Dissimilar thicknesses are welded together all the time.  The bore of the thicker pipe is usually machined to match the thinner pipe on a taper.  The code of costruction should have some minimum taper requirements in order to reduce transition problems.  (For instance, B31.3 figure 328.4.3)

If you can't get to the thick pipe to taper the inside of it then you might want to use a transition fitting.  There will be a minimum fitting length.  You can have one end thick and the other thin with a nice uniform taper.

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Per the input from GRoberts, this is common and the applicable construction or fabrication code or stabdard will often specify the maximum allowable mismatch in thickess.  Greater mismatch is usually accommodated by:
1)  taper bore at an angle or range of angles specified by the code or standards or
2) backwelding so that the weld metal depositied on the pipe ID provides the smooth transition from thick to thin pipe or
3) transition fittings (short piece of pipe of intermediate thickness) with thickness selected so that the change in thickness at the weld joint on either end of the transition fitting is within the allowable thickness difference specified by the code or std. or
4) some combintation of the above.

mcinco50,
         I would ask myself "why are the pipes of different thickness?" and "Is the thinner pipe adequate for the pressure?" before consideration of welding them together. Are the materials of each pipe the same needs to be considered also.

DSB123,

This is for a condensate line with a TLR, problem occurs every now and then I decide to do something instead of just doing a repair evrytime. Existing pipe size is 6' that branch in a tee to 4" in sch 40 pipe. Replacing piping in the affected area extra strong to a sch 120 pipe hoping at least it'll last 5 year before another reapir will be done.

mcinco50

To follow up along the vein of DSB123's question, why is the failure/problem occuring?  Is it a velocity issue?  Corrosion, erosion, etc?

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website: www.oil-gas-consulting.com

GregLamberson,

It is a velocity issue.

Velocity mean you've had failure due to erosion, erosion-corrosion?

S.

http://www.corrosionist.com

mcinco50

As strider6 mentions, it could be due to erosion or eronsion-corrosion which is accelerated by high fluid velocities, presence of sand or other particles, corrosive contaminants such as C02 and H2S, high wall shear stresses.

We don't know what type of flow you have nor of the product flowing, but you probably should look at developing an optimum velocity for the system.  

I know this has gotten away from the original question.  There have been some good discussions on velocity thresholds in the some of the other forums, you might want to check out some of them.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website: www.oil-gas-consulting.com