Mill Tolerance

[tabbymulla]

Hello Everyone,

I was modeling on AutoPIPE when I came across mill tolerance.

According to my understanding, Mill Tolerance is the tolerance for variation in the thickness of pipe from nominal pipe thickness which is 12.5% according to B31.3. i.e for NPS 10" and thickness 6.35 mm it is 0.794 mm

Now from the ASTM material specification sheet of A358 we have

"8.1.4 Thickness—The minimum wall thickness at any point
in the pipe shall not be more than 0.01 in. [0.3 mm] under the
nominal thickness."

I understand that this means a Mill tolerance of 0.3mm.

Question is can I use this Mill tolerance of 0.3mm instead of the B31.3 mill tolerance of 0.794mm.

Or these two things are not same? Please.

Regards,

Tabby

 

[metengr]

First, you need to review ASTM A 999 as a reference document to A 358 to understand the definition of minimum wall thickness and nominal wall thickness. Mill tolerance cannot exceed 12.5%.

t[sub]min[/sub] = 0.875 x t[sub]nom[/sub]

for example t[sub]nom[/sub] is 0.300" (7.62mm)
t[sub]min[/sub] is 0.262" (6.65mm)

So, depending on how pipe is ordered in the PO or contract document can be either minimum wall (absolute value) or nominal wall thickness (up to 12.5% tolerance but not below minimum wall).

Now, going back to ASTM A 358, by this specification the minimum wall thickness at any point in the pipe shall not be more than 0.010" (0.3mm) under nominal thickness.

 

[tabbymulla]

Lets say order is an A358 pipe with Nominal Wall Thickness of 7.62 mm.

Then if the thickness at a place is found 6.65 mm, it is within the B31.3 code limitis.

But is it within the ASTM A358 specifications? Please

Regards,

Tabby

 

[metengr]

According to ASTM A 358, the minimum thickness measured cannot exceed 0.3mm under nominal thickness. So, this means if nominal is 7.62mm, the tolerance is much tighter for minimum thickness at 7.32mm. So, if you have material produced to A 358 and a minimum thickness measures 6.65mm, you have a nonconformance.

 

[tabbymulla]

So should I use the Mill tolerance of 0.3 mm for a B31.3 Piping Design using A358 Pipe?