CODE CASE 2596 - Cold stretching 1

[claudiosanga]

Good morning everybody,
I have a question for you.
After a cold stretching performed on the internal vessel of a criogenic tank, we found that the real (measured) thickness is lower than the calculated one considering the allowable design stress according to the table 1 of the code case 2596-1.
Particularly:
Measured thickness: 9.58 mm
Calculated thickness: 9.72 mm
In my opinion this procedure (real thickness < calculated thickness) is not correct and I didn't accept the tank.
The manufacturer claims that this procedure is correct because the code does not say expressly that the thickness after deformation must be measured and compared with that of computing.
What do you think? You would accept a real thickness lower than the one calculated according the code case 2596-1?

 

[metengr]

claudiosanga;
Read the Code Case. There is nothing incorrect about this methodology. I have a copy of the latest code case revisions from ASME C&S connect and it clearly states under Design 4.0, "Wall thicknesses shall be calculated according to the applicable rules of ASME Section VIII before cold stretching. The maximum design stress values are given in Table I."

So, the bottom line is before cold stretching a minimum design wall thickness is calculated by using the Code Case allowable stress values. The allowable stress values were based on controlled cold stretched procedures to take advantage of work hardening, which increases tensile and yield strengths. Yes, the wall thickness will become thinner because of the controlled deformation from cold stretching.

Knowing the wall thickness will become thinner after proper cold stretching and the material increases in strength, what is the point of re-calculating wall thickness after stretching???

PS; There is a detailed file on cold stretching of pressure vessels which is over 74 pages. The technology file is detailed and describes the method and its limitations. Many of the cryogenic tanks in Europe and other locations have used this technology for years. Also, attached to this file is Swedish Cold-Stretching Code from 1975.

 

[claudiosanga]

Metengr, thanks for your prompt answer.

The code case is really clear and it states that for the cold stretching operation you can use the same formula before the stretching. It's logic because you don't know the stretched thickness a priori and so you use a well known value: the one before cold stretching.
Take the case I reported above: the supplier calculated the thickness considering an allowable stress value of 410 MPa (according the table 1 of the code case) and the minimum thickness results 9.72 mm. Then the supplier measured the thickness and found 9.58 mm. In my opinion to accept a value of 9.58 it means to accept an higher allowable stress value. And this is not possible.

What if the thickness after the cold stretching was 6 mm (I write this very low value only for exempla gratia) instead of 9.58?
Since normally the reduction of thickness for the stretching is between 3%-5% (see EN13458-2) it should be better to consider this percentage while purchasing the material to avoid that the real thickness is thinner than the calculated one after the cold stretching.

Whhat's file you are talking about? where can i find it?

 

[metengr]

claudiosanga;
The file I mentioned is only available to code committee members. The reason I mentioned it is because there was significant effort into drafting the code case.

In my opinion to accept a value of 9.58 it means to accept an higher allowable stress value. And this is not possible.

Do you realize the tensile strength properties of the cold stretched material will be increased by as much as 30% over annealed base material? I have seen actual data from cold stretched vessels that were subject to destructive examination, sapart of the item for code case development.

 

[claudiosanga]

thanks Metengr for your clarification.
I know that a Technical Inquiry has been sent to the ASME Committee to clarify this point. If it could be of your interest I'll return to you as soon as I have the answer.