Circular flanges for pipes, valves and Fittings This Section of BS 1560 has been prepared under the direction of the Piping Systems Components Standards Policy Committee and constitutes a new Section of BS 1560 for Class designated cast iron flanges. For PN designated cast iron flanges, reference should be made to BS 4504-3.2. This Section of BS 1560 specifies requirements for circular grey and malleable cast iron flanges having Class 125 and Class 250 designations and ductile cast iron1) flanges having Class 150 and Class 300 designations. The flanges are in the range of nominal sizes in to 24 inches.