Ductile iron pipe is measured by the nominal thickness of the wall of the pipe. In order to know which size to order for a particular application, the manufacturer assigns the pipes to pressure classes. A pressure class is determined by calculating the rated working water pressure of the pipe and is expressed as psi (pounds per square inch). There are five pressure classes ranging from 150 to 350 psi; these classes span pipe diameter sizes from 3 to 64 inches wide. The pressure class and pipe diameter size are used in conjunction to calculate the nominal thickness in inches of the wall of the pipe.
Pressure Class 150
Pressure class 150 is used for ductile iron piping with the largest diameters, ranging from 30 to 64 inches. The diameter range varies by 6 inches for each incremental pipe size. The lowest nominal thicknesses for pressure class 150 piping ranges from .34 inches for a 30-inch pipe to .54 inches for a 64-inch pipe.
Pressure Class 200
Pressure class 200 is used for piping with diameters ranging from 24 to 64 inches. The pipe diameter range again varies by 6 inches for each incremental pipe size. The lowest nominal thickness for pressure class 200 piping ranges from .33 inches for a 24-inch pipe to .56 inches for a 64-inch pipe.
Pressure Class 250
This pressure class is used for piping with a wider range of diameters, from 14 to 64 inches. The 14- to 24-inch sizes range by 2 inches for each incremental pipe size; the range changes to 6 inches for 30- to 64-inch pipes. The lowest nominal thickness for pressure class 250 piping ranges from .28 inches for a 14-inch pipe to .72 inches for a 64-inch pipe.
Pressure Class 300
Pressure class 300 is used for piping with the same range of diameters as the 250 class; the range change increments are also the same. The difference is that the lowest nominal thickness ranges from .30 inches for a 14-inch pipe to .80 inches for a 64 -inch pipe.
Pressure Class 350
This final pressure class has the widest range of of diameters and includes the smallest piping at 3 inches to the largest piping at 64 inches in diameter. The range change increments are 1 inch when the pipe sizes are 3-4 inches, 2 inches when the pipe sizes are 4-24 inches and finally 9 when the sizes are 30-64 inches. The lowest nominal thickness for pressure class 350 piping ranges from .25 inches for sizes 3- to 8-inch pipes all the way to .87 inches for a 64-inch pipe.
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