API 650 Sec. E.6.2.1.1 1

[hamidun]

Dear All,

In API 650 Ed. 12th 2016, it states that The tank is self-anchored providing the following conditions are met:
(4) The required annulus plate thickness does not exceed the thickness of the bottom shell course.

Based on the above statement, if the tank is self-anchored, the thickness of bottom annular plate can not exceed the thickness of first shell of the tank. Am I translating this, right?

If I am right, I would be confused then, because I have seen an API tank with annular bottom thickness is greater than the thickness of first shell of the tank.

Please help me with this section of API.

Thanks!

 

[LittleInch]

Yes you are reading this correctly.

Section 6.2.1.1.2 the note makes it clear.

If the annulus plate thickness is greater then it may need to be anchored.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IFRs]

hamidun - As master LittleInch has noted, you are reading it correctly. Perhaps your confusion can be alleviated as follows:

A tank with 1/4" shell can have a bottom that includes a 3/8" annular ring and the tank can still be seismically stable without anchors as long as the REQUIRED thickness of the annular plate is less than the tank shell.

The tank resists uplift by the sum of several masses, including a portion of the contents of the tank. The volume of this portion of the tank contents is calculated using some radial width of an annulus of liquid inside the tank. This radial width is determined by the thickness of the bottom plate next to and under the shell. This thickness is limited to the thickness of the bottom shell course for this calculation.

So, you calculate the seismic uplift, calculate the resisting moment and include in that moment an annulus of contents whose width requires an annular plate thickness less than the tank shell thickness. Good to go, your tank is stable and self anchored.

Then you increase the annular plate thickness for some other reason. That's OK and actually quite common.

My opinion only, remember you read it on the World's Wildest Web

 

[hamidun]

IFRs,

Based on your statement "Then you increase the annular plate thickness for some other reason. That's OK and actually quite common."

Have you experienced designing a self-anchored tank, but with annular bottom thickness is greater than the first course of the shell? And what is the result of the tank? Is that ok?

Because, the senior here keeps insisting to not have annular bottom thickness greater than the first shell course. He argue that it would disrupt the flexibility of the tank when experiencing earthquake. He says, if we have a tank with 1/4" thick of first shell course, the annular bottom plate must be 1/4" or lesser than the thickness of first shell course. Is He right? His argument regarding flexibility of earthquake is acceptable?

Help please! Thanks!

 

[IFRs]

hamindun - Your senior associate is in all likelihood correct and I have no issue with his or her thinking. They have far more knowledge than I of your particular tank, operation, risk and local loading. If they are saying you can't exceed the shell thickness for this tank, have good reasons and technical basis, I would not use the "I read it on the internet" as your basis for discussion.

I am reading the language of the standard as best I can. Think of it this way: in an area of zero seismic activity, can the bottom be 3/8" for a shell that is 1/4"? I think so. In an area of mild seismic activity, if the tank is self anchored just by the shell weight can the bottom be 3/8" for a 1/4" shell? I think so. In an area of moderate seismic activity, if the tank is self anchored using an annulus of liquid supported by an annular plate 1/4" thick can the bottom be 3/8"? I think so. In an area of high seismic activity if the tank needs a 3/8" thick annular ring to be self anchored it is not OK because the shell is only 1/4" thick. As long as the thickness of the bottom necessary to resist seismic loading is less than the tank shell you are OK. After that you follow the thickness to width rules for annular plates and I don't see other restrictions to annular plate thickness in relation to the shell thickness.

In answer to your other question, yes - I have seen a very large population of tanks with 1/4" thick shells that have 5/16" bottom plates. Some owners always use 5/16" bottoms for corrosion resistance and many of their tanks are small enough to have 1/4" thick shells.

Your tank may be a special case, perhaps a high risk tank in a high seismic area and it may need to be considered carefully, perhaps in greater detail than the API "cookbook" standard. The corner joint is the most critical one on the tank. Interaction between the shell and the bottom is complex. You have an expert that is counseling you in one direction. take their recommendations seriously.

These are my thoughts on interpreting the API standard only, without understanding the particulars of your situation at all. Do the right thing here, protect the public safety. Sometimes that means exceeding the minimums or interpreting the standard more critically.

 

[hamidun]

IFRs,

I am totally with your statement "If they are saying you can't exceed the shell thickness for this tank, have good reasons and technical basis" And to be honest, I am not satisfied with my senior's reason/argument about flexibility of the tank. And I believe, you have far more and better knowledge than my senior regarding storage tanks.

That's why I ask to this forum to get a better explanation on this case. What I still doubt about the API statement
"The required annulus plate thickness does not exceed the thickness of the bottom shell course"
is that I have seen a drawing of a tank with annular bottom 8mm and the first shell course thickness is 6mm, that's why I am still looking for a good reason regarding that API statement.

 

[IFRs]

Sir - Your expert is most likely quite correct in his/her understanding of the dynamics of the shell-to-bottom joint. While I appreciate praise, I hesitate to compare myself favorably to anyone whom I have not met, especially one with knowledge and experience. I'm an internet nobody.

That said, your original and re-stated question is regarding the interpretation of the statement in API "The required annulus plate thickness does not exceed the thickness of the bottom shell course".

I would interpret that statement literally, in the context of the section it is in and would not apply that statement to any other section of the standard. To me it says that when evaluating seismic stability, your ability to thicken the bottom plate under the shell to increase the resistance to uplift is limited to the thickness of the tank shell. If your calculations for seismic stability include an annular plate that is less thick than the tank shell, stop there, check that box and move on.

There is no other section of the standard that restricts bottom plate thickness to that of the tank shell. For instance, in figure 5.3c, note 2 says "thinner of the shell or annular bottom plate thickness" clearly anticipating either being the thinner. 5.1.5.7 has similar language. Section 5.4 is silent on comparing the bottom to shell thickness or imposing a limit on bottom thickness. Section 5.5 also does not mention any limit for the annular plate thickness but goes to great lengths to establish minimum thicknesses.

My reading of the standard is that the bottom can be thicker than the tank shell as long as the bottom thickness needed to resist to seismic uplift is a less than the tank shell thickness.

At this point, if your senior engineer is still insisting that the bottom be no thicker than the tank shell, I'd be inclined to go with his or her judgment since clearly they are convinced of the validity of the technical basis for their statement and may have knowledge of this particular situation in far greater detail than some random internet voice. The API standard is a very good reference for building tanks but intimate knowledge of the actual design conditions may suggest to a careful and thoughtful engineer that conservative decisions are appropriate.

 

[hamidun]

IFRs,

After looking at your profile, I still believe you have far more and better knowledge than my senior regarding storage tanks. I am so sorry for this.

And after all the discussion we have been through, it has come to one remaining question in my mind, as per statement of API 650 Sec. E.6.2.1.1, "The required annulus plate thickness does not exceed the thickness of the bottom shell course", but why there are storage tanks that have bottom annular plate thickness greater than the thickness of the bottom shell course.

Does the word "required" means the corroded thickness? Then I can assume, storage tanks having bottom annular plate thickness greater than the thickness of the bottom shell course might use the thickness of annular bottom plate + corrosion allowance. Is that something like that?

For example, the required/corroded thickness of annular bottom plate is 1/4" and the first shell course is 1/4", but then the designer adds corrosion allowance to the annular bottom thickness to be 3/8".

 

[IFRs]

Your question has remained the same from the start. I have attempted to respond as best that I can. After all the posts, you have stated the same question again.

I am doing a very poor job of responding to your specific question. My apologies, I'll try again.

The words "required thickness" in the context of E.6.2.1.1 mean the calculated thickness needed to resist overturning. Nothing more, nothing less. Unrelated to any other section of the standard.

It does not draw from any other section of the standard. There are no other sections of the standard that refer to this thickness calculation.

Your question: "Does the word "required" means the corroded thickness?"

Since it is unclear, go back to the beginning of the section where variables are defined. In this case the variable is ta and for the definition of ta it says "ta Thickness, excluding corrosion allowance, mm (in.) of the bottom annulus under the shell required to provide the resisting force for self anchorage. The bottom plate for this thickness shall extend radially at least the distance, L, from the inside of the shell. this term applies for self-anchored tanks only."

That is your answer.

Your question: "For example, the required/corroded thickness of annular bottom plate is 1/4" and the first shell course is 1/4", but then the designer adds corrosion allowance to the annular bottom thickness to be 3/8"."

If the required (calculated) thickness is 1/4" and the shell course is 1/4" so you have satisfied this portion of E.6.2.1, your calculations resulted in an annular plate required thickness not greater than the tank shell. Your tank is self-anchored. Check that box, move on.

If the designer adds corrosion allowance and the result is an annular plate thicker than the tank shell, this is unrelated to E.6.2.1.1 and does not present a conflict of any sort.

 

[hamidun]

IFRs,

Alright, I understand about the thickness stated in E.6.2.1.

But, if the required (calculated) thickness of annular bottom is 3/8" and the shell course is 1/4", what about that? Should I increase the thickness of the shell course? Or is it any other solution for this?

Sorry for the late reply!

 

[IFRs]

If your bottom thickness needed to satisfy E.6.2.1 is greater than the shell thickness, either accept that the tank is not self anchored and add mechanical anchors or redesign the tank so it becomes self anchored. This may include thickening the tank shell ring(s).

 

[hamidun]

Alright IFRs,

I can conclude that there are two solutions for this annular bottom thickness case, either we add mechanical anchors or increase thickness of tank shell.

But my senior here come up with a solution which is decreasing the thickness of annular bottom to be equal to the first shell course thickness, earlier we have annular bottom thickness of 5/16" and the first shell course thickness is 1/4" and then he reduce the bottom annular thickness to be 1/4" as well. What do you think of that solution?

I know if we see it economically, it is the best solution but in terms of technical aspect, is this how we decide a solution for this type of case? Or is there any discussion about this before and agree on a specific solution?

 

[IFRs]

Sir -

If the required thickness of the bottom under the shell is less than the shell thickness the standard says you don't need mechanical anchors for seismic conditions.

If the required thickness of the bottom plate under the 1/4" thick shell is less than 1/4" and you decide to use 1/4" for other reasons, the standard says you are OK.

If the required thickness of the bottom plate under the 1/4" shell is less than 1/4" and you decide to use 5/16" for other reasons, I think the standard says you are OK.

Capital expense is always one criteria for selecting material grades and thicknesses. Usually it is not the only criteria, sometimes owners choose to pay more now for lower life-cycle costs.

 

[hamidun]

IFRs,

If the consultant recommends to use annular bottom plate thickness of 5/16" under 1/4" thick shell. But, the senior here insist to use annular bottom plate thickness of 1/4" under 1/4" thick shell based on the statement of API 650 Section E.6.2.1.1. What do you think of that?

 

[IFRs]

My opinion on those choices of bottom plate thickness is they both are compliant with the standard.

I don't think that E.6.2.1.1 imposes a thickness limit on the bottom plate and still read it to say that to demonstrate seismic stability you can only use in your calculations a bottom plate thickness up to the thickness of the shell. Apparently your senior engineer does not agree and that is certainly OK.

Since there is a difference in interpretation of E.6.2.1.1 may I suggest that you submit a technical inquiry to API for their consideration?

 

[hamidun]

IFRs,

Yea, I was thinking to submit a technical inquiry to API, but I am still not sure about my inquiry itself .

Let me ask you this, if you're the owner of a company which want to build a tank and working with a consultant who recommends to use annular bottom plate thickness 5/16" under 1/4" thick shell, but then the contractor based on their experience recommends you to use 1/4" thick annular bottom plate under 1/4" thick shell and the contractor gives you the statement of API 650, E.6.2.1.1 as their principle to recommend so.

What would you do? Would you stick to your consultant or contractor?

 

[IFRs]

At this point you are aware of exactly my opinion on this matter. My opinion is that I don't think API E.6.2.1.1 restricts the bottom to the shell thickness, only the thickness used to resist seismic loading. I don't know why the consultant wants 5/16" but you do. I don't know your regulatory environment or your relationship with those and any other entities that review and/or approve your project.

If you are the owner, you have the final say, it is your risk, your insurance, your investment, your valuable equipment. Evaluate the costs and benefits, the risks and rewards. Make a decision using good engineering judgment. Inform all stakeholders and document the decision process. Move on.

 

[hamidun]

IFRs,

After all the discussion we have had, I think I will suggest to use the required annular bottom thickness of 1/4" and then add 1/16" thick of corrosion allowance under 1/4" thick of shell.

Thanks IFRs!!!