B31.8 PIPE THICKNESS 2

[B.L.Smith]

HI

In B31.3 C.A. and Mill tolerance are applied for calculating pipe thickness but in B31.8, I have not seen these parameters in calculating pipe thickness(suppose the pipe material is API 5L). Shall we use C.A and Mill tolerance for pipe thickness calculation for a gas pipelines? If yes, why B31.8 doesn't mention them?

 

[BigInch]

You should add the corrosion allowance to the calculated wall thickness, but not the mill tolerance.
Mill tolerance for pipe made and certified according to one of the secifications listed by B31.8 is already included within the safety factor associated with the DF required for the respective area classes. That is stated within the code. Specific corrosion allowances are not required by the code, however if you have knowledge or reasonable suspicion that corrosion, or any other agent or effect not specifically covered or included in the code by name, that could have some reasonably possible detrimental effect to the design resulting from following the B31.8 code, you have an additional responsibility to make the design just and adequate to accommodate those effects as well.

Independent events are seldomly independent.

 

[LittleInch]

B 31.8 specifies this in the definitions of nominal wall thickness in 804.5 that manufacturing tolerance is allowed for when using the line pipe and materials codes specified in the code, i.e APIL 5L. B 31.8, as its title suggests is intended for use for gas transmission and distribution. This gas should really be dry and water free. Section 802.1 b 6 actually states that the code does NOT APPLY to, amongst other things, flowlines, which is where most corrosion allowance issues occur. I will accept it is often used for this purpose, but it was not designed for this use and hence corrosion allownace is not a key considereation. Big Inch is totally correct that apart from under tolerence, everything else need to be added to the nominal thickness generated from the code.

 

[BigInch]

Also applies to B31.4. Interpretation 4-76.

Independent events are seldomly independent.