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