AN Tube Flares
TYPE
Seamless (MEETS ASTM A213 & A269) could probably get away with single flare - but I personally would still go with double.
Seamed - would definitely only go with double flare as the tubing will tend to split at the seam at flare area. (as an example: most of the tubing you buy in auto parts store for brake lines is seamed and requires double flare)
MATERIAL
If the tubing will show I would go with 316 stainless tubing (can buy from McMaster Carr or Swagelok) then you can also polish it for the 'bling' factor.
Aluminum or copper tubing - go with double flare regardless of tubing wall thickness.
Stainless or plain steel tubing - as stated before, if wall thickness is .035, .049, or .065" you could possibly get away with single flare - but I would still use double.
When choosing whether to single or double flare - the type of tubing (seamless/seamed) takes precedence over material used (i.e. if you have a SS tube with heavy wall thickness, but it is of seamed construction, go with double)
-Jay-





