI B16.1 Class 125 Lightweight flanges are flat face cast iron flanges designed for low-pressure service in water distribution, HVAC, and general utility piping. These flanges use a full flat face gasket that spans across the entire flange surface, providing a reliable seal for pressures up to 150 psi in non-shock cold water service and 75 psi for steam or gas applications.
The lightweight designation reflects a thinner flange body compared to standard Class 125 flanges, reducing both material cost and weight while maintaining adequate strength for the intended pressure and temperature ratings. Sizes range from NPS 1 through NPS 96, with bolt circle diameters and bolt hole counts increasing proportionally.
These flanges are frequently paired with ductile iron or gray iron pipe fittings in municipal water systems, cooling towers, and fire protection lines. When bolting Class 125 flat face cast iron flanges to raised face steel flanges, the raised face must be removed to prevent uneven loading that could crack the brittle cast iron body.
