Mill Tolerance and ASME B31.8

[Abdel Halim Galala]

Sir! Good to see you here too

Basically, one of the approaches I recently went through was that "Mill Tolerance effects are built into the design factor", and so don't need to be incorporated by dividing the t by 0.875. The reference is a book "Piping and Pipeline Engineering Design, Construction, Maintenance, Integrity, and Repair By George A. Antaki". According to 4.1.2 Pipeline Design Equation, "90% SY x 0.875 /1.1= 0.72 SY" where 90% is Hydrostatic Mill Test Pressure, 0.875 [= 100% - 12.5%] and 1.1 is the "Over-Pressurization Factor". So by this, we get 0.72 factor, and all the remaining factors w.r.t different locations will be less than 0.72 as defined by Table 841.114A ASME B31.8.

Against this claim, I wanted to find the relationship between Mill Tolerance and Design Factor in somehow more detail.

Please have a look at this text in the referred book and thanks in advance for helping me out in clarifying this.

AlBaigMughal

From the text book: "Piping and Pipeline Engineering Design, Construction, Maintenance, Integrity, and Repair By George A. Antaki", Page 106,

"There are two equally plausible versions of the origin of 0.72 Sy. The first explanation is that 0.72 SY goes back to the early days of fabrication of steel line pipe. In the mill, the pipe was tested to a hydrostatic pressure causing a hoop stress PD/(2t) of 90% Sy. In service, the pressure was limited to 80% of the mill hydrotest pressure, or 80% x 90% Sy = 72% SY. The second explanation is that the 90% Sy hydrostatic test was reduced by 12.5% for fabrication tolerance on underthickness, then further divided by 1.1 to compensate for the 110% overpressure transient allowance (as was the common practice for water pipelines), which leads to 90% SY x 0.875 /1.1= 0.72 SY".

It is just an explanation for two cenarios, and if that is true, the ASME 31.8 must clearly indicate that instruction. What will you do if you considered only the 1st cenario?
I recoomend to use a mill tolerance 12.5%.