Operating pressure for ductile pipe , & joints for BURIED application

do you recommend the use of WELDED STEEL PIPES instead of Ductile pipe ???

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sowhatso

Maybe I'm doing my math wrong, but I get that 25 bar is 363 psi -- which is above the maximum working pressure you give. (I haven't taken the time to confirm the maximum pressure for ductile pipe, I'm just going off what you quoted.) If that's the case, then it doesn't seem that ductile pipe is the right choice for your application.

You don't say what standard you're building this too; what limitations does your standard or specification put on you?

As far as it being bolted versus welded, I've seen lots of commercial water systems that use bolted joints for ease of repair. You don't say exactly what your application is, or how long the piping is or where you're located. These might all influence whether bolted or welded is better.

Patricia Lougheed

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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 ]

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.

rconner and vpl , Thanks alot :

The project's specifications call for pipes that need to be designed in accordance with AWWA C150 and C151 , Except that pipe dimensions including wall thickness shall conform to ISO/DIN 2531 Class K9 . I think it is better to go for the choice of Class K9 as it has higher working pressure comparing to AWWA C150 AND C151 pipes , as the standard product for AWWA C150 allow for 350 psi WP , but Class K9 have a working pressure more than 33 bar. I attached adatsheets for one of the manufacturer that produce Class K9 pipes with high working pressure for pipes of small diameters ( 3" - 12").

It sounds like the developer of these requirements is basically asking you to employ inch-sized pipes and with all the arguably some more conservative design principles of ANSI/AWWA C150/A21.50, but e.g. to plug in the slightly higher than 350 psi specified operating pressures into the C150 internal pressure etc. formula (but not necessarily be limited in class selection to “Pressure Class” pipes or a standard 350 psi rating).

This would not necessarily mean something exactly “K9” thickness, but in the larger sizes will e.g. obviously mean at least slightly thicker pipes than e.g. minimum Pressure Class 350 ductile iron pipes. In this regard, there are a currently several heavier standard classes to choose from, including the upper end of the seven Special Thickness Class selections 50-56 per AWWA C150. These are feasible calculations, and I suspect the results per all your project details will be formidable engineering structures.

While as far as I know there is nothing in the standards or past history that would prevent such an approach, in any case it may be appropriate to check with prospective vendor(s) providing at least some of your actual project details (for their evaluation as previously recommended in the references I cited). While metric-sized ductile iron pipe could be near indistinguishable from the inch-sized pipe in metallurgical and other aspects, the inch-sized pipes are dimensionally some larger than the metric pipes (and perhaps also more readily available also from large and proven vendors in the USA if this is desired).

rconner ,

I think we will go for the choice of AWWA products , but we need to choose a special class of larger thickness that will ensure the working pressure is highre than 350psi.


again , thanks alot

Go to a DI Special Class 56 and you will have your maximum wall thickness. This pipe is still 'rated' to 350 psi, but most manufactueres will tell you its good for more. I had this discussion with several DIP manufacturers and I think they can only rate pipe to 350 psi, so you will not find a pipe that is 'rated' higher than 350 psi.

US Pipe offers a pipe called TR Flex (other manufactueres have thier own version) which is a restrained joint push on pipe. It utilizes a weld on the PE and 'ears' that lock the segments together. We just used this pipe in an application where surge pressures could exceed 500 psi (working pressure of 150 psi +/-). About a mile of 20 and 24 inch water main for an industrial application. The installation was actually very easy; however, once the pipe segments are locked, they need to be 'extended' back apart to fully engage the restraining mechanism to ensure that the maximum protection against high pressures is provided. Once the installers figured that out, they were able to get a few hundred feet in per day. Pressure test early though to make sure they are installing correctly.