Agitator dynamic loads on tanks

[pdosreis]

Hi,

I'm designing a tank, and after doing all the calculations following API 650, the tank doesn't need anchorage. Everything fine for me.

When I delivered the calculation report to my client's consulting company, they made a lot of questions about the fact of not having anchor bolts, that I managed to answer based in the API 650 and and national portuguese standards. As agreed in contract.

Now, the consulting company, very well known german company, is asking me to prove that due to the fluid pressure on the tank baffles when the agitator is running it's not going to make the tank spin!!!!!!!

The agitator supplier gave me the baffles area and the dynamic load of on those baffles (800 n/m2).

What calculation can I make to certify that the tank doesn't need or needs the anchorage? Or what standard should I take into consideration regarding this?

Thanks.

 

[JStephen]

Assume the tank shell can't spin unless the floor does. API-650 has a section on checking a tank for sliding that could be applied to the spinning as well. Figure vertical load on floor (including fluid), multiply by coefficient of friction to get allowable shear on the floor, integrate shear force across area to get total torque.

This is a quick approach to show it's not a problem, and hopefully, the resistance is twenty times the applied load. If it is actually marginal in stability, you'd probably want to be a lot more conservative in the design.

I'm assuming, too, that the mixer is not attached to the tank itself. If it's on a bridge attached to the tank, you may overstress the shell or bridge, but the net torque on the tank would be zero, with the possibly exception of some dynamic effects.

 

[pdosreis]

Thanks JStephen.

But refering to your last phrase, the mixer it's attached to the tank roof by a mounting flange, of course, with the needed reinforcements.

 

[dig1]

If JStephen's approach doesn't provide the stability you need it may be possible to install a shear key instead of anchor bolts to handle the shear load from the torque.

It may be easier and less expensive than bolts for both the tank manufacturer and owner (assuming that the owner is installing the foundation).

I recently finished a project in a high seismic zone where the anchor bolts were not allowed to resist seismic shear (The national seismic design code did not permit) so shear keys were used instead.Obviously bolts were used for the uplift.

If you look at friction, as was suggested earlier, make sure that you examine a variety of fluid levels including the lowest one that the agitator may be operating at (not necessarily the design level). Do not forget to include the governing wind or seismic shear with the normal operating shear from the agitator to determine the total shear.

By the time you are all done, it may be easier just to supply a shear resisting device rather than fight with the consulting engineer who will keep coming back with various other requests.