What is the purpose of the “service allowance” used in Ductile Iron pipe wall thickness design?
The “service allowance” used in the design of Ductile Iron pipe is a holdover from the old Gray Iron pipe days. During that early period, it was called a “corrosion allowance” to offset any initial corrosion or minor surface imperfections that might occur. With the advent of Ductile Iron pipe and polyethylene encasement for corrosion control, the corrosion allowance was retained for similar general conservatism but renamed as a service allowance. The addition of a 0.08-inch service allowance, which is unique to Ductile Iron pipe, ensures that the actual wall thickness will always exceed the design thickness, thereby providing an additional margin of safety and dependability.
