Hello everyone. I was hoping to receive some opinions from people regarding a buried section of a ductile iron, cement lined piping system located within a chemical facility.

We are using ductile iron push-on joints for the buried piping section, thrust blocks have been designed by the civil group for the elbows. The service is water (public water) at ambient conditions.

For the aboveground sections, the temperature used for the stress analysis is 50 deg C, which is based on the client's standard for pipe temperature due to solar radiation and no flow.

For the buried sections, we would perform the analysis at ambient conditions, therefore we are assuming no expansion effects in the buried sections. Based on this assumption, we would not be analysing any stresses at all as the buried section is continuously supported and would also not experience any high sustained stresses. The buried section will not experience any external loads due to vehicle crossings etc.

I have discussed this with my supervisor and we have concluded that the only issue will be effects due to pressure thrust (we are assuming no effects due to settlement as it's small). As thrust blocks have been designed, this should only be reflected in radial stresses, or stresses caused by the pipe wanting to bow (if that happens at all).

Ultimately, we would prefer not to analyse the buried sections as it is a water service at ambient conditions, and we don't believe there would be any stress issues. However we recognise that there are other issues due to the push on joints.

Can anyone comment on the above? I'm not looking for answers, just interested in the path that others would take, or have taken, so I can re-assess my situation. I have not analysed this kind of piping before but I have spent quite a bit of time trying to research it. Yes, I am a novice but I'm not looking for someone to solve it for me.

Thanks.

I would suggest you to read "DIPRA" publication on the design of ductile iron pipe. It may give you some guidance on what you are looking for. Following link is for the thrust restraints, and you may find other chapters valuable to read on the same website.

http://www.dipra.com/pdf/thrustRestraint.pdf

Regards,

Ibrahim Demir

Thanks for your suggestion. I have read all the relevant information from Dipra, Ductile Iron Society, ACIPCO, and many other ductile iron resources, however I haven't read anything related to stress analysis of slip joints.

If I have missed something on one of those sites I would appreciate any specific links.

Having said all that, my current thinking is that if the civil group designed the thrust blocks correctly, then I should be able to assume that there will be no joint separation or pipe bowing due to pressure thrust.

I think that you are going the right direction but do beware of pressure thrust .... If there is any possibility of piping being misaligned within containment of thrust blocks, a large lateral load can build up. The most obvious risk would be in buried pipe following ground terrain which rises then falls ... it could lead to upheaval buckling and dependent upon backfill, the line could burst out of the ground.

I haven't involved in any analysis of this kind of piping. However, I do not think that you need special stress analysis for the slip joint at all.

Additionally, the piping with ductile iron is designed for low pressures if I understand correctly.

Slip joints are there to ease the manufacturing and handling the pipe and provide sufficient sealing against the internal pressure. The seal groove is designed to provide sufficient strength to back the seal against the static diferantial pressure accured in the localised area. Why the pipes do not separate at the joints? Because they are in the trenches allocated, and after the installation, these trenches filled with the required filling. The codes are requiring certain depths for different kind of piping to provide sufficient external pressure for guiding and against the bowing action, thus the pipe sections will not separate and bow at joints. There will be small displacement due to the thrust loads, however, the joint will cover these small displacement and continue the sealing. Pipe lengthwise will be restricted by the thrust blocks at both end or will be restrained in other manners. The thrust load due to direction change will be taken by the thrust blocks or restraints.

The last word is the seal and joint are not designed to take the thrust loads, and it will not separate due to the guidance of surrounding filling and the thrust blocks in the system.

I hope this helps.

Ibrahim Demir

An example slip joint model is shown in the Applications Guide, starting on page 5-21. Note, you need to get actual numbers from your vendor - don't use the values in our example.

Thanks guys, all good advice again. I did see the guide in the Caesar manual and it's very good, however I was met with blank looks from the vendor when I requested breaking friction for the slip joint. The closest thing to actual friction values is to assume they are frictionless.

"Tests have shown that this frictional resistance in the joint is unpredictable, varying widely with installation conditions and other factors that are insignificant in other respects. Thus, these joints should be considered as offering no longitudinal restraint for design purposes."

The above is taken from the DIPRA document "Thrust Restraint Design for Ductile Iron Pipe". However I remember reading one of the board member's posts about some underground steam lines groaning very loudly when the joints moved, so a frictionless joint seems unlikely.

Anyone who has taken a physics lab that explores friction learns that it is a nebulous number subject to scatter such as fatigue data.


This is why the people who ask the simple question gee what should I use for friction are met with silence and in some cases silence with a chuckle....

What to do? Pick a number which provides a conservative solution identify the basis and the unknown of the situation and move on.

Further to Mein Großvater's (Herr Luf) comments, friction restraint against axial movement of the joint is a step function - not a smooth transition. The compressive force builds until the force overcomes the resistance of the friction at the packing then it moves the joint (stopping when the compressive force is exhausted (released)). A similar movement occurs when the axial force becomes tensile.

I recently viewed an installation of ductile iron water pipe that was designed to run underground and to use the guiding force of the soil to keep the centerline of the pipe straight (to assure stability). The "interesting" aspect of the installation presented itself when the pipeline got to a bridge. The installation contractor mindlessly suspended the exposed bell and spigot sections from the bridge with no provision for keeping the pipe sections in a straight line (no guides, no stability). The pipeline bowed and the joints came apart as any rational engineer would assume.

http://www.ductile.org/didata/

Regards, John.