CSA Z662 Manufacturing Mill Tolerance in WT Calculation

[qwerpoiu]

I would like to understand why manufacturing mill tolerance is not required to be accounted for in the design wall thickness calculation in CSA Z662.

CSA Z662-11 Clause 4.3.5.1 provides an equation that can be used to calculate design wall thickness. Clause 4.3.11.1 states that “the nominal wall thickness shall be not less than the design wall thickness calculated from the formula in Clause 4.3.5.1…”, with a note indicating that “in determining the nominal wall thickness, manufacturing tolerances need not be considered.”

However, seamless pipe such as ASTM A106 Gr. B and A333 Gr. 6 can have an under-thickness tolerance of 12.5%. When design under other codes (such as ASME B31.3), manufacturing tolerance will need to be accounted for in wall thickness calculation.

From the definition section of CSA Z662, the nominal wall thickness is defined as “the specified wall thickness of the pipe purchased”. Does this mean that there can be chance the purchased pipe may have a thinner wall than the calculated design wall thickness due to mill tolerance? (Typically the next schedule pipe is procured, but there is also chance that the calculated design wall thickness is exactly the same as a standard schedule wall).

How does CSA Z662 account for this under-thickness tolerance from the manufacturing process? I am wondering if someone won’t mind explaining, thank you ahead for the help.

 

[BigInch]

The wall thickness of a pipe as delivered may in fact be up to 12.5% thinner than the nominal wall thickness requested before it must be taken into consideration in the pipe wall thickness formula.

Allowance for the -tolerance has already been considered and included in the wall thickness' formula design factor parameter.
This is the same provision for wall thickness tolerance you will find in US PIPELINE codes ASME B31.4 and B31.8