How to decide between a slip on / weld neck flange? 2

[arksurat]

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

 

[UtilityLouie]

Both flanges are rated by ASME if installed properly, so there shouldn't be an issue.

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.

 

[prex]

Slip on generally costs less than weld neck, so it should be preferred wherever it is allowed or acceptable.
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

http://www.xcalcs.com

Online tools for structural design

 

[BillBirch]

For 600# service, I would not consider a slip-on flange, primarily for the ease in which weld necks can be radiographed. Also, at elevated temperatures or if PWHT is required, unwanted pressure could be built up in the air gap between the two fillet welds. I have actually seen a nozzle neck "balloon" inwards in this area due to PWHT.

Regards,
Bill Birch

 

[kstaylor]

The use of slip-on flanges (and also socket-weld flanges in smaller diameter piping) should be avoided in cyclic service. So, another consideration is how often is the separator going to be opened and then closed up (repeated mechanical loading) or what is the frequency of operating cycles (repeated pressure cycles)? If this facility runs all the time, and is only brought down for maintenance/cleaning/inspection once every few years, and the system stays pressured up most of the time then slip-on flanges may be appropriate. My company's standards, however, would allow slip-on flanges only in ASME 150 and 300 Class systems.

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.