API-650, Appendix E, Equation E-20

[TankDude]

It appears to me that equation E-20 is missing a reduction factor. The resultant calculation for this center of mass is much greater in all the trials that I have ran than that calculated in equation E-19. The value has often came out to be larger than the tank shell height. Under the right parameters, just a slight increase shell height could bump you to a D/H<1.33 which, at least in my calcs, almost doubles the slab moment. Has anyone else had similar findings or have any thoughts on the validity of this? Thanks in advance.

 

[JStephen]

It may be a typo.

Generally, with a large diameter low tank, it should give you a moment much larger than the shell moment. And it could give you an effective height higher than the shell. The vertical centroid of the forces acting on the shell has got to always be less than the shell height. But this is the moment on shell plus moment on bottom, divided by effective mass acting on the shell. That doesn't imply there's really a force up there acting, just a way to calculate the moment.

 

[TankDude]

I have learned since posting my question that there in fact is a typo in equation E-20. The equation should read:

Xis=[0.500 + 0.060(D/H)]H

This will provide a moment much more reasonable and closer to the ringwall moment.

 

[richay]

TankDude,

Can you say where you obtained this correction?

Richard Ay
COADE, Inc.

 

[TankDude]

Sorry, I can't divulge the exact source. I can tell you that it came from someone who sits on the API-650 committee and is a credible source. I think that if you play with the numbers you will see that it's valid.

 

[IFRs]

I think there are enough other errors to make one queasy about using Appendix E until API issues an errata.

 

[JStephen]

Check the new AWWA D100-05, which includes similar equations. This particular one should also be available from the old TID-7024 document.

 

[notnowiambusy]

There are also errors in Equations E-18 and E21.

For Eq. E-18 the placement of parentheses includes the "-1" term as part of the argument of the cosh function. This is not consistent with other published sources (AWWA D100-05, Philip Myers' book "Aboveground Storage Tanks", and the original paper by Wozniak and Mitchell). The numerator should read:

cosh(3.67H/D) - 1

The same is true for the "-1.937" term in Eq. 21.