Weld.

But not always practical and makes replacement of repair of items very difficult.

Why do you ask?

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

Welded, but high temperature and high pressure applications may have issues with some dissimilar metal welds that can , in some cases, be better served by the use of a "compact flange".

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

Welded or press-mounted joints.
Flange bolted joints will have to be very large and thick to withstand high pressures.

* Finding a solution is great * Knowing how to implement it is fantastic * Believing it is the only one and best is naive ?

Nobody has mentioned cost. Each joint requires purchase and installation of of 2 extra pieces, with probably 1 additional weld and many bolts, nuts and gaskets. You should have a reason (see above) for buying them.

Flanges should also be avoided where leaks are undesirable, or dangerous for many reasons.

I have a few cents to add based on recent experience but I’ll wait for the OP to first joint the discussion and show some appreciation for the input provided so far.

Huub
- You never get what you expect, you only get what you inspect.

We have installed many large pressure vessels and it was easy to convince the owner not to have flanges. Just asked them how many times they indent to actually unbolt the flanges and answer was usually never. They then saved on the cost of the flanges and no future leaks to deal with.

More details, better answers.

As this is a cost/benifit calculation, NOT a pure technical one, we need a lot more information. 'High pressure process conditions' in a forum for 'pipeline and fluid Mechanics' can be valves for anything from gas, water, oil to anything else. (Not only petroleum related as some always seems to assume.) Pressure class and diameters might also vary widely.

From my experience you may note down some general points:

Smaller sizes, not very often operated, non critical for continious operation, repairable without demounting from pipeline
larger number equal valves: all separately lean in favour of welding ends.

We must take into consideration the requirement for continous uninterrupted operation time, cost for repair and shutdown. I have supplied high pressure class valves up to several tons. (Hydroelectrical and drinking water pipelines if you wonder). For high pressure class valves of larger dimensions the process of welding in place, and not at least removing and replace, would be costly and highly impractical.

End types regarding to class, size, fluid and precess requirements and cost have to be considered for each case.

Linua Jr.
What pressure and process we are talking here? The design will depend on the risk to environment, personal and economic loss.

GDD
Canada