Common joints for gas/hydrogen

The usage of the welded vs flanged joints on the pipeline may be considered as below,
- potential leaking issue due to the flange joints
- use weld joints for high P & T system
- use flange joints on the lined piping
- use flange joints as needed for connecting the equipment nozzle, instrument, sectional isolation, etc.

What type and diameter?

"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....


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

My experience has been in high purity H2 lines.
All Stainless Steel and 99.995% welded.
When we couldn't weld we used double ferrule compression fittings up to 2".

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P.E. Metallurgy, consulting work welcomed

Distribution and filling systems for high pressure H2 (200 & 300 bar - 6 mm up to 18 mm tubing) does not qualify as a pipeline but if that is still interesting then here is my few cents:

In those cases described above, welding is rarely used if it can be avoided (oftentimes it cannot, then you weld). Joints brazed with silver are somewhat popular and is great for high pressure systems. Metallic seals are often used and so are non-metallic seals like O-rings for quick connections (often in specialized polyurethane compounds). You often see these employed in the industrial gas industry (cylinder valves, fittings, pressure regulators ect.) In other cases you would use tube fittings (front end/back end ferrules) in combination with a special quality grade SS tubing that is polished to ensure a leak-free seal after installing the fittings.

OP,
Hydrogen gas is highly flammable, explosive and the fire is invisible. This makes it dangerous if not handled correctly.
ASME B31.12 Code provides piping/pipeline specification for Hydrogen service.

GDD
Canada