Plate Material and Mill Undertolerance

Revised..

My required thickness is 21.47 mm (including corrosion allowance)

Lets check using mill undertolerance 6% of nominal thickness (lets say: 25 mm)

Mill undertolerance = 6% x 25 = 1.5 mm

Then, my thickness is 25 - 1.5 = 23.5 mm

Since the thickness is 23.5 mm (bigger than the required thickness, 21.7 mm), so my thickness is safe.

Thanks all..

If you order an ASME spec pressure vessel plate that references SA-20 (where I can't think of one that doesn't ....) - it will have an undertolerance of 0.3mm (0.01").

What that clause states is this:

For example - let's say that your minimum required thickness, including corrosion allowance is 24.999 mm - you may order/use 25 mm plate (that references SA-20) and be fine - there is no need for the check that you have used as an example.

Indeed. If your required thickness is 21.47mm per the standard, you may order a 21.5mm thk plate or so (if it was available...) if the supplier assures you the 6% undertolerance or if he sells you the plate per the spec. That's because the standard has enough safety factor built in to take it in account. Rgds.

Thanks Mr XMech & Cobar..

So, let me conclude this matter, that it is not necessary to add 6% of the ordered thickness.

If the required thickness is 21.47 mm, i simply order the greater thickness that available in the market, let's say 21.5 mm. So, i do not need order 23 mm thickness of which fulfills the ASME code - greater 6% of the ordered thickness (6% x 23 mm = 1.38 mm, then the thickness 23 - 1.38 = 21.62 mm, safe thickness because 21.62 mm is little greater than 21.47 mm)

Am i correct Sir?

Then, what does ASME mean on that Para 4.1.3.2?

Any comments would be excellent. Thank you..

P.S: The material is SA-516 Grade 70 (for propane storage)

The responses to date don't seem to be taking the value of 0.01 in. (0.25 mm) into account and instead only focus on the 6% value.

I'm not a user of ASME, however, the quote states "... not more than the smaller value of 0.01 in. (0.25 mm) or 6% of the ordered thickness ...". Therefore, to my way of thinking, the allowance is the smaller of 0.25 mm or 1.5 mm (6%) and 0.25 mm is smaller than 1.5 mm i.e. your allowance is 0.25 mm.

I initially posted this as a guest in error. Just setting the record straight.

The responses to date don't seem to be taking the value of 0.01 in. (0.25 mm) into account and instead only focus on the 6% value.

I'm not a user of ASME, however, the quote states "... not more than the smaller value of 0.01 in. (0.25 mm) or 6% of the ordered thickness ...". Therefore, to my way of thinking, the allowance is the smaller of 0.25 mm or 1.5 mm (6%) and 0.25 mm is smaller than 1.5 mm i.e. your allowance is 0.25 mm.

That is not quite accurate -

From SA-20 the undertolerance of SA 516-70 is 0.3 mm.

Your minimum required thickness is 21.47 mm.

Therefore you may order the next commercially/readily available thickness - 22 mm. (If not readily available go next step higher 25 mm)

6% of 22 mm = 1.32 mm.

Quote from Div 2, with my highlights and [notes]

"Vessels made of plate furnished with an undertolerance of not more than the smaller value of [3] 0.3 mm (0.01 in.) [1] or 6% of the ordered thickness [2] may be used at the full maximum allowable working pressure for the thickness ordered [4]."

[1] this is your case - SA 516-70 has an undertolerance of 0.3 mm

[2] 6% of 22 mm is 1.32 mm

[3] "the smaller value of" → 0.3 mm is smaller than 1.32 mm; "not more than the smaller value" → from [1]: 0.3 mm = 0.3 mm, that is - 0.3 mm is not more than 0.3 mm

[4] you may order plate at 22 mm and calculate MAWP as a thickness of 22 mm.

Ok i understand Sir.. Thank you very much.. Mill undertolerance of my SA 516-70 is 0.3 mm, because 0.3 mm is smaller than 1.32 and 22 mm - 0.3 mm is STILL less than my required thickness. Is the underlined statement correct?

Sir, let's take another example, not related anymore to the previous case.

What if the ordered thickness is 3.4 mm and my required thickness is 3.36 mm. Then 6% of 3.4 mm is 0.204 mm

Let's combine it with quote from ASME, "not more than the smaller value of 0.3 mm OR 0.204 mm"

Then, the undertolerance is 0.204 mm right?

BUT, since 3.4 mm - 0.204 mm is 3.19 mm (greater than the required thickness, 3.36 mm), i shall increase the ordered thickness to 3.6 mm.

6% of 3.6 mm is 0.216 mm. Because the 0.216 is smaller than 0.3, the undertolerance is 0.216 mm.

Let's check, 3.6 mm - 0.216 mm = 3.38 mm.

Since the 3.38 mm is greater than 3.36 mm, the ordered thickness of 3.6 is ACCEPTABLE.

Is that correct Sir?

Sorry for asking so detail.

I am just afraid to make mistake in translating ASME code statement.

Thank you very much..

I do not understand your underlined statement. But I will say this, following the checks in post #7, you DO NOT need to worry about any undertolerance. Do not bother subtracting⁄adding⁄..... anything!!! Order 22 mm plate and use the exact value 22 mm in your MAWP calculation. That´s it, you´re done, finished, end of story, no more discussion, ......... except for: thinning due to forming, bending, rolling. You must account for that and that is not an exact science - talk to your steel supplier.

Now for the second part - (I think it is pointless to discuss it)

You will probably find it difficult to get SA516-70 in anything less than 5mm - so this reasoning⁄checking is a moot point. 6% of 5mm = 0.3 mm.

If you need to go that thin you would most likely be looking at a different product form such as sheet, which would have different tolerances.

Sorry for the late reply..

Thanks Sir.. I understand, i just want to make sure..

Cheers..