21 Apr 12 23:41
I've completed a wall thickness calc. using CSA Z662 Clause 14 and Annex I for an NPS 20, A106-B line and found that I get a thinner minimum wall thickness using Clause 14 vs. Annex I. This is counter-intuitive since I've always assumed that Annex I always gives lower wall thickness values (usually used for higher grade, larger pipe diameters). I'm curious if the Clause 14 calc. is inherently less conservative than Annex I or if I'm only getting a thinner allowable wall thickness since I'm using a low yield strength material (A106-B) in Annex I (which is permitted). Or perhaps I've made an incorrect assumption in my calc. (which I would also appreciate if someone can point out). Note that Annex I is based on methodology of ASME B31.3 Chapter IX.
Here are my design parameters:
• P = Design pressure, MPa = 11 MPa
• T = Design temp., deg. C = 344 deg. C
• D = Outside diameter, mm = 508mm
• S = Allowable stress from Table A-1, B31.3 (i.e. 1/3 elevated temperature tensile strength), MPa = 117 MPa at 344 deg. C (used for Clause 14 calc.)
• S = 1/3 Yield strength at elevated temperature from Table Y-1, ASME Section 2 Part D, MPa = (1/3)*179 MPa = 119.3MPa at 344 deg. C (used for Annex I calc.)
• J = Joint factor = 1 (for seamless pipe)
• CA = Corrosion allowance, mm = 1.6mm
• WM = Minus mill tolerance = 12.5%
• MA = Mechanical allowance, mm = 0mm
• Co = Sum of mechanical allowances, mm = 0mm
• t = Pressure design thickness, mm = Find using Clause 14 and Annex I calc.
• tm = Minimum required wall thickness, mm = Find using Clause 14 and Annex I calc.
Clause 14 Eq'n:
Annex I Eq'n:
• Clause 14, tm = 28.9mm
• Annex I, tm = 30.9mm
Again, I'm very surprised with this result (i.e. surprised to calc. a thicker minimum wall thickness using the Annex I formula). Can anyone spot an incorrect assumption? Possible with using 1/3 of the elevated YS found in Table Y-1, ASME Section II, Part D for the "S" value in the Annex I formula? Any comments on this would be greatly appreciated.
Background on Annex I (originally called a variance on ABSA Information Bulletin No. IB05-001)
"This variance is provided to allow the advance use of proposed changes to Clause 14 of
CSA Z662-03, Oil and Gas Pipeline Systems. Twenty years experience with the high
yield strength materials used for oilfield steam distribution pipelines provides a high
confidence level as to the properties that can be guaranteed in these steels. Flow rates
and transmission distances currently required for oilfield steam distribution pipelines
mandate the use of larger diameter lines, the wall thicknesses of which, under the existing
design formula, exceed the capacity of the mills to manufacture the pipes. There is a
definite need for the pipeline companies to be able to take full advantage of the high yield
strengths that have been demonstrated for these materials over the years.
Because of the long lead times that are required for booking mill space and getting pipe on
order, this Variance is needed on an urgent basis. The CSA standards-development and
publishing process means that CSA would not be able to publish the proposed changes
until sometime in 2007."