pipe& fitting thickness mill tolerance

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13 years 10 months ago - 13 years 10 months ago #7843 by ven
hi, in pipes and fittings thickness calculation(as per ASME B31.3) for semless pipes we go for mill tolerance of 12.5% in general. what is the value if it is welded pipe? from where i can take that tolerance value ?it is in "%"or in "mm" or "inch" will it varies from material to material ?kindly help me out.

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13 years 10 months ago - 13 years 10 months ago #5800 by 2ndMoment
Replied by 2ndMoment on topic Re: pipe& fitting thickness mill tolerance
Hi Ven,

Common practice, in the absence of any references, is to assume the mill tolerance for wall thickness for seamless piping to be 12.5%. This is, in fact, the figure quoted by ASTM A53 a standard which incidentally applies to both seamless and welded pipe. This means that the piping wall thickness may vary by 12.5% or, stated differently, the ratio of the thinnest portion of pipe wall to the nominal (published) thickness is no less than 0.875. This does implicitly suggest that the ratio of thickest portion of the pipe to the nominal (published) thickness should not exceed 1.125.

A53 provides the following equation:

tn x 0.875 = tm; Where: tn is the nominal wall thickness and tm is the minimum wall thickness allowed

A further interesting requirement indirectly affecting minimum wall thickness relates to the weight and the variation permitted thereof. A53 requires that the weight - as measured per bulk load or bulk lift of pipe - may not deviate by more (or less) than 10%. Now this affects the pipe wall thickness indirectly in the following way: even though Mr. Supplier guarantees that his wall thickness will not be less than 12.5% at any place around the pipe, he may actually be trying to sell a pipe that is 0.875 of the nominal wall thickness all or most of the way around i.e. not necessarily having a pipe with an equal thicker and a thinner portion. Thus if the wall thickness is quite constant the 10% figure would apply to bring the average weight within limits

Although the 12.5% rule applies to wrought fittings’ wall thickness too, I don’t believe that fittings would ordinarily have the same requirements for weight, as this could, quite possibly, be difficult to reconcile with, say, elbows having a necessarily thicker intrados.

The above relates to A53 pipe which I chose as being relatively common and broadly representative. Each piping material standard will have its own requirements and allowances, and this MUST be referenced before blindly assuming a “rule of thumb”. As an example, API 5L mirrors the 12.5%, but only for piping thicknesses above 4mm and less than 25mm. Below 4mm an allowance of 0.5mm is permitted.

A recurring phrase appears throughout most pipe material standards, example ASTM, BS, etc. Typically: "The wall thickness is permitted to deviate by X% or Ymm, or by whichever is agreed by purschaser and supplier, or which appears on the purchase order". This is why many seasoned engineering contractor firms will have (seemingly duplicate) in-house specifications for pipe material which is governed by international standards anyway. If the firm requires more stringent tolerances they will reference their own specification on the purchase order and / or piping material specification datasheet, instead of simply stating A53 shall apply.

Hope this explanation suffices
2M

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13 years 10 months ago - 13 years 10 months ago #5801 by ven
Dear Sir,
Thank you.

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