Design of Bottom Transition Plate as per API clause 5.7.8.1

[aamir2003]

I am currently working on a API 650 storage tank, client has said that our use of transition plate thickness is less as we have used the minimum value as per API 650 clause 5.7.8.1 e) of 13 mm.

The computed thickness of annular plate is 6 mm and we have used the thickness of annular plate as 10 mm adding 3 mm corrosion allowance and bottom transition plate for flush type nozzle as 13 mm? How do we justify to client that our design is correct . There only question is what will happen if bottom transition plate corrodes and goes below 13 mm


An urgent reply is required ?

Regards,
Aamir

 

[blawn]

mr.aamir,

If you meet all the design criteria,performed all necessary calculations and if your material meets all the requirements stated in the code,then no one should go against it.Present your client all your calculations and documents that your design meets all requirements.
If the plate got corroded,it will not immediately fail or cause leak depending on the severity of corrosion and leak caused by metal thinning will not happen instantly.It will take years for these to happen.

regards,
blawn

 

[IFRs]

Is there an annular plate on this tank? If so, you only have to be that thick. Otherwise, 13 mm is your minimum thickness. The code does not call this a nominal thickness or a corroded thickness so it is unclear what to do with corrosion allowance in this case. A technical inquiry may be a good idea, the API.org web site has a link to this form.

 

[aamir2003]

Dear All,

Please note our client reply and advise, we are actually in a fix now due to schedule constrains

I wrote to them after consulting above points, my reply is in pint 1 to 3

Hi Aamir,

Please find our comments below on the proposed reasoning.

Since we can’t afford to wait for further API committee response, please change the design of the transition plate for both tanks to 16 mm thk.

As per info we received, API committee may take up to 1 year to response to an inquiry.



1.) As transition plate has to joint with other annular plates with backing strips, the first shell course shall sit evenly along annular plate as well as transition plates. (API has no drawing showing how to design between 1stshell course and different thickness of annular plates.)

Client : Agree that the first shell course shall sit evenly along annular plate as well as transition plates. To achieve this, we could also follow weld designs as per API 650 figure 5.14.




GP-09-04-01 para 5.8 refers to annular plate (not bottom-transition plate) which is already considered 10mm thick plate ---->based on API 650 table 5.1a, minimum required annular plate thickness is 9mm considering 3mm corrosion allowance. (6mm + 3mm = 9mm < 10mm), hence it can be concluded that the required minimum thickness of bottom and annular plates are greater and not less than of the values computed

Client :As per our US based consultant, the bottom transition plate minimum thickness requirement is 13 mm as per API 650 5.7.8.1 e. This is exclusive of CA as per para 5.3.2 implies. Furthermore API do not deal with CA as CA is highly depended on the external environment and corrosiveness of stored liquid and bottom soil.

While API 650 5.7.8.1 e mentioned thickness of annular plate, the thicker of both options should govern. We cannot establish the suitability of the thinner plate section when a minimum thickness has already been specified. Further query to API could confirm whether there are enough margin in the 13 mm min requirement which can be considered as CA.



2.) The nozzle external load are handled by bottom reinforcing plate (not to transition plate). The load exerted on transition plate is same as other annular plate. The thickness of annular plate and hence transition plate is more than required, it would have been a concern if annular plate thickness was e.g 14 mm and transition plate used was 13 mm . We requirement of 9 mm annular plate thickness and have used 13 mm for transition plate which is higher .

Client :As per our US based consultant, the 13 mm min requirement in API for bottom transition plate is determined by certain mechanical limitation (thicker plate is intended to reinforce the opening created by the flush nozzle). CMSB statement need to be clarified with API or proved by calculation or finite element analysis. OR else take 13 mm as min requirement which is specified in para 5.7.8.1 e. FYI, one of our existing crude tank, TK-1 has 19mm thk transition plate.


3.) API says only “min thickness 13mm”. Not says “ min thickness 13mm excluding corrosion allowance”. In the clauses where we need to take care of CA it is clearly spelled out in API , refer to snap shot below .


Client :API doesn’t deal with CA as it is highly depended on the external environment and corrosiveness of stored liquid and bottom soil. If acid water or solution is being stored, higher CA will be required than storing Lube oil for instance. Thus para 5.3.2 implies that CA to be applied on top of any calculation or minimum dimensional requirement

 

[IFRs]

API 650 5.7.8.1.e reads like this:

"The thickness, ta, of the bottom-transition plate in the assembly shall be 13 mm (1/2 in.) minimum or, when specified, the same as the thickness of the tank annular plate."

API generally intends that their statements are read as much as possibly literally, without "reading between the lines" or trying to figure out what they really meant or adding words that are not there. Statements with more than one clause should not be taken apart, the entire sentence should be read as one.

If they meant the greater of 1/2" and the annular plate thickness, they would have said that. If they meant the lesser of 1/2" and the annular plate thickness, they would have said that. If they meant "nominal thickness" or "corroded thickness" or "thickness exclusive of corrosion allowance" they would have said so. It is not up to us to try and figure out why the section was written as it was, but to follow it as much as possible exactly as it is written. If we think there is an error or a confusing statement, we submit an inquiry.

You could substitute colors for the selections, perhaps that would clarify it. The sentence then reads: "The thickness, ta, of the bottom-transition plate in the assembly shall be BLUE or RED. BLUE is 1/2" minimum and RED is when specified, the same as the thickness of the tank annular plate. Why did they allow you to increase BLUE beyond 1/2" by using the word "minimum" ? We don't know.

The section reads 1/2" or the annular plate thickness if an annular plate is specified. I read this exactly as it is written: if an annular plate is specified, use that thickness, if not, use 1/2". Pretty clean and clear. They used the word "or" and I take it to mean "or", not "the greater of" etc. Corrosion allowance is not mentioned so it is not intended to be applied. Note that corrosion allowance is a factor when determining the annular plate thickness so if an annular plate is specified and the transition plate is the same thickness, it has the same corrosion allowance. If an annular plate is not specified, then the minimum thickness is 1/2". You can make it thicker but not thinner.

On the other hand, the client is the final arbitrator on issues and questions, you may have to do as they ask.

 

[aamir2003]

Client has written this back

Dear Aamir,



We should still refer this to the API committee for correct and final interpretation and intent.



For now if we maintain the 13 mm as it is and proceed with fabrication, and if your interpretation is not as per API committee reply, CMSB need to make the necessary correction in the field. The complexity, cost and schedule impact to change the design will depend on at which stage the construction has completed. And worst possible scenario is for the mistake to be corrected after the tank has been commissioned.



If we change to 16 mm and if your interpretation is not as per API committee reply, no correction will be needed. The schedule impact to change the plate now is minimal considering we can make a door sheet at the flush type nozzle location, while erecting the higher courses.



We should not take the risk. Please change the design for both tanks to 16 mm thick bottom transition plate.

 

[IFRs]

I'm glad it was resolved, and have submitted an inquiry to API.

 

[aamir2003]

The matter is still not resolved as we have no reply committee

 

[Duwe6]

Rule #1 -- the Client is always right [assuming his checkbook is open]

Rule #2 -- When the Client or the Client's 'expert' cannot interpret the rules correctly, please refer to Rule #1.

and I agree with IFRs and your reading of the Code. The book says what it says - 'reading between the lines' is not allowed.