"When are welding techniques employed versus the use of flanges?"
When you can weld, weld. Only in pretty rare circumstances do you joint some other way.
Typically these are are mk3223 say, plus Water pipelines are often Cast Iron or Ductile Iron which is a bitch to weld and often low pressure so they use push fit joints or sometimes flanges.
Flange joints are considerably more expensive than welded ones. You have two welds per joint compared to one, you have the cost of two flanges, bolts and gaskets plus the time taken to actually joint them up as well as the weld them. If you bury them you can very easily get crevice corrosion in-between the flange faces as it is very difficult to seal the gap.
When considering all pipelines, what percentage are welded, and what percentage utilize flanges?
"All pipelines" is a very vague and encompassing term so you need to separate that out.
Low pressure (<2-3 bar) water lines and old gas lines are often Cast or Ductile Iron and are usually push fit, not flanged. but some above ground lines might be flanged.
Newer water and gas lines are commonly PE and are fused / welded together without flanges.
GRE pipelines can often be flanged due to the difficulties of jointing GRE pipes together, or use push fit.
Steel pipelines of any type of service are usually welded - my guess would be 98% to 99% or more.
you only see flanges where you need to connect a valve ( though not always) or instrument or something.
Some understanding of why you're asking would help....
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