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:
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.