Corrosion Allowance 3

[aiwa7777]

Hi Fellows,
We have issued a purchase order for a ASME U stamped pressure vessel. Specification states that SI units shall be used.

Vendor has provided calcs in imperial units with a corrosion allowance of 1/8". We have specified a corrosion allowance of 3.2 mm in purchase order.

Vendor replies that 1/8" CA is acceptable for 3.2 mm as 1/8" is imperial equivalent of 3.2 mm.

Owner is not accepting this as Owner says that 1/8" is not equivalent to 3.2 mm as 1/8" = 3.175 mm which is less than 3.2 mm.

Please provide your thoughts.

Thank you.

 

[blacksmith37]

Unfortunately you are dealing with an incompetent owner/ customer; and the "customer is always right". Maybe 3.3 mm will make him happy. The disagreement results from "hard" vs "soft" conversions; hopefully it is specified in the contract.

 

[jamesl]

Seems to me that the owner's engineer is nitpicking. When we work on vessel data sheets, 1/8" CA is equivalent to 3mm. This is also the recommended conversion by the code. The difference between 3.175mm and 3.2mm is negligible in vessel design.

On the other hand, if the spec requires SI units, the vendor should provide a calc in SI units.

 

[vesselfab]

Have him use 0.1259842519685"

or .126"

.001 should not affect the overall design much at all.

will make the client happy

if he uses commercial software he can do his calcs in si units ant then convert to imperial for his people

 

[EngAddict]

They did not specify 3.200 so what you have is correct for 3.2 to 1 dp. I bet the reason their spec says 3.2 is because the person who wrote it had rounded 3.175 to 3.2 thinking it was too precious to need to specify 3.175. Guess they has no idea who would be interpreting it down the track.

 

[MrBTU]

The customer may not always be right, but the customer is always the customer - and at the end of the day, you have to keep them happy or lose them. I would re-do the calcs, and if possible, make all the c.a. come out to 3.2 or greater, and if not, let the customer know which components do not meet it and what they do meet and ask them where they want you to go from here. Chances are after they see that you intend to please, they will be OK with what you have designed.

 

[trottiey]

Your customer is right based on paragraph U-4:
"any conversions necessary for verification of Code compliance and to ensure that dimensional consistency is maintained, shall be in accordance with the following:
(a) Conversion factors shall be accurate to at least four significant figures.
(b) The results of conversions of units shall be expressed to a minimum of three significant figures."

But yes, I would agree with everyone else that this is the kind of error that reasonable people usually overlook.

 

[r6155]

Please, see ASME VIII Div. 1 where

1/8 in.= 3 mm
1/4 in.= 6 mm
1/2 in.= 13 mm

Regards
r6155

 

[trottiey]

r6155, could you point out where in ASME VIII Div. 1 it says that? I'm aware of Appendix GG explaining that the code committee used those rough equivalencies to convert limits in the code, but users are held to U-4 as far as I know.

 

[TJOrlowski]

I'm not sure how this is a problem??

Is the vessel fabricator specifying the fabricated thickness on a component at 25.4mm when the required minimum thickness (including the CA) is 25.376mm? Just have the fabricator add .025mm to the required minimum thickness of all components with a corrosion allowance and be done with it.

You're talking about a difference of less than one-thousandth of an inch. Find a solution that doesn't change geometry or submitted component thicknesses and move on.

-TJ Orlowski

 

[r6155]

trottiey

In PDF electronic edition of ASME VIII Div.1 use the search in your computer with 3 mm, 6 mm, or 13 mm

(ie.: UG-43;UG-83;UW-9; UW-16..........an a lot !!!!!)

Regards
r6155

 

[EdStainless]

I'll be honest, when I see a design that is native SI, then I do all of my work in SI.
When I see one that is obviously a conversion, then I work in Imperial and convert at he end.
Is this right, probably not. But I usually end up with a better 'fit' than if I do it the other way.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[r6155]

TJOrlowski
You should specify "Minimum required thickness=25,376 mm" as additional requirement in the Ordering Information.

Regards
r6155

 

[waterpipe]

What's the acceptable tolerances of the steel sheet manufacturing in your case? is there a chance to mention that tolerances and satisfy this microscopic costumer? it's am engineering work not an academic lab test.
And where this corrosion allowance come from? Perhaps getting back to its calculation and could show that 0.025 mm is negligible. What about the tolerances of the measuring tools?

 

[trottiey]

r6155, please see Appendix GG and paragraph U-4. It's true that ASME converted code limits of 1/8" to 3 mm, but they require code users to apply more precision to design values. They don't practice what they preach.

 

[roca]

Are we making a watch here or a fabricated pressure vessel?

 

[roca]

what about the plate tolerance? Doesn't this exceed the missing 0.025mm (0.000984")corrosion allowance?

 

[metengr]

The bottom line is what the contract states, and the Owner is correct if SI units were specified. You can argue until the cows come home. Obviously, the vendor did not read the contract specification carefully.

 

[Duwe6]

A recalc will 'find' the missing 0.001" and bring the C.
A. to 3.2mm. There is ALWAYS a little extra between the minimum delivered plate thickness and the pressure-calculated Tmin plus C.A. Talk nice to the engr that did the original calc, and he will 'find' you your needed 0.001".

 

[TVSN]

hi aiwa7777,

i hope after adding the corrosion allowance to the calculated vessel thickness, definitely you would have rounded off it to have a nearest integer or to obtain the commercially available plate thickness, then in that case you are increasing the plate thickness more than what actually required (including the corrosion allowance).