Actual thickness and stress relief

[Syahar1975]

Good morning Gents,

Tank Engineers design calculation + CA shows the required thickness 12.5 mm. The material is Group IV.

We have purchase order and mill certificate shows 12.5 mm thickness.

When the plate arrived and inspected we found all thickness above 12.5 mm. Most of them above 12.7 mm.

The client insist us to stress relief all nozzle greater than 2" to satisfy API 650 requirement.

I argued with the client since the design requirement is 12.5 mm where stress relief is not required and there is also steel mill tolerance which is not relevant for stress relief requirement.

I am not sure why the tank designer does not aware this border line. I mean it is impossible to order to steel mill exactly 12.5 mm.

Any comments is highly appreciated.

 

[IFRs]

You are OK if you are using API 650.

API 650 Section 5.7.4.3 code says that "when the shell material is Group IV, IVA, V, or VI, all opening connections requiring reinforcement in a shell plate or thickened insert plate more than 13 mm (1/2 in.) thick shall be prefabricated into the shell plate or thickened insert plate, and the prefabricated assembly shall be thermally stress relieved..."

Note: the threshold thickness is "over 13mm" so you are good at 12.7mm

 

[Syahar1975]

IFRs,

The problem is th client refer to API 650 where 1/2" is 12.5 mm not 13 mm like in the latest edition. Maybe that is API 650 11th edition.

We are keep maintaining our position that stress relief is not required.

 

[IFRs]

Hopefully API will clarify this soon.

 

[valleytank]

You are permitted an under-run of not more than 0.3 mm (0.01 in.) from the computed
design thickness or the minimum permitted thickness of plates ordered. Now this does not say anything about being over... Personally I would agree that Stress-Relieving is not required for this thickness. Both 11 and 12th show 13mm as the cutoff.

 

[Syahar1975]

Thanks IFRS and Valley,

I found till 10th edition is 12.5 mm.

From 11th edition till latest one is 13 mm.

I will bring this issue to the meeting.