AWWA rating design and practice is in general very conservative, and is arguably an outgrowth of gray cast iron pipe design that existed decades ago. The markedly greater strengths and toughness of ductile iron pipe have nevertheless allowed it since to be applied to many VERY high pressure applications worldwide, with pressures far beyond the maximum AWWA ratings currently ascribed to even PC350 pipe and fittings per AWWA standards, for many decades.
Page 4 of the publication “Ductile Iron Pipe” and others available at
state e.g. that 6” PC350 ductile iron pipe manufactured per AWWA C151 standard actually has an approximate bursting pressure of more than 3,500 psi (>240 bars), perhaps putting an understanding of much higher operating pressures in perspective.] Page 5 of the publication “Design of Ductile Iron Pipe” accessible at the same portal discussing AWWA ductile iron pipe states, “Ductile Iron pipe can be utilized for water working pressure greater than 350 psi and is available in thicknesses greater than Pressure Class 350. Contact DIPRA member companies on specific requirements…” This publication also contains the further guidance , “For extraordinary design conditions not shown in the table, such as extremely high internal pressures or extreme depths of cover, it may be advisable to consult DIPRA member companies for recommendations to maximize system design.” Similar statements and qualifications show up in the AWWA standards and manuals regarding ductile iron pipes.
I think at least some of the reasons for this special attention is that all elements of the system and installation must be considered, e.g. any valves, very special fittings or flanged joint piping accessories, all thrust restraints etc., pipe that must be installed exposed or in unstable, shallow cover soils etc. (that might not necessarily have the innate physical strengths of straight pipe) or any needs for special testing or control of manufacture or installation etc. While it is my understanding ductile iron piping has been used for field operating pressures in excess of 1,000 psi (68+ bars), a chain in general is of course only as strong as its weakest length. [If necessary or desired, some ductile iron manufacturers can even provide special extremely high pressure joints and/or steel items/fabrications, compatible with joints etc. of ductile iron systems.]
You may be interested also that ISO 2531 and EN 545 standards, developed specifically for ductile iron pipe (decades after the A21 committees issued previous iron pipe standards since the early 1900’s), e.g. currently ascribe standard allowable operating pressures excluding surge for all sizes up to 300mm (~12”) Class K9 ductile iron piping of far more than 33 bars (~480 psi). Likewise, e.g. say standard Class K9 ductile iron push-on pipes produced per such standards in even very large sizes up to say 1600mm (64”) rate at least 27 bars (<~392 psi) maximum allowable operating pressure exclusive of surge. [Examples of such higher standard operating ratings can be seen e.g. at pages 6-2 and 6-3 of the manufacturer’s publication at
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With regard to your question regarding preferences for bolted mechanical etc. vs push-on ductile iron joining choices for buried service, it is probable that systems involving both mechanical and push-on systems have successfully worked in the past at the very slightly greater than 350 psi pressure you describe and far beyond; however, it is my opinion that more modern push-on or push-on restrained joints (at least where they can be obtained for the piping elements involved) might in general be less labor-intensive and labor-reliant for good performance (and in general the push-on joints are similar in operation to o-rings, in that the seal just gets tighter as very high pressures are applied). This may perhaps be particularly true for very high pressure service, or with system movements that might in general be more pronounced with high pressure than low pressure etc.
