Hi there

API 650 Appendix E says that nozzle and piping should be designed to accomodate displacements mentioned in table E-8. But it does not say anything about the low-type tank nozzles. I have seen in some of the stress specs they do not consider these movements in piping and the only displacements considered is settlement or expansion of the tank.

I was wondering if this should be considered regardless of the EL of the tank nozzle.

Thanks
Bobby

Hello again

Can any one please help me with this? I went through all the references, specs and books that I had but I couldnt find any thing that says table E-8 displacements should be considered for in analysis of pipe and nozzle

thanks

Hello again

Can any one please help me with this? I went through all the references, specs and books that I had but I couldnt find any thing that says table E-8 displacements should be considered for in analysis of pipe and nozzle

thanks

The way I read Section E.7.3 and Table E-8, low tank nozzles are NOT excluded. This information seems to address ALL tank nozzles.

As to whether or not you should include these displacements in your piping analysis, API-650 indicates "yes", but the ultimate decision is between you, the owner/client, and the inspector.

API 650 seismic "overturning" calculation assumes the development of two plastic hinges, one at the junction to the shell and the other at some distance inward from the shell. As a result of this model, a part of bottom and shell can be lifted off the ground and the weight of the content will resist overturning.
Details are given in a famous paper "Basis of seismic design provisions for welded steel oil storage tanks", authors Wozniak and Mitchell and In "Aboveground Storage Tanks" by Myers, chapter 6.1.5.

Based on this model, one can calculate the effective lift-off; see E.7.3.1 Method for Estimating Tank Uplift.
As you can see, API 650 does not consider a correlation of such value with Earthquake magnitude, vertical acceleration, etc, so I would consider E.7.3.1 gives the maximum tank response vertical displacement.


Returning to your question, the lift off is vertical (of course!) and theoretically this vertical deflection has nothing to do with nozzle's centerline elevation. I would add that the horizontal displacements should be increased to account for drift of the tank, and the drift depends on that elevation - see the case when the nozzle is top shell located.

As EC.7.3 PIPING FLEXIBILITY explains, the design displacements of Table E-8 "are a compromise of practical design considerations, economics and the probability that the piping connection will be at the point of maximum uplift. If one "estimated" the tank uplift using the simplified model in the appendix, the uplift will often exceed the values in Table E-8 unless the tank is in lower ground motion regions".

Thanks guys. It is a little hard to say then what type of stresses are induced in the piping. I saw another post here that had it in EXP case, as displacement range stress, while the cause of the displacement is occasional. Which one do you think we have to check against?

Thanks

On the subject they are many opinions.
See for example this one
"Strain (displacement) that does not produce collapse would be considered "secondary". The stress they produce is limited to yield. The mode of failure is fatigue.
It gets sticky though when this stress component aggravates other stress caused by force-based loads (primary stress)."

However in this particular case the values given by Table E-8— Design Displacements for Piping Attachments are referred as "ASD Design Displacement mm (in.)".

Also API 650 E.7.3 PIPING FLEXIBILITY says
" Unless otherwise calculated, piping systems shall provide for the minimum displacements in Table E-8 at working stress levels (with the 33% increase for seismic loads) in the piping, supports and tank connection. "

Please note other requirement in the same paragraph:

"The piping system and tank connection shall also be designed to tolerate 1.4Cd times the working stress displacements given in Table E-8 without rupture, although permanent deformations and inelastic behavior in the piping supports and tank shell is permitted."

Thanka Mario. The first E.7.3 you mentioned was the reason I thought the post here was not right. I think it should be checked against OCC stress.

Perhaps it may be helpful to cross-reference API 650 Appendix-E to the provisions of ASCE-7 section 15.7, and notably subsection 15.7.4 "Flexibility of Piping Attachments", also Table 15.7-1 "Minimum Design Displacements for Piping Attachments".