#71584 - 04/26/18 09:19 AM
Calculation of Ts for Effective Section Modulus B31.3
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Member
Registered: 06/08/15
Posts: 11
Loc: Alberta, Canada
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Hello, I need your valuable advice. Actually I am calculating Ts for effective section modulus Ze as per ASME B31.3 for a reducing welding Tee 30"x18". The data for the Header and Branch is as follows:- OD Branch =18", thickness=.5", Corrosion allowance=.25" OD header =30", Thickness= .375", Corrosion allowance =.0625".
As per code Ze= (pi)(r.square)Ts where r is the mean radius of the branch.
As per Code for "Section modulus used to compute the stresses shall be based on nominal pipe dimensions less allowances". Ts =min(Thickness of header, i*thickness of branch) In my case i=3.93
So now for selecting Ts should I deduct the corrosion allowances from Branch thickness and Header thickness before using them in the eqation, i.e
Ts= min(.3125, i*.25) = .3125"
Do I need to further deduct Corrosion allowance from Ts i.e Ts=.3125-.25 =.0625
Or I have to use Ts=min(.375, i*.5) = .375 and now deduct the branch corrosion allowance from it i.e Ts=.375-.25=.125
Please advice , as I am little confused over there.
Thanks
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#71585 - 04/26/18 10:23 AM
Re: Calculation of Ts for Effective Section Modulus B31.3
[Re: SandeepTiwana]
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Member
Registered: 06/05/00
Posts: 151
Loc: Middle East
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I'm afraid your question is not so clear, you do not specify what Code stresses you want to calculate. Your statement: < > is correct only for SUStained and OCCasional stresses.
If you go back to B31.3 Code, para. 319.4.4 ("Flexibility Stresses"), and read CAREFULLY all definitions of the involved notations, you'll find the following definitions:
<< Z = section modulus of pipe; see para. 319.3.5 >> << Ze = effective section modulus of branch = pi * (r2)^2 * TS; see para. 319.3.5 >>.
Going to Para. 319.3.5, you'll find:
<<319.3.5 Dimensions. Nominal thicknesses and outside diameters of pipe and fittings shall be used in flexibility calculations.>>
So, you DO NOT NEED TO SUBTRACT any allowance to calculate Z and Ze WHEN YOU CALCULATE DISPLACEMENT STRESSES (EXP). Expansion Stresses are calculated for Piping Nominal (Un-corroded) conditions.
BUT, if you calculate SUS (or OCC) Stresses, yes, you'll need to use the CORRODED dimensions.
At Para. 320.1 it is stated:
<<...The loads due to weight should be based on the nominal thickness of all system components unless otherwise justified in a more rigorous analysis. Section moduli used to compute the stresses in this paragraph shall be based on nominal pipe dimensions less allowances, i.e., the sum of mechanical (thread or groove depth), internal and external corrosion, and erosion allowances. Areas used to compute the stresses in this paragraph assume nominal pipe dimensions less allowances affecting the inside diameter of the pipe, i.e., the sum of mechanical and internal corrosion and erosion allowances....>>
Later, at Para. 320.2, it is stated:
<< Z = sustained section modulus. Z in eqs. (23b1) and (23c) is described in para. 319.4.4 but is computed in this paragraph using nominal pipe dimensions less allowances; see para. 320.1. Ze = sustained effective section modulus. Ze in eq. (23b2) is described in para. 319.4.4, using ii from Appendix D in Ts calculation, but Ze is computed in this paragraph using nominal pipe dimensions less allowances; see para. 320.1.>>
In such cases, when calculating Ze, the allowances for TS are applied ONE TIME ONLY, to the ORIGINAL/REAL thicknesses Th and Tb only, and not to TS. Note that the r2 mean radius is also affected by corrosion.
_________________________
Dorin Daniel Popescu
Lead Piping Stress Engineer
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#71587 - 04/26/18 11:37 AM
Re: Calculation of Ts for Effective Section Modulus B31.3
[Re: Dorin Daniel Popescu]
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Member
Registered: 06/08/15
Posts: 11
Loc: Alberta, Canada
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Thank you Dorin for taking the time to reply.
I am calculating Ze for Sustained Stresses and I was matching my manual calculations against CAESAR-II results.
As you have advised that allowances for Ts are applied to the Original thicknesses Th and Tb only and not to Ts.
If I do that my Ts=min(.3125, 3.93*.25)= .3125 r2= 8.875, Ze=77.36
By this result my stresses does not match with CAESAR-II results.
But If I deduct allowance from Ts ie Ts=(.375, i*.5)-CA = .125 r2=8.875, Ze= 30.08.
With Ze=30.08 my manual calculations matches with CAESAR-II results.
If I use Ze=77.36 my bending stress become way less and does not match with CAESAR-II results.
Thanks
Edited by SandeepTiwana (04/26/18 11:58 AM)
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#71590 - 04/27/18 09:46 AM
Re: Calculation of Ts for Effective Section Modulus B31.3
[Re: SandeepTiwana]
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Member
Registered: 12/29/99
Posts: 197
Loc: Houston,TX,USA
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Just for your information, B31J-2017 eliminates the use of Ze. B31.3-2016 does not yet mandate the use of B31J, but B31.3-2018 which will be issued typically in January 2019 may change the game.
_________________________
Bob Zimmerman, P.E. Vice President of The Piping Stress International Association (The PSI)
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#71594 - 04/28/18 10:45 AM
Re: Calculation of Ts for Effective Section Modulus B31.3
[Re: SandeepTiwana]
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Member
Registered: 12/13/99
Posts: 6226
Loc: Houston, Texas, USA
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Sandeep,
Please open a support ticket on Smart Support and send in your job file, we need to see this. Make sure you indicate what version of CAESAR II you're running.
_________________________
Regards, Richard Ay - Consultant
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