API 650 Uplift / Overturning and Tank Anchoring

[xema77]

I am looking at the decision tree (Fig F-1) in Appendix F of API 650. The tank internal pressure is less than the weight of the shell & roof, however when I calculate Pmax it is less than the design pressure. F.4.2 indicates not to exceed Pmax. What needs to be done, I assume that anchoring the tank and following F.7 is an acceptable solution? Please advise.
I am also having trouble understanding why Pmax is lower than the design pressure (resulting in uplift) yet my 5.11 calculation for wind overturning indicates no anchors are required. Both are based on wind and internal pressure, why the discrepancy?

Thanks

 

[DSB123]

xema77,
Sounds like you have done the calcs incorrectly!!

 

[xema77]

No, I don't think so. I was looking for a more constructive response.
I believe that it is because 5.11.2 load case 1 uses 0.6Mw and appendix F does not factor the wind moment. It is difficult to tell with the factors that API applies to formulas in Appendix F. Ayone else have any input?

 

[rab1944]

It is rare for an Appendix F tank to not be anchored.

Have you checked out Section 5.2.1(k)2: The design uplift presure on the roof (wind plus internal pressure)need not exceed 1.6 times the design pressure P determined in F.4.1.

Also of interest are the 30 May 09 posts regarding wind overturning stability check and apx F moment definitions.

Hope this is helpful.

Best regards

 

[JStephen]

In App. F, there is a division into two different categories depending on whether the pressure exceeds the weight of the shell and roof.

If you're in the first category, you may still need to anchor the tank to avoid wind overturning, but you don't have to comply with F.7. There may be an Interpretation that states this. You also have to check pressure based on top angle area, and if that is inadequate, either increase the area or increase the roof slope.

A couple of addendums back, API revised the wind loading by adding uplift on the roof. This dramatically increased the calculated wind overturning. At the same time, they revised the overturning criteria to include part of the tank contents, recognizing that tanks are not likely to be completely empty, which offset a lot of the increased moment. However, some of these changes didn't get reconciled between the wind overturning criteria, the anchorage requirements, the Appendix F requirements, and the Appendix E requirements. The most recent addendum did have some corrections to the anchorage force equations, but I think there are still some discrepancies, and you have to use your judgment to figure which section to use when they don't agree.

 

[LSThill]

xema77 (Mechanical) have you check the following

Please review TID-7024 NUCLEAR REACTORS AND EARTHQUAKES

1) Sloshing Frequency for Fluids in Tanks
2) Wave Motion Periodic Loading on Tanks
3) Slug Loads

5) Seismic/Periodic Time History Loading
6) Falling Fluids
7) Tank Fill Simulations
9) Slug Breaker Design
10) Loads on Flow Obstructions
11) Deceleration of Tank
12) Branch Flow Splitting
13) Flow around Baffles

L S THILL

 

[xema77]

Thanks for the input