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

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.

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

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.

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.