Awarded as the best online publication by CIDC

# What does the pipe thickness of ductile iron pipes generally larger than that of steel pipes?

Both steel pipes and ductile iron pipes use hoop stress equation to model internal pressure design. The difference in pipe thickness arises as a result of more conservative approach in DI pipes.

For ductile iron pipes, surge pressure is considered as part of design pressure and they are added together before applying a safety factor of 2 as follows:

t=[F(P+S)D]/2Y
where t = Pipe thickness
F = Factor of Safety of 2
P = Working pressure
S = Surge pressure
Y = Yield strength of ductile iron

For steel pipes the design of working pressure is based on 50% of steel yield strength (i.e. a factor of safety of 2). The presence of surges could be allowed to increase the stress in pipe to 75% of yield strength. The design is based on the following steps:

(A) If surge pressure is less than or equal to one-half of working pressure, the pipe shall be designed using working pressure only with 50% yield strength as allowable stress.

t= PD/2Y where Y = 50% of yield strength

(B) If surge pressure is more than or equal to one-half of working pressure, the pipe shall be designed using working pressure and surge pressure only with 75% yield strength as allowable stress.

t= [(P+S)D]/2Y where Y = 75% of yield strength

For case A, the use of 50% yield strength is essentially the same of adopting a safety factor of 2 in DI pipe design. However, as surge pressure is not considered, the thickness calculated is smaller than that in DI pipe design.

For case B, the use of 75% yield strength is essentially the same of adopting a safety factor of 1.33 in DI pipe design. As such, the thickness calculated is smaller than that in DI pipe design.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.