Tank Overturning: Main Body vs App F

[yosamitysamm]

Hi All,

I am trying to resolve a descepancy I have come across several times with small API 650 tanks. The tank in this example is a 3000bbl tank (23.5'D x 40'H).

Per the main body there are 2 checks for overturning:

5.11.2 item 1 This formula can be rearranged to find the maximum pressure for the given tank. In this case the max pressure without anchoring is 0.15 psi.

5.11.2 item 2 yields a max pressure of 1.84 psi without anchors.

Therefore I am limited to 0.15 psi on my tank if I want to avoid anchors.

Per App F the internal pressure of 0.15psi is greater then the weight of the roof plate but less then the weight of the shell. Therefore per Fig 4-1 the tank does not require anchors but the conditions of F1-F6 must be met.

F4.2 will also calculate a maxmium pressure to limit uplift of the shell. In this case however the maximum pressure comes out negative.

I am unsure how to inperpret this negative maximum pressure (The max pressure per F4.1 is 2 psi). Is the intent that the tank now requires anchors even thought the pressure is less then the weight of the shell?

Another question regarding F4.2. Why is there no consideration given to liquid holddown? Per 5.11.2 point 1 API 650 is looking at an empty tank and assigns a factor of safety of 1.5 on overturning (No product in tank means less risk if tank shell fails). Per 5.11.2 point 2 API 650 is looking at a half full tank and assigns a factor of safety of 2. App F4.2 gives a safety factor of 2.5 on uplift. My understanding of this increased safety factor was becasue this was looking at a full tank. However there is no allowance given for liquid hold down.

F4.2 rearranged
0.67 Mw + 0.4 Mpi < Mdl / 2.5

Anyone looked into this before? Your comments would be appreciated.

 

[JStephen]

First off, there are, or have been discrepancies in some of the equations. I think if you look at the history of the revisions, the problems were that one section was updated, but a related section was not updated to match. So use your own judgment in applying it. I think the App. F equations have lagged the main body of the standard, if that helps.

I don't have the standard here in front of me, but generally, if the tank requires anchorage for pressure alone, it falls under F.7. If it does not require anchorage for pressure alone, it may still require anchorage for wind+pressure or seismic+pressure, or for wind or seismic without pressure, for that matter.

Under the older standards, wind moment was calculated based on projected area. A couple of revisions back, they added wind uplift on the roof, but when they did, they also confused the wind moment about the centerline with the wind moment about the edge of the tank. This may be the source of the discrepancy you are seeing. Note that in the Addendum 2 anchor bolt load equations, they separate out the wind uplift and the moment due to projected area.

When they added the wind-uplift, they also added provision for liquid hold-down. This assumes that the maximum wind and maximum emptiness are not likely to occur at the same time (and it is not based on a full tank, but some fraction of full). Anyway, if the pressure can be applied to an empty tank (as might happen with blanketing, or where P/V vents are used), then it would make sense to omit the liquid hold-down when checking for anchorage.