Top Angle, Stiffening Ring, Compression Ring API 650 1

[hamidun]

Dear All,

I need help in translating the sentence of API 650 para 5.1.5.9.e) Except as specified for open-top tanks in 5.9, for tanks with frangible joints per 5.10.2.6, for self-supporting roofs in 5.10.5, and 5.10.6, and for tanks with the flanged roof-to-shell detail described in Item f below, tank shells shall be
supplied with top angles of not less than the following sizes...

For the sentence above, does it mean that when We have open-top tank, para 5.1.5.9.e is not applicable?

And please help me to understand the difference between Stiffening ring and Compression ring. From my understanding, Stiffening ring is used for wind load and compression ring is used for internal pressure. Therefore, when we design an open-tank, we only need stiffening ring since the tank will not be subjected to internal pressure? Am I right?

Help please!

Thank you.

 

[DSB123]

See a lawyer!!!

 

[IFRs]

I read it this way: Top angle are required and their minimum size shall be as shown in the table but you need to check out the four exceptions listed:
.
See 5.9: for open top tanks a top angle is not needed if if the top stiffener is within 0.6M (2 ft) of the top of the tank
.
See 5.10.2.6: the top angle may be reduced in size if needed to meet frangibility
.
See 5.10.5: the top angle may be larger if needed to meet the participating area calculation in 5.10.5.2
.
See 5.10.6: the top angle may be larger if needed to meet the participating area calculation in 5.10.6.2
.
See 5.1.5.9(f): if the tank is small and the roof-to-shell junction is flanged
.
Your open-top question is answered with the combination of 5.1.5.9 and 5.9 - a top angle is required if the top stiffener is more than 2 feet below the top of the tank but can be eliminated if the top stiffener is closer to the top of the tank than 2 feet.

 

[hamidun]

DSB123,

I need an experienced engineer!!!

IFRs,

Thanks for the answer!

But I am wondering why would a tank designer put the stiffener below 2 feet? I mean, is there any reason to put the stiffener below 2 feet from the top of the tank?

And I just want to make sure if stiffener ring is the same as wind girder, right?

 

[IFRs]

So it can be used as a walkway. Very common.

 

[IFRs]

Stiffener ring and wind girder are terms that are used interchangeably and in the context of section 5.9 are equivalent in my opinion.

 

[JStephen]

The unstiffened height of the shell is figured to the wind girder so it is advantageous for shell design to move it down some.
If it is to be used as a walkway, it is advantageous to put it 42" below the top of the shell so the shell acts as a handrail on the inside edge.

 

[hamidun]

IFRs,

I see!

What about top angle? Is it the same as top wind girder/stiffening ring?
Because I have this weird case, where I found an open-top tank using both top angle and top wind girder. But when the tank is analyzed using formula from API 650 (para 5.9.6)/AWWA D100 (Sec. 3.5), the minimum required section modulus is achieved by using only top wind girder to accommodate the wind load... My point is, what is the use of top angle where top wind girder is sufficient to accommodate the wind load?

For your information, the open-top tank has 25.23 m Diameter and 10.97 m Height.

JStephen,

Thanks for the answer! I just found that it is stated in API 650 para. 5.9.4

 

[IFRs]

The top angle is not a stiffening ring in the same sense that a wind girder is.
It does keep the upper shell course round but there is no calculation for it specifically - you simply use the table and associated referenced sections to size it.

On your open top tank with both top angle and wind girder, this is not unusual at all, I'd wager that the wind girder is more than 24" below the top. It does not matter if the wind girder is sufficient by itself, there must be a top angle if the wing girder is more than 24" below the top of the tank.

 

[hamidun]

IFRs,

So, as you said in earlier post, a top angle can be eliminated if the wind girder is placed within 24" of the top of the tank.

And for my case, it is possible to eliminate a top angle when the calculated wind girder is placed within 24" of the top of the tank, right IFRs?

 

[IFRs]

Normally I'd say yes, you can eliminate the top angle if the wind girder is within 24" of the top of the tank.
However, this question has been raised several times and that leads me to suspect there is something unique about this tank that may justify its being treated specially.
I'm going to qualify the "yes" with "unless the tank is special for some reason" and leave it to you for using good engineering judgment.

 

[BJI]

For the open top it likely assists with fabrication to pull the edge of the shell into roundness before it is welded and jacked up. In the case of a roof, the top angle is also convenient for fabrication as it allows greater tolerance on welding your roof plates to your shell. Not always required by design but I like to have a top angle on all tanks.