Suppose the tank settlement is defined by a displacement in vector D1.

Should D1 be added for the sustained stress check? That is, create a sustained case like SUS(W+P1+D1)?

Although displacement is a secondary type loading, in this instance it remains in the system like a sustained loading and does not cycle with temperature.

Another Aspect -

For example if we have OPE((W+P1=T1=D1) minus SUS(W+P1), isn't the result for the EXP stresses going to be very conservative because the D1 component does not cycle?

Paragraph 319.2.1.c (of B31.3) considers settlement as a single cycle effect that does not influence the fatigue life significantly.

How should the load cases for the EXP stresses be setup?

I'll appreciate your answers and insights.

Somnath

Tank base cycles with the level of the fluid in the large ground supported tanks. Tank base moves between maximum and minimum settlement values depending on the design of the civil work under the tank. In some cases you can provide thick concrete slab for relatively small tanks, so there could be insignificant settlement in case the piping is on the same slab. Most cases in large plants you may see the pipe rack sattlement is differently than the tank settlement since their civil work are different.

You need to see all the loads on the piping as well as the settlement cycles if available.
The same can be said for the thermal loads. The thermal loads are cyclic since you cannot maintain any equipment on the exact temperature, there are ups and downs within control limits. But we use the minimum and maximum to consider in the analyses.

So, we are the designer to consider all the load requirement. Both thermal and settlement cycles will interract which will be the worst case. Thus we consider both in the same analysis under the same load case to simplify the analysis.

I trust this answer your question.

Somnath,

Read ASME B31.3 Para. 301.8 and 319.2.1(c).
It is clearly stated that settlement represents a displacement induced stress - quite a "single cycle effect" displacement stress, so that, based on engineering judgement, an increased allowable stress range might considered. In this regard, ASME B31.1 indicates 3 times allowable "hot" stress as stress range limit.

Athough your inquiry includes some logical judgement (e.g. there is no "un-loading - return" cycle), B31 Codes approach is clearly stated and documented, and has been used long time for storage tank piping design purpose.

Of course, as stress engineer, it's up you to employ a more conservative approach than provided by design Code and consider Settlement together with dead weight and pressure as Sustained Load Case - however, be aware to consider SIFs instead of SSIs for this load case, since settlement induces significant bending bending stresses that should be treated properly.

My opinion is in such case, you need to document this conservative approach by a Design Basis document that should be approved by Owner/Client, since the design solution might be more sophisticated and expensive than expected...and we are leaving now bad times, isn't it?, when industry is lead by accountants and not any more by real engineers...and reducing costs (i.e. his highness "the profit") represents the main target for any Project, sometimes despite quality/safety are affected...

Best regards,