pipe stress due to Tank Settlement 1

[mylexicon]

Hi gents,
When performing pipe stress analysis in accordance with B31.3 code, one would usually combine the "displacements" due to settlement of a supporting structural foundation or a Tank foundation with the Operating Load and end up with the following load combinations to check the loads and stresss vs. code stresses:

1) W+P+T+D1+D2 (OPE) where D1= anchor movements due to thermal expansion
D2= anchor movements due to settlement
2) W+P (SUST)
3) L1-L2 (EXP)

In a situation where the piping is connected to a Nozzle of a Tank with a foundation which has settled down into the soil over time, the above combination would see the settlement induced stresses summed up with the thermal expansion stresses in the 3rd case. Is this correct in your opinion to check the stresses of the 3rd case vs. the Allowable Exp. stress of the code?
In addition to the combinations that are listed above, isn't it necessary to look at the displacements due to settlement as a sustained condition? Since the long term settlement of the Tank is now a permanent & sustained condition, don't we have to have a case to check the Sustained stresses including the displacement due to settlement as follows:

W+P+D2 (SUST) where D2= settlement displacements

look forward to hear your comments.

drno

 

[NozzleTwister]

mylexicon,

If you are installing new piping that will be routed to a settled tank, you can ignore any settlement that has already occurred and just consider the anticipated future settlement.

If you are analyzing existing piping and the tank has settled and you are not correcting the pipe routing due to the settled nozzle location then you need to consider that displacement in your thermal (secondary stress) analysis.

For me, settlement is never a sustained loading but it does need to be considered when your pipe is ambient as well as hot.

You have three cases that you need to consider displacement stresses.
1) System hot, with nozzle displacements and NO settlement.
2) System cold, with settlement. You can run operating with a temp T2 set to 70 deg. F. (or your ambient temp)
3) System hot, with nozzle displacements and settlement.

You may not need to analyze cases 1 & 2 if they can be judged to be less severe than 3.


NozzleTwister
Houston, Texas

 

[mylexicon]

NozzleTwister
Thanks for the reply. I was actually thinking within the context of designing a new process plant. The settlement that I was talking about was meant to be future long term settlement of a Storage Tank. i.e.; due to long term (say 2 years after the completion of construction) settlement of the tank foundation, a nozzle that is connected to the tank shell will pull its connected piping down with it.

regards,

drno

 

[NozzleTwister]

mylexicon,

That settlement should be treated as a thermal loading. Even though it's not thermal, the additional stresses caused by settlement are secondary in nature.

Many plants in coastal areas are built on poor soil and the settlement can be quite large. Talk to your civil engineer about the settlement numbers. Maybe the total settlement will be 4" but many times after a new tank has been built, it is filled with water a left full for 30 days or more so the tank can settle before piping is hooked up. This initial settlement might be 2 1/2" leaving only an additional 1 1/2" settle over the next 30 years. Since your piping is not hooked up until after the initial settlement has occurred, you have much less settlement to deal with for your piping system. The numbers I gave are only examples, your civil engineer will be able to give you numbers for your tank location based on his soils reports.

Another thing to consider on large tanks is that nozzles near the bottom rotate downward as the tank fills up. Upon filling, the tank bulges out slightly with the bottom staying fixed thus causing a rotation of the nozzle. This is covered in Appendix P of API-650.


NozzleTwister
Houston, Texas

 

[mylexicon]

NozzleTwister
Thanks again for your comments on treatment of settlement induced displacements in our pipe analysis. And You're right, the best place to go for getting reliable numbers on long or short term settlement is a civil/struct. engineer. I'll do that.

best regards,

drno

 

[MJCronin]

mylexicon....

This response doesn't directly address your question, but I believe is worth noting....

The expected tank defelection and rotation must be considered in the piping design of a new process plant either through the use of piping loops or offsets; alternately ball type swivel joints can be used.

Settlement of large tanks is nothing new....

FE Pipe, an excellent tool by Paulin Associates, uses the "big analytical hammer" approach to evaluating settlement stresses....

http://www.paulin.com/prg/paper/FP2004.pdf

There are others that suggest the use of expansion bellows designed for large lateral defelections at tank nozzle locations.

http://www.dmeexpansionjoints.com/universal-bellows.htm

also note this discussion:

http://www.coade.com/cgi-local/ultimatebb.cgi?ubb=get_topic;f=1;t=000585;p=1#000002

-MJC

 

[mylexicon]

MJCronin
Thank you for the helpful references you provided. However, I believe NozzleTwister did answer my question on whether to treat long term settlement induced stresses as Primary or Secondary code stresses.
Thanks again.

drno