Anchorage requirements for Pf Uplift

[merton]

If calculations show that no anchorage is required for Net uplift, seismic and wind loading do they still net to be installed to resist internal uplift at the calculated failure pressure?

Thank you

Vance

 

[JoeTank]

No.

Joe Tank

 

[JStephen]

If you'll notice in the API-650 anchorage requirements, there is a note for the failure pressure that says "only applicable when section F.7 applies" or something to that effect, in which case the tank is anchored anyway.

 

[gdubyuh]

The note in Table 5-21b specifically states that 'failure pressure only applies to tanks falling under F.1.3 only', which means the tank must be subject to net uplift, in order for failure pressure to exist. Additionally, failure pressure, Pf, as defined in F.6, is only applicable to frangible tanks (API-650 5.10.2.6). So, if the tank diameter is less than 50 ft (or if any other 5.10.2.6 criteria are not met), failure pressure does not exist.

 

[JStephen]

It is not clear to me from the wording of F.6 that the failure pressure requirement of Table 5-21 is only applicable to frangible-roof tanks.

 

[gdubyuh]

I cannot say with 100% certainty that failure pressure is only applicable to frangible roof tanks. That has just been my interpretation. I adhere to that based on what I would call logical progression. Table 5-21b describes Pf as failure pressure, and references App. F. Pf as described in App. F is discussed in F.6.

F.6 begins with, "In tanks that meet the criteria of 5.10.2.6, failure can be expected to occur"... (paraphrasing) at the compression ring / roof juncture.

I stop at that point and take that as the definition of failure pressure. It could also mean, but is not stated, "In tanks that do NOT meet the criteria of 5.10.2.6, failure can be expected to occur at the anchorage attachment area" i.e. take off like a missile.

In that case you would still have a failure pressure that would occur, just not at the compression ring area, and therefore Pf would not = the equation provided by F.6

 

[JStephen]

I did send an inquiry into API on this.

Assuming that a tank falls under F1.3, there is a minimum pressure involved and thus a minimum top angle area per F.5.1. But it seems this area will always be greater than the maximum area allowed under 5.10.2.6, such that F1.3 tanks are inherently non-frangible. I think the significance is that 5.10.2.6 is based on avoiding uplift at the shell, while F1.3 is limited to the uplift case. If failure pressure is limited to frangible tanks falling under F1.3, then that would not include any tanks.