Weld Neck vs Slip-on Flange Application

[pneveu]

What is the reasoning behind using 150# and 300# weld necks vs. slip-ons for welded piping systems? My feelings have been that weld necks are the better choice for fittings which allows bolting clearances; and slip-ons for straight run pipe. Is there a common practice which states that or a code reference?

 

[Unotec]

I have not seen a code on that. We try to use weld necks as much as possible (fittings, straight runs, etc...) and slip-ons only when the first would be too cumbersome, i.e. the flange inside a vessel nozzle.
I guess our reasoning is more on the reliability of a weld neck vs a slip-on than anything else. It also helps that weld necks are done in a shop and slip ons in the field, there is a cost factor difference there as well.

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[pennpiper]

The verification of weld quality has a major baring on the choice.

The Weld-neck flange is attached to pipe or fittings with a Butt-weld. The Butt-weld can be X-rayed to insure the weld quality.

The slip-on flange is attached to pipe or fittings with two fillet welds (one on the out side and one on the inside). These welds do not allow for proper X-ray weld quality verification.

 

[IFRs]

For plant piping, we use slip-ons unless the flange is being welded directly to a fitting (elbow, tee, etc). Much less expensive, and can tolerate more misalignment.

 

[Gator]

IFRs, what kind of pressures and temperatures and fluids do you typically do this with?

Back in the "old days" when pipers would exchange "borrowed" design details on paper I remember seeing one that depicted welding slip-ons to buttweld elbows.

Like attaching coconuts to swallows, it MIGHT work, but it's just plain wrong.

 

[IFRs]

Our crude and #6 oil temperatures max out around 140 DegF in insulated and heat traced lines. The pressures are generally limited to 100 PSI. The pipes range from 8" to 36" in diameter, some runs are about 5,000 to 6,000 feet long. Slip-ons "can" be welded to buttweld fittings I guess, but we don't as a rule do that.