Although there are differing opinions on this subject, a conservative maximum velocity for design purposes is 7 fps (feet per second).

The AWWA (American Water Works Association) standard for thickness design of ductile iron pipe is C150. The exercise for calculating the required thickness based on internal pressure includes a 100 psi allowance for surge pressure and a 2:1 safety factor. The surge pressure allowance is based on a 50 psi pressure rise for each foot of extinguished velocity, and the fact that most domestic water systems operate at approximately 2 fps.

Ductile Iron pipe may be rated as high as 350 psi service. A pipeline operating at 7 fps velocity could account for a 350 psi pressure surge (7fps X 50 psi/fps). Adding a potential surge pressure equal to the pressure rating of the pipe encroaches significantly on the safety factor. Exceeding 7 fps velocity could produce potentially damaging surge pressure.