"Actual thickness" acc. to ASME

[Lukasz1]

Hello

Can someone be so kind to explain to me what is the definition of term "actual thickness" acc. to ASME Code? I am specific interested in Section IV HF-301.1(c)(3) and (c)(4) requirements.

Is it referring to thickness before fabrication (oredered material thickness) or to thickness measured after fabrication of completed vessel (including thinning due to welding acc. to HW-820.1)?

Best Regards

Lukas

 

[GenB]

It should be the mom less c.a. less thin out, if pipe less mfg tol. Genblr.

 

[racookpe1978]

You're using mom as minimum ordered material?

 

[Lukasz1]

Thank you for your reply.

I have tried to search for definition of "actual thickness" in the Code but couldn't find any. If Code states 2,5mm (actual thickness) i would like to understand what does it mean.

For me this may be understood in two ways:

1) Nominal thickness (ordered) minus corrosion allowance and minus manufacturing undertolerance (acc. to material specification). This means that ordered material need to have nominal thickness thick enough that when we reduce its thickness by the mentioned ealier factors it will be still thick enough to be equal to or above minimum required actual thickness. That is why i asked ealier if "actual thickness" is reffering to thickness of ordered material before fabrication stage. Or....

2) "Actual thickness" of considered part shall be measured after fabrication of completed vessel. This means that we need to consider not only factors mentioned above but also factors that may happen during fabrication stage, like: thinning due to forming or thinning due to welding. This would mean that at any point of considered part when the thicknees will be measured it thickness shall not be smaller than minimum required actual thickness. This would make nominal thickness of oredered material even more thicker.

I am not sure which one of this two options is correct. I hope someone will help me to understand this.

Regards

Lucas

 

[gr2vessels]

Lukasz1, the actual thickness is the material thickness you purchased and you are using it to fabricate the equipment. You have calculated the minimum thickness, including all the allowances, then you selected the next available material thickness. The code does not state the actual thickness, because that's subject to availability, particular manufacturing tolerances, etc. You might calculated the minimum thickness in accordance with the code say, 6.2 mm but the available material is 10 mm, because the 8 mm was sold out. The 10 mm plate is the actual thickness. Conversely, if the minimum plate thickness after forming the dished end is 8 mm, the manufacturer of the dished end will advise the actual material thickness that he is using is 1/2" (12.7 mm). In extreme case, you might have to accept 16 mm plate actual thickness, because it is the only available plate at the time of purchase.

 

[racookpe1978]

OK, so - to make sure I understand what you've said:
If the minimum calculated thickness (pressure resistance + corrosion allowance for example) = 6.2 mm, then I'd need to specify 8 mm or 10 mm as you pointed out. But, if that 8 mm purchased thickness shrunk to 6.0 mm during the pressure forming, I would be under-strength at that thin point: maybe Ok everywhere else, but not at that particular place along the tank head.

Thus, in this case, I'd need to actually specify 10 mm, and the actual thickness might be 10, or 12 or even 14 mm. NOT 10-2 mm shrink, 12 - 2 mm shrink, or 14 - 2 mm shrink, right? (Nor 10.0 - 0.1 mm (measured tolerance) either, nor 10.0 + 0.1 mm measured tolerance.)

 

[GenB]

Rakookpe. I meant. Nom. For nominal less...

 

[Lukasz1]

Thank you for all your help.

According to what gr2vessels wrote i understand that "actual thicknees" is reffering to thickness of purchased material which is used to manufacture the equipment. This means i need to calculate minimum required thickness for pressure and add all required allowances to obtain the final minimum thickness which need to be purchased. Which allowances i need to consider to be in accordance with "actual thickness"? What is the difference between actual and nominal thickness of the material?

Don't get me wrong but i'm little confuesed here. For me it looks like "actual thicknes" may depend on many different factors and due to the reason there is no clear definition of its meaning it may be understood differently by people. I think that this term exists in the Code along with term nominal thickess for a reason and i would like to understand what is this reason.

When i calculate min. required thickness for pressure I can add corrosion allowance, manufacturing tolerance, thinnig due to forming (if calculating a head for example) but do i need to add allowance for thininig due to welding (which may happen during fabrication) if acc. to Code such thinning is acceptable within 10% of nominal thickness?

Racookpe gave a good example. Let me try to extend it a little more:

If someone is calculating a shell made from a rolled SA-240 plate (longitudinally welded with LBW process "without" additional filler metal) and adds only corrosion allowance and manufacturing undertolerance (acc. to material specification) to obtain thickness of material which needs to be ordered... is it "actual thickness"? During fabriacation stage other things may happen like: thinning due to rolling, thinnig due to welding (remember i mentioned welding with laser without additional filler metal). Do i need to also consider them?

If Code states 2,5mm (actual thickness) and i buy 3mm plate:

1) If i only consider corrosion allowance ( which is equal to 0) and manufacturing undertolerance i am OK and 3mm thickness will be sufficient.
2) If i also consider allowances mentioned above like thinnig due to rolling and welding i may drop below 2,5mm during fabrication. I will be still above minimum required thickness due to pressure but i will be below required actual thickness. This could mean i need to buy 4mm plate thickness.

I am worried that "actual thickness" may be verified after a vessel is completed. I do not want a situation where AI will demand from us to verify the thickness of pressure part under consideration and it will be below the required actual thickness due to production process.

I hope you understand me.

Regards

Lukas

 

[gr2vessels]

Lukas, you are really going out of rails. Try to understand plain English.
There is a code (you need to read it first) detailing you how to calculate the minimum thickness of a material to resist the design conditions. Full stop.
Then you have to buy the material with thickness no less than the minimum calculated thickness. Full stop.
The material you buy is coming with standard under-tolerance, or thinning allowance specified by the material manufacturer or even the code specified under-tolerance for the standard pipe wall thickness. That is the actual thickness of the material. The material you substract from the thinning allowances or fabrication allowances has an initial, tabulated/listed thickness called nominal thickness. When the actual thickness is measured, it is the nominal thickness minus/plus the fabrication tolerance, depending on the fabrication standard. You need to read separately the code requirements, then read the various materials (plate, pipe wall thickness/schedule and their fabrication tolerances). For a dished formed end, the fabricator has to use a plate with a nominal thickness sufficient large that after forming the final thickness will be not less than your calculated minimum thickness. That nominal thickness varies depending on the forming process, manufacturing technology or even the fabricator ability to maximize the use of plate material. Hope this helps.