Significance of "-Mill Tolerance" in the Piping Stress Analysis

In the tube and pipe industry, the weld seam is nondestructive tested for defects larger than 12.5% of the wall thickness. Essentially, the spec allows for a wall thickness reduction of 12.5% due to weld inclusions and incomplete fusion. While it is obviously undesirable to have any weld defects, they are a fact of life that will eventually occur. Perhaps the allowance to reduce the wall thickness by 12.5% is related.

good luck

Any input on the corrosion allowance as a seperate reduction in tolerances. Even API 5L calls out the 12 1/2 % for line pipe, but working on pressure vessels or power piping we also factor in the corrosion allowance.

Dear Sir,

Regarding wall thickness, i need help. I have to design 48" API 5L X52 pipe as per ASME B31.3 (process piping) to be used in skid. Design press: 92 bars, design temp: 80 deg C, corrosion allowance: 3mm.

For wall thickness calculation, shall i consider hydrotest pressure (1.5 times design press) also? since we have do hydrotest for whole skid, or should i consider only design press + corrosion allowance + mill tolerance of +12.5% for wall thickness calculation?

Please help

In addition to Mr Abdel's answer, while calculating stresses for sustained load you should consider least section modulus (tn -12.5% of tn- CR ) and while calculating thermal loads (forces and moments) take higher thickness (nominal thickness + 12.5% of tn) and resisting section modulus is the same as for sustained loads.

Dear Guest,

Please note the following paragraph UG-16(d) of ASME code, Section VIII, Division 1, which explains where we don't take that tolerance in such a specific calculations for nozzles of pressure vessels fabricated from pipes/tubes in what we called "nozzle area of compensation", detailed at UG-37 REINFORCEMENT REQUIRED FOR OPENINGS IN SHELLS AND FORMED HEADS:

Quote

UG-16(d) Pipe Undertolerance. If pipe or tube is ordered by its nominal wall thickness, the manufacturing undertolerance on wall thickness shall be taken into account except for nozzle wall reinforcement area requirements in accordance with UG-37 and UG-40. The manufacturing undertolerances are given in the several pipe and tube specifications listed in the applicable Tables in Subsection C. After the minimum wall thickness is determined, it shall be increased by an amount sufficient to provide the manufacturing undertolerance allowed in the pipe or tube specification.

Unquote

Therefore, for pipe wall thickness calculation, we have to take the mill tolerance into consideration to find the nominal thickness, tn. But in nozzle area reinforcement stress analysis, we don't deduct the mill tolerance from the pipe wall nominal thickness (tn - 12.5%tn), only we take the corrosion allowance CA into consideration, (tn - CA).

The following statement was made by a previous poster:

"This tolerance is very important for designers when started to proceed the piping design, they have to take into consideration the worst case, I mean when the pipe supplied with a tolerance by minus 12.5% tn, to select the proper pipe wall thickness to compensate for any expected reduction in thickness of the pipe."

I think that statement is grossly off base. The piping Designer may do the design in Feburary, the stress analysis may be done in March, the total PDS (or PDMS) piping model is approved in June, the Isometrics downloaded in July, The Isometrics approved and issued to a pipe fabrication shop in August with a start of fabrication in September.

Now just how do we guarintee that a specific piece of pipe with a specific wall thickness will be used for that specific Isometric?

I would say that the stress analysis must be based on the wall schedule specified in the piping material spec and it must consider a margin of error for the mill tolerance.

Sorry, may be you don't understand quietly my statement: "The expected reduction in thickness of the pipe", it means that the pipe when procured as a new and you found that its actual thickness is less than the nominal pipe wall thickness, tn. For this expecting, we have to take the worst case for the mill tolerance -12.5%tn. And, as I mentioned at a previous post, the corrosion allowance (other than the pipe mill tolerance) must be taken into consideration. I mean you have to know the deterioration period of your piping system as a years, and you have to know the corrosion rate per year (depending on pipe material, temperature, and type of fluid and its concentration) from which you can evaluate the total corrosion allowance during the whole life, and this corrosion allowance must be added also to the pipe wall thickness like mill tolerance.

Therefore, I don't understand what do you mean with your question nor the relation between the design and fabrication stages with our case. In the same time, I think my explanation is very clear and familiar to whom interesting and experienced in piping design, fabrication, and erection.

Great clarification.

Thank you qaqcpipeman,

Great Clarification

Dear Sir,

I understand the mill tolerence, Thanks for your clarification,

Please specify the corrosion allowance, How we find out this .

What is the corrosion allowance of API 5L GrB . service is caustic soda.

Thanks and Regards

Vani.

The attached graph is Caustic Soda NAOH Service Graph, which is extracted from handbook "Corrosion Data Survey - Metals Section - 6th. Edition by NACE". The graph is a relation between the concentration of NAOH (% by weight) and its temperature which creates and indicates an areas A (carbon steel, no stress relief necessary), B (carbon steel, stress relieve welds), and C (application of nickel alloys to be considered) defines the type of metal recommended at each area.

So, for proper selection of type of metal and its corrosion rate, you have to define the concentration and temperature of NAOH.

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Dear

Understand piping first not a modelling

Mill tolerance reduce section modulas (calculated along with corrosion allowance) used in stress calculation.

Mill tolerance increase wt. + thermal forces based on potentially thicker wall.