CSA Z662 Wall Thickness Calc. (Clause 14 vs. Annex I)

[auzie5]

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:
• t=PD/2SJ


Annex I Eq'n:
• t=((D-Co)*(1-exp(-1.155P/S))/2


Results:
• 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."

 

[auzie5]

Sorry forgot one more eq'n...

Minimum required wall thickness Eq'n:
• tm = ((t+MA)/(100-WM))*100+CA

 

[BigInch]

Never calculate a value for which you do not already know the approximate result.

MPa = (1/3)*179
MPa = 119.3 MPa

I get 60 MPa, rounded up from 59.66666666666666666666666666666666666666666666667

What would you be doing, if you knew that you could not fail?

 

[auzie5]

MPa = 119.3 MPa is correct.

My typo was typing (1/3) when I meant to type (2/3) of elevated temp. yield as per Annex I

Annex I, Section I.2.1.1:

"S = not more than the lower of two-thirds SMYS at room temp. and two-thirds of yield strength at design temp for materials listed in ASME Section II, Part D".

Elevated temp. yields are found in ASME Section II Part D, Table Y-1 (pages 540-543)

A106-B:

At 650 deg. F (~344 deg. C), yield strength = 25.9 ksi (~179MPa).

Thanks BigInch for catching that typo.

 

[auzie5]

Just noticed one more minor typo (corrected below).

Annex I Eq'n:
• t=((D-2Co)*(1-exp(-1.155P/S))/2

• Co = Sum of mechanical allowances, mm = 0mm anyways so this does not have any affect on calculated values.

 

[BigInch]

Well, it looked like some kind of mistake. I'd try to help with checking the rest, but I'm not familiar with those CSA codes, so better left to someone that's up on them.

What would you be doing, if you knew that you could not fail?

 

[BigInch]

BTW I would expect B31.3 based calculations to generally give thicker walls. I think I understood you to say you were expecting the reverse, B31.3 giving thinner walls. I would not expect that.

What would you be doing, if you knew that you could not fail?

 

[auzie5]

Thanks BigInch. You are correct that the Annex I (i.e. B31.3 Ch. IX) eq’n is in fact more conservative.

The benefits of using Annex I are encountered when we begin to use higher strength materials (i.e. CSA Gr. 550 Q&T material; ~API 5L X80).

From mechanical test it has been shown that at 344 deg. C YS of Gr. 550 Q&T pipe = 520 MPa. Minimum TS at both room temp. and 344 deg. C are the same, TS = 645 MPa.

As you can see, this thermo-mechanically control processed (TMCP) pipe maintains a relatively high YS at elevated temperatures which is why Annex I (i.e. design using 2/3 YS) is used in these instances to significantly reduce material costs (savings in 1000’s of tons on a pipeline).

CSA Gr. 550 Q&T: WT Calculations using CSA Z662 Clause 14 vs. Annex I
• Room Temp. YS : Elev. Temp. YS = 520 MPa/550 MPa = 95%

• 2/3*Elevated YS = (2/3)*520 MPa= 346.7MPa (used to calc. WT using Annex I) – slightly more conservative WT formula but much higher allowable stress. Therefore smaller minimum acceptable WT calculated.

• 1/3*Elevated TS = (1/3)*645 MPa= 215 MPa (used to calc. WT using Clause 14) – slightly less conservative WT formula but much lower allowable stress. Therefore larger minimum acceptable WT calculated.



Whereas, a low yield strength ASTM material (i.e. A106-B) does not maintain as good a YS at elevated temperatures.

ASTM A106-B: WT Calculations using CSA Z662 Clause 14 vs. Annex I

• Room Temp. YS : Elev. Temp. YS = 179MPa/241 MPa = 74%

• 2/3*Elevated YS = (2/3)*179 MPa= 119.3 MPa (used to calc. WT using Annex I) – slightly more conservative WT formula with approximately the same allowable stress. Therefore slightly larger minimum acceptable WT calculated.

• 1/3*Elevated TS = 117 MPa (B31.3, Table A-1) (used to calc. WT using Clause 14) – slightly less conservative WT formula with approximately the same allowable stress. Therefore slightly smaller minimum acceptable WT calculated.


I think I now have a better understanding of how/when Annex I should be used and how it relates to the less conservative WT formula from CSA Z662 Clause 14. However, I still welcome any further comments.

Thanks Everyone!