Does it is important to mention every time "C" dimension calculated using B16.25(page-3 of 2003)in weld end preparation of two pipe spool even though pipe has same nominal OD and same nominal thickness and using fig.2a/2b.
C=A-OD tolerance-2*tmin-0.25.
Where,
A-Nominal outside diameter.
OD tolerance-undertolerance on OD(This value come from SA530 Table-1)
In case of A106 or A335 pipe above can be modified as-
C=A-0.79-2*tmin-0.25
C=A-0.79-2*0.875*tnominal-0.25
Here
tnominal=tmin/0.875.(12.5% mill undertolerance on pipe thickness)
For example -
Nominal OD-508mm
Nominal thicknes-S60(20.62mm)
So,C dimension
C=A-0.79-2*0.875*tnominal-0.25
C=508-0.79-2*0.875*20.62-0.25
C=470.875mm.
So,for higher thickness does we have mention mention "C" dimension rather than nominal inside diameter for every weld.
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Also,as per ASME B31.1,in case of inside thickness difference is greater than 2 mm internal trimming has to done.(explained below)
As per SA350 Table-1,(for 20" NPS pipe)
Over tolerance on OD,+3.2mm
undertolerance on OD,-0.8mm
and considering 12.5% under and over tolerance on pipe thickness,
NOMINAL WALL (mm),from B36.10
t
n
20.62
ACT MIN WALL (mm),-12.5% mill tol.
T
min
18.04
ACT MAX WALL (mm),+12.5%mill tol.
T
max
23.20
MAXIMUM OUTSIDE DIAMETER (mm)
511.20
MINIMUM OUTSIDE DIAMETER (mm)
507.20
MAXIMUM INSIDE DIAMETER (mm)
475.12
MINIMUM INSIDE DIAMETER (mm)
460.81
SO,if we minus MAXIMUM INSIDE DIAMETER (mm) and MINIMUM INSIDE DIAMETER (mm),475.12-460.81=14.31mm,14.31/2=7.155 which is higher than 2mm (as per ASME31.1).In this case we have do trimming.
Conclusion
So,do we have to do trimming for each and every weld in case of higher pipe thickness as every time we will get inside thickness difference more than 2 mm and what dimension we will be showing on weld end preparation,either "C" dimension or "B"(nominal inside diameter) dimension from B16.25.
"C" Dimension Requirement in Butt Welding
