Can someone please guide me about why we don't consider Mill Tolerance while calculating pipeline wall thickness using ASME B31.8? Is there something to do with the Design Factor?
Member
Can someone please guide me about why we don't consider Mill Tolerance while calculating pipeline wall thickness using ASME B31.8? Is there something to do with the Design Factor?
Again my dear friend AlBaigMughal, where you go, you wll find me.
My answer shall be the same of my post at CR4:[link Point to another website Only the registered members can access]
Member
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,Originally Posted by AlBaigMughal
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
"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%.
Last edited by Abdel Halim Galala; 07-19-2011 at 06:57 AM.
Member
Dear Sir,
Sorry for being so late. I was out of town and so, couldn't respond.
This is all right what you tried to tell me. I am myself of the view that we should include the mill tolerance but i believe ASME B31.8 doesn't recommend us. You can see the definition of "t" in section 804.222 as:
"Nominal wall thickness, t, is the wall thickness computed by or used in the design equation in para. 841.11 or A842.221 in Chapter VIII. Under this Code, pipe may be ordered to this computed wall thickness without adding allowance to compensate for the underthickness tolerance permitted in approved specifications."
Now, this is again something very confusing. Anyways, I still recommend to incorporate Mill Tolerance so that we may have a safer result.
Thank you very much for your continued support. Hope to have this always,
AlBaigMughal
"Anyways, I still recommend to incorporate Mill Tolerance so that we may have a safer result".
That is good.
"Anyways, I still recommend to incorporate Mill Tolerance so that we may have a safer result".
That is good.
Posting Permissions
Reply With Quote
Bookmarks