Corrosion Allowance
Corrosion Allowance
(OP)
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.
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.





RE: Corrosion Allowance
RE: Corrosion Allowance
On the other hand, if the spec requires SI units, the vendor should provide a calc in SI units.
RE: Corrosion Allowance
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
RE: Corrosion Allowance
RE: Corrosion Allowance
RE: Corrosion Allowance
"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.
RE: Corrosion Allowance
1/8 in.= 3 mm
1/4 in.= 6 mm
1/2 in.= 13 mm
Regards
r6155
RE: Corrosion Allowance
RE: Corrosion Allowance
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
RE: Corrosion Allowance
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
RE: Corrosion Allowance
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
RE: Corrosion Allowance
You should specify "Minimum required thickness=25,376 mm" as additional requirement in the Ordering Information.
Regards
r6155
RE: Corrosion Allowance
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?
RE: Corrosion Allowance
RE: Corrosion Allowance
RE: Corrosion Allowance
RE: Corrosion Allowance
RE: Corrosion Allowance
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".
RE: Corrosion Allowance
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).
RE: Corrosion Allowance
Regards,
Mike
RE: Corrosion Allowance
RE: Corrosion Allowance
Business was done with a handshake, and if there were clerical, or actual quality issues, they were resolved without a customer holding payment hostage in order to get service.
We had a customer last year request a $750.00 credit for "engineering time" because we submitted a U-1 form with a typo on it: we listed a 14" nozzle when the approved drawing, as built drawing, and actual unit had the correct 12" nozzle. I wonder if it was the same guy who says 1/8" will not work for 3.2mm.
-TJ Orlowski