RE: MILL TOLERANCE

From: <James>
Date: Thu Apr 08 2004 - 06:01:00 EDT

-----Original Message-----
From: <a href="/group/PipingDesign/post?postID=hTlv-bFeW2t4py0ApUidpgr95IcSajv27dHhpiMHIWWvpFgZ1fjFik-39FK5IXuYITYyCFrwHcYJ1rmz4xHlqHlIWw">Jagarlamudi.Suresh@ril.com</a> [mailto:<a href="/group/PipingDesign/post?postID=hTlv-bFeW2t4py0ApUidpgr95IcSajv27dHhpiMHIWWvpFgZ1fjFik-39FK5IXuYITYyCFrwHcYJ1rmz4xHlqHlIWw">Jagarlamudi.Suresh@ril.com</a>] Sent: 08 April 2004 10:31
To: <a href="/group/PipingDesign/post?postID=fo7zQDDtua4ubuLkehVJp-vNB6lVy2sM5nFgI4l3xxH_JAjp4ISQBD008vfkTEVRvzDSAdrO3bhsarx0TW1_EHXCbvyG">PipingDesign@yahoogroups.com</a> Subject: [PipingDesign] MILL TOLERANCE

I have basic doubt
whether any piping analysis software takes care regarding mill tolerance?
I am working on CAESAR II,it's mill tolerance 12.5% negative. but if I am
using less thickness than required thckness (1.125times the calculated thickness)
[James, Barbara]

You have mad a very basic and common error: the mill tolerance is the manufacturer's tolerance. The 12.5% should be taken from the nominal size. ie it is the minimum permissable thickness that the manufacturer can supply to you.

For example a 6" Sch 80 pipe - Nominal wall thickness is 10.97mm. 12.5% of 10.97mm is 1.37mm.
If this is taken from the nominal thickness, the minimum wall that is within tolerance is 9.60mm.

Suppose your scantling calc shows a wall thickness requirement of 9.70mm 12.5% of 9.70mm is 1.21mm
If this is added to the basic min T of 9.70mm the total is 10.91mm

You can see that the second calculation shows that the Sch 80 pipe is acceptable, whereas the first example shows that minimum manufactured thickness could be less than the calculated required thickness.

CAESARII correctly uses the first method to calculate the minuimum manufactured wall thickness and then compares it to the calculated required thickness from Eq 3a or Eq 3b of ASME B31.3 (or the equivalent from the relevant code if you are not using ASME B31.3).

Your scantling calculation should be based on design pressures and temperatures, whereas your analysis may be based on operating conditions which are usually less than the design conditions.

Regards

Barbara

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Received on Thu Apr 08 06:01:00 2004