How to decide between a slip on / weld neck flange?
How to decide between a slip on / weld neck flange?
(OP)
I have a 24" SA 106 -Sch 100 Seamless pipe(Max Working pressure=1440 psig @ 25 Deg C, petroleum crude) for vertical seperator with top head as 600# RTJ flange of SA 105 and bottom as formed end of SA516- GrB.
How to decide between a slip on flange or weldneck flange
How to decide between a slip on flange or weldneck flange





RE: How to decide between a slip on / weld neck flange?
Not sure how much welding would be required for each to compare on installation costs, but that could be a factor. Initial cost of each flange in 24" could also be a factor.
I would buy slip ons if I was retrofitting something and space wouldn't allow a weld neck. Otherwise, I usually prefer weld necks. I do think the welding is a little easier on the slip ons, but I think I just have a irrational leaning to the weld neck.
One problem I always have with slip ons, is that sometimes with 150#'ers on non-code systems, the contractor "forgets" to make the face weld. With a 600# system, I would guess that there would be an AI present with all of the testing that goes along with it.
Bottom line is that this is one of those choices in engineering that both are correct, but whatever you choose, it will be the wrong one.
RE: How to decide between a slip on / weld neck flange?
Issues for the choice are:
-fatigue resistance is much better with weld neck; this may be of importance especially with elevated operating temperatures
-with lethal or highly explosive fluids slip on is either forbidden or should be avoided
-where 100% RX is required, weld neck should be used to allow for such testing
-client's specification may require weld neck due to its higher quality.
You don't seem to be in any of these conditions, so I guess you should choose slip on.
prex
motori@xcalcsREMOVE.com
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RE: How to decide between a slip on / weld neck flange?
Regards,
Bill Birch
RE: How to decide between a slip on / weld neck flange?
Another issue is whether the flange is in a service where hydrogen could build up in the void between the flange ID and the pipe OD. This doesn't just apply to actual hydrogen service. Any service where atomic hydrogen is liberated as part of the corrosion mechanism and where contaminants exist that inhibit the formation of molecular hydrogen will result in atomic hydrogen penetrating into the wall of the pipe or vessel (and through-wall if the driving force is high enough). Hydrogen pressure in this annular space can buckle the pipe wall and crack the flange attachment welds. If you have this type of service either prohibit the use of slip-on flanges or drill a small vent hole in the hub of the flange or the outer fillet weld (note this hole is a leak source if the inner fillet weld corrodes or cracks!) to prevent hydrogen build up in the annular space.