What is the difference between pressure class and pressure rating?

AWWA C900 has a "Pressure Class" design approach based on a 2.5 safety factor. Pressure Class is formed using a 2.5 safety factor versus the 2.0 typically used in design. In addition to the elevated safety factor, a surge allowance equal to surge pressure created by instantaneously stopping a column of water traveling 2 feet per second in the system has been allowed for in each pressure class. The motivation for this design approach is to supply a piping product that is intended for use inside the "looped" perimeter of an urban water system where piping system geometry is complex.

AWWA C905 has a "Pressure Rating" design approach based on the 2.0 safety factor. AWWA C905 is intended for use as water transmission piping where long straight runs are the norm and system geometry is more simplistic. Surge pressures are easily predictable and should be accounted for in design.

ASTM D2241 has a "Pressure Rating" design approach based on the 2.0 safety factor. Once again, surge calculations are the designer's responsibility.

ASTM D1785 schedule pipe (40, 80 & 120) does not conform to the same design approach as the above-mentioned products. The products listed above offer a pressure capacity independent of pipe size, whereas the schedule product pressure ratings vary between different pipe diameters.