Reinforcement Calculation for Welded Branch Connections:
Any pipe having an opening, for branch connection will be weaken, if pipe wall thickness will not sustain the pressure. In this case, pipe reinforcement is necessary to provide. The amount of reinforcement required to sustain the pressure, shall be determined in accordance with below reinforcement calculation.
Added reinforcement is required to meet the criteria as under,
Available Area (A2+A3+A4) shall be always, greater than Required Reinforcement Area (A1).
i.e. A2 + A3 + A4 ≥ A1
If above criteria is not meeting, then reinforcement is required.
Further we will see details of calculation of all areas like A1, A2, A3 & A4.
Required Reinforcement Area (A1):
The reinforcement area, A1, required for a branch connection under internal pressure is,
A1 = th x d1(2- sin β)
Where,
th= Pressure design thickness of pipe reflecting to header, which is actual calculated thickness excluding mill tolerance, corrosion allowance etc.
d1 = Effective length removed from pipe at branch.
β = Smaller angle between axes of branch and run.
Further d1 = [Db – 2 (Tb – c)]/sin β
Where,
Db = Branch diameter in mm
Tb = Branch pipe thickness (measured or minimum per purchase order) in mm, excluding mill tolerance.
C = Corrosion Allowance in mm
Area resulting from excess thick-ness in the run pipe wall (A2): Area A2 is the area resulting from excess thick-ness in the run pipe wall,
A2 = (2d2 – d1) x (Th – th – C)
Where,
d2 = Half width of reinforcement zone
d2 = d1 ‘or’ (Tb – c) + (Th + c) +d1 / 2 ………Whichever is greater
Th = Run pipe thickness in mm
Area resulting from excess thick-ness in the branch pipe wall (A3): Area A3 is the area resulting from excess thick-ness in the branch pipe wall,
A3 = 2 x L4 (Tb – tb – C) x sin β
Where,
L4 = Height of reinforcement zone outside of run pipe.
L4 = 2.5 (Th – c) or 2.5(Tb – c) + Tr …….. Whichever is less.
Tr = Minimum thickness of reinforcing ring or saddle. (0, if there is no reinforcing ring or saddle)
Area of other metal provided by welds and properly attached reinforcement (A4):
Now the area A4, is the area of other metel provided by welds and properly attached reinforcement.
A4 = (Do – Db) x th = (2d1 – Db) x th
Finally if A2 + A3 + A4 ≥ A1 then no further reinforcement is required.
Example:
Calculation:
Required reinforcement area A1 required for a branch connection under internal pressure:
A1 = th.d1 (2 – sinβ
where,
d1 effective length removed from pipe at branch.
d1 = [Db – 2 (Tb – c)]/sinβ
sinβ is angle between axes of branch and run = sin90°
d1 = 364.4 mm
Now A1 = 7944.2844 mm²
Area resulting from excess thickness in the run pipe wall:
A2 = (2d2 – d1) * (Th – th – c)
where
d2 = half width of reinforcement zone
d2 = d1 or (Tb – c) + (Th + c) +d1 / 2 whichever is greater
Selected value d2 = 364.4 mm
Now A2 = 1165.7156 mm²
Now the area A3, is the area resulting from excess thickness in the branch pipe wall:
A3 = 2L4(Tb – tb – c) sinβ
where
L4 = 2.5(Th – c) or 2.5(Tb – c) + Tr
L4 = 62.5 mm
Therefore calculated value of A3 = 328.125 mm²
Now the area A4, is the area of other metel provided by welds and properly attached reinforcement:
A4 = (Do – Db) th = (2d1 – Db) x th
A4 = 7340 mm²
Total available area = A2 + A3 + A4 = 8833.8406 mm²
Required area = A1 7944.2844 mm²
Conculusion:
The available area for reinforcement is greater than required area hence proposed pad is acceptable. However area available in the fillet welds attaching pad to shell & pad to branch are not included in the calculation of available area, hence reinforcement is 800 mm²
