Whether bypass valve should be Socket welded or flanged one? Also tell, is there any requirement of upsteam & downstream pipe spool piece (between valve & run pipe ) or not.

Regards
PME

PME,
If you're asking whether a bypass valve should be socket welded or flanged, then the answer is that it should match what is specified in the piping specification. That may be socket welded, screwed, flanged, or otherwise connected. The method chosen should depend on the pressure, temperature, materials of construction, valve/pipe size, and various other factors. There is no requirement for any up/downstream spool pieces under normal circumstances.