ANSI/ASME Pressure Class
Most flanged valves and fittings for water works service in North America have flanged ends meeting one of four flange “Classes” as defined by the B16 Committees of American Society of Mechanical Engineers (ASME) and accredited by the American National Standards Institute (ANSI). Class 125 and Class 250 flanges and flanged fittings are made from cast iron while Class 150 and Class 300 are ductile iron, steel, or stainless steel. However, the bolting pattern is the same for both Class 125 and 150 so, regardless of material they will bolt together, as is also the case for Class 250 and 300.
Cast iron flanges are usually supplied “flat faced” and should only be bolted to a flat faced mating flange to avoid breaking the cast iron flange when tightening the bolts.
Pressure and Temperature
The ANSI/ASME pressure ratings for a given class and size of flanges and flanged fittings change based on the material from which it is made and/or the temperature at which it operates.
The nominal flange “Class” (e.g., Class 125) generally represents the maximum working pressure at the temperature of saturated steam. However, when used at lower temperatures, the maximum working pressure is higher than the nominal class.
The American Water Works Association
The American Water Works Association (AWWA) has committees charged with addressing the specific needs of the water works industry.
Considering the ANSI/ASME ratings too conservative, through burst testing the AWWA C110 committee determined that flanges and fittings made from high strength cast iron with dimensions conforming to ANSI/ASME B16.1 Class 125 could be rated for a working water pressure of 250 PSI with at least a 3.0 safety factor.
Mistakes sometimes occur because cast or ductile iron flanged fittings may be marked “250” (indicating a 250 PSI pressure rating) but the flange dimensions and bolt pattern conform to ANSI/ASME B16.1 Class 125. These will not bolt to an ANSI/ASME B16.1 Class 250 flange!
Valve Pressure Ratings
ANSI/ASME B16 and AWWA C110 pressure ratings are based on flanges and flanged fittings with standardized geometry, dimensions and wall thickness. However, waterworks valves are available in countless types, shapes, configurations and geometries that vary from one manufacturer to the other.
Although its connecting flanges may conform to ANSI/ASME dimensions, the valve’s pressure rating may be different than the “class” of its flanges. It is the responsibility of the valve manufacturer to establish the pressure ratings for their products. The design professional should consult the manufacturer’s published information.
The pressure ratings of many flanged valves used in waterworks service are defined in AWWA standards applicable to that type of valve. The Pressure Class (or rated pressure) listed in the AWWA valve standard may differ from its flange pressure rating, leading to even more confusion.
One should remember that “a chain is only as strong as its weakest link” so every component of a piping system should be able to withstand the highest pressure that could be realized at the location where it is installed.
GA Industries will be happy to provide assistance on any application involving our valves.