Nozzles slip-on vs RFWN

[Unotec]

While designing tanks we always specify raised face welded neck (RFWN) type. However, one of the manufacturers suggested using slip on type, as they are less expensive. The mechanical stresses the nozzles are subject to are not considerable. In any event, the welding for the slip-on type should have more mechanical resistance anyway.
Would it be better to change to slip-on instead or RFWN?

 

[IFRs]

I usually see slip-on for tank nozzles and weld-neck for piping after the tank.

 

[SnTMan]

unotec, You can use either, the difference is that weld necks use a full penetration weld to the pipe, where slip-on do not. It depends on the service whether this is important. In some classes of work you will never find a slip-on, in others you will not find weld necks.

Regards,

Mike

 

[nickelkid]

agree with above, if you do not require full penetration welds you will not need weld necks

 

[JStephen]

It's usually just an economic decision if no one specifies a particular style. For atmospheric tanks, the pressure is normally just a fraction of the allowable.

You can get weld neck and slip on flanges in both flat face and raised face- the face style isn't dictated by the weld style (although availability may vary). You can get slip-on flanges in both plate style and hub-type- see API-650 or AWWA C207.

 

[corrosionman]

unotec, If its tanks, which might be lined, slip-ons are best because with a weld neck the weld bead is a short distance down the pipe and can be tricky to line correctly whereas the S/O flange only has a weld at the flange face which is easier to handle.
Secondly If the pipe system is made up from flanged straights and flanged bends / tees then the bends & tees must be Weld necks , partly cos the items then remain conforming to standard dimensions and also, because often physically, you cannot put a s/o flange onto a tee or a bend.
Cheers DW