Thickness Tolerance of Large size pipes and Fittings

[Santha]

Hi Everybody

I have a question on the tolerance you apply for pipes and fittings of size greater than 24". The mill tolerance on pipes and fittings upto 24" is minus 12.5%. But for sizes >24", invariably they are rolled from plates and so the thickness tolerance is not always minus 12.5%. By and large it is 0.01" and so on. But for the fittings above 24" size, per ASME B16.9, it is still minus 12.5% only. Now while specifying the pipes and fittings for 26" and up, we do end up specifying a thicker fitting to include a higher tolerance. Does this make sense. I want your opinion on this.

Regards
Santha

 

[TerryOsser]

If you are concerned about the mill tolerance on fittings (in that it will affect the minimum wall thickness, and therefore increase your actual wall thicknesses) write an additional clause (or addendum) to your Piping Specification or whatever document you are using as a project reference / procurement specification and within that document state what negative tolerance YOU require. Get the supplier to confirm this prior to the order. Check during final inspection prior to shipping. This is reasonably common pratice, especially for exotic materials.
Alternatively accept the ASME standard negative tolerances.

 

[Santha]

Hi TerryOsser
Thanks for your immediate response. Writing a specific addendum requiring a lower tolerance is a good solution. But in doing so, there is a strong possibility that you will end up paying more for the fittings as these are made to order and may not be available off the shelf. I want to know what is the practice followed by materials specifiers in a situation like this. Do they go for a special lower tolerance for the fittings or procure the standard fittings thereby buying a fitting thicker than the corresponding pipe. Does the CODE give any guideline on this.

Thanks
Santha

 

[TerryOsser]

It depends where you using the pipe - offshore or onshore, and what the material is.
Offshore weight is prime importance - there reduce tolerances and reduce weight. Same principle for exotic materials - reduce tolerances and reduce weight = reduced cost if the items are being made to order. For 'off the shelf' items (costing potentially less) standard thicknesses / available thicknesses are used. You can always specify one wall thickness and accept a thicker wall thickness if it is more widely available.
Looking through the ASME B16.9 interpretations re-tolerances it seems that if the fitting is to B16.9 the tolerances must be adhered to. Doesnt mean (as discussed in earlier thread)you can't be more stringent.

 

[NozzleTwister]

Let's resurrect this old thread about mill tolerances for piping components >24".

Let's say I have a general Piping Spec, Class 300, 1 1/4%Cr-1/2%Mo for pipe up to 48" dia. Piping Code: ASME B31.3

Above 24" dia.:
PIPE: ASTM A691 Gr. 11 CL.22
MILL TOL.: 0.01" Under Nominal Wall
FITTINGS (EBLOWS & TEES): ASTM A234 Gr. WP11W
MILL TOL.: Per ASME B16.9, Minimum thicknes is 87.5% of Nominal Wall

For talking purposes, let's say I require 1.00" nominal wall thickness pipe in the 48" size. Obviuosly with the difference in Mill Tollerance a 1.00" nominal wall elbow can't be rated for the same pressure.

My concern is not so much about how I will buy these fittings today, but about someone using this piping specification and procuring fittings in the future. What is the normal practice for dealing with this situation when writting Piping Material Specifications.

A) Specify Pipe & Fittings by calulated nominal pipe wall and ignore the differing tolernaces of the fittings.

B) Specify one nominal wall for the pipe and a different nominal wall as/if required for the fittings.

C) Specify the required nominal wall for the pipe and the required minimum wall for the matching fittings as suggested above.

I've looked at some old pipng specifications and I've seen 'Bore to Match Pipe' on fittings in this situation with no notes about tolerances or minimum wall and I don't think that is correct.

Please advise about what is the most common industry practices the dealing with this situation.

Best regards,


NozzleTwister
Houston, Texas