Overturning stability - Unanchored tanks

[sajk14]

Hello All,

I have the latest copy of API 650 11th Edition, June 2007, Addendum 1 Nov 2008.

I am designing a cone roof tank that is atmospheric and have a few queries with regards to the formula for working out MF and Liquid weight WL.

In the Code clause 5.11.2 states that unanchored tanks must satisfy equations:

1. 0.6Mw + Mpi < MDL/1.5
2. Mw + 0.4Mpi < (MDL+MF)/2

Mpi = 0 as tank is atmospheric.

With regards to finding the value for MF am i correct in saying that we use the formula:

WL = 59.tb x SQRT Fby.H (N/m)

This gives us the force from a band of liquid acting at the shell per meter....which we then multiply by the circumference of the tank to get the total force acting around the shell of the tank.

You then multiply this total force by the radius of the tank to get the value of MF which we use in equation 2?

Also the code states that WL shall be the lesser of 0.90HD or the mentioned above formula for WL.

Is this an error as if i use 0.90HD i get a figure that is completely out in comparison.

Many Thanks for your help

 

[JStephen]

You are correct in your application. The 0.90HD is in error. I think it is the US-unit equation. The equation for the amount of liquid resisting uplift is based on the equation given in App. E for seismic design. You should be able to work out the comparable metric limitation based on the App. E equations.

Usually the first equation is what determines if it is anchored.

 

[sajk14]

Thanks JStephen,

based on the equations in appendix E i have calculated that for SI units it should be 141HD

Im assuming that the equations in Appendix E do not apply in my case as i am not designing the tank for seismic conditions and that the formula for WL stated in my original post is the correct one to use.

Thanks again for your help

 

[IFRs]

API is currently revising this section, a sub-committee ballot on the new equations is due soon.

 

[gwkwong]

I just wonder what is means by "band of liquid". To determine the anchorage requirement, shall we assume that tha tank is empty. Where is this "band" of liquid comes from?

 

[JStephen]

In recent revisions, they added the uplift on the roof, which has a major effect on overturning. At the same time, they added the effect of the tank contents. I think the idea is to recognize that the maximum wind plus minimum liquid level are not likely to occur at the same time. This is similar in idea to the approaches used with maximum venting and overturning, and in combining impulsive, convective, and vertical seismic acceleration.