In API 650 5.12 tank anchorage calculation 1

[BZ29]

Hi. In API 650 5.12 tank anchorage calculation... for design pressure case, instead of using input design pressure, it uses maximum calculated design pressure from Annex-F.

Doing so, causes the anchorage to increase as we increase the compression area (increase in compression area increases maximum design pressure).

Tank is obviously restricted in operation, to the input design pressure, then why anchorage is being designed as per maximum design pressure of annex-F?

And this implies that we should not increase compression area too much and keep it near to just required one, to optimize the size and number of anchors?

Thanks.

 

[JStephen]

Where exactly are you looking? In Table 5.20A and B, Pi is "design internal pressure". Are you maybe looking at some program for the calculation? If so, see if that program gives you the option to toggle design pressure as input maximum or maximum allowed per Annex F (note the different approaches in F4.1 vs F5.1).

 

[BZ29]

yes, i understand that Pi in table 5.20 is design internal pressure. but that design internal pressure is of Annex-F, which we calculate. This is not the input design pressure we get from client.

i am looking for the reason/logic behind it.

 

[HTURKAK]

- When you look Figure F.1—Annex F Decision Tree,
Does internal pressure exceed the weight of the shell,roof and attached structural?

If yes , Look to the F.7 Calculated Failure Pressure.. API 650 implicitly does not rely on some instruments to limit the max. internal pressure which may develop. Literally , if internal pressure can develop , build-up and exceed the weight of the shell,
roof and attached structural, the failure of ROOF TO SHELL junction is preferred rather than rupture of shell or shell to bottom junction..

- My question is, consider you did not provide anchor bolts and if the internal pressure exceeds the weight of the shell + roof, does the tank fly like a balloon?









Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[JStephen]

I still don't understand the issue. If you have the tank geometry already fixed, you CAN calculate a maximum pressure per Annex F. But if the customer specifies a design pressure, that is what is used for "design pressure", not the maximum calculated under F.4.1.

 

[BZ29]

Thanks a lot HTURKAK. Clearly understood the point now.

 

[BZ29]

Sorry to drag. But there is another confusion now.

As per 5.10.2.6 (d), even though after providing anchors, it still asks to meet the requirement 5.10.2.6.a.6 (maximum area for frangibility). It means that if that area requirement is not met, then regardless how many anchors we provide, we cannot make the roof frangible. This is confusing. (The anchors are already provided due to wind load)

In my case there is a huge difference between 5.10.2.6.a.6 area and required compression area as per annex-F. So it means i cannot make it frangible regardless whatever i do, or is there some way to do this by increasing anchorage?

 

[HTURKAK]

I may answer in detail tomorrow.

My short answer is,

-5.10.2.6 is for Frangible Roof requirements and clause 5.10.2.6 (d) is for tanks with frangible roof..

- When your tank is not frangible ( could be,and a valid option ) you are required to provide EMERGENCY VENTING ..( pls look 5.8.5.3 Emergency venting und API 2000)

I hope this respond answers your question



Good luck ..




Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[BZ29]

Yes your answers are very accurate and precise and clearly responded to my question, but confusions keep coming on...

Just to understand conceptually...
Although because of enough anchorage there will be no uplift and most probably the shell to roof joint will fail earlier, but still we cannot take it as frangible roof.

is it because in high anchorage, may be anchorage does not fail and no uplift is produced but still shell to bottom weld joint fails prior to roof to shell joint (and for that we do not make any calculations)? so that's why we cannot assume it as frangible?

Thanks a lot for such naïve questions

Regards

 

[IFRs]

The tank can fail either at the roof-to-shell joint or the bottom-to-shell joint.
API 650 wants the tank to fail at the top during an emergency but it needs to be strong enough to withstand your design pressure.
If the tank is anchored it will not fail at the bottom.
If the top joint does not qualify as frangible it does not matter what the design or maximum internal pressure is or can be, you need to provide emergency venting.
So, you do the calculations: is the top joint stronger than your design pressure, is it frangible.

 

[BZ29]

Understood the point. thanks a lot