SW Flange and Slip On Flange 2

[PAN]

From B31.3, Appendix D, SIF for socket weld flange is slightly higher compare to slip on flange. Should we use slip on flange instead of SW flange in small bore piping (2" or less)? Please comment.

 

[KernOily]

Are you an owner or design-engineering contractor?

Here are some thoughts to help you decide. You do not say what your process service is. If I knew that I could be more helpful.

Most owners already have specifications in their piping specs as to what flange shall be used on what line size in what service. (Poor grammar... it's Friday and I don't feel like correcting it ;-) )

For a slip-on flange, the B31 codes allow, I think, 30% of the fatigue strength/life as a weld neck.

Usually the SIF is not the controlling factor in making this decision.

As far as I know, neither of your choices (socket weld or slip-on) allows you to do x-ray NDT so you will have to be satisfied that the other NDT methods will help you sleep better at night. Both SW and SO use a fillet weld which is not a full-penetration strength weld, as is a circumferential buttweld.

Most owners that I've seen use socketweld in this service, not slip-on. Most owners only allow slip-on in certain process services, e.g. 15 psig or less and 100° F or less, for example, in atmospheric plant drain lines, utility water, low pressure condensate return, etc.

Thanks!
Pete

 

[PAN]

Thanks Pete,
In this case, I'm in owner's side.

The line is steam (pressure > 15 psig, temperature > 100F). The specification shows SW flange for small bore piping. For line size 3" and over, slip on flange is specified.

I found the case during line check. This is not conformed to our project specification. However, I will not comment to cut and re-weld the SW flange.

 

[PAN]

I would like to update the status for above case.

The additional PT for weld soundness at the inside weldment of SO flange is our solution.

 

[TangoCleveland]

I'd suggest PT or MT on the OUTSIDE weldment of the SO flange. This is the weld that carries all the load. The inside weld is essentially only a seal weld.

Larry

 

[Scipio]

Something else you have to watch for using slip on flanges on small bore piping, is the bore diameter if you're using spiral-wound gaskets.

On lines 1" NPS and smaller (I think - it's been a couple years since I found this out the hard way), the OD of the pipe, and hence the ID of the flange facing, is greater than the inner diameter of a spiral wound gasket. If your inside fillet weld isn't machined square to the flange face, the spiral actually protrudes inside the raised face portion of the flange and will unwind. This is one of the reasons why you'll see line classes with slip-on flanges switch to socket weld for small bore.

 

[PAN]

Thanks for all comments.

TangoCleveland,
PT was performed on the outside weldment. For the inside weldment is our additional requirement.

Scipio,

I agree with the reason as your comment. Please clarify more about the machining square to the flange face. Can I find the detail in any references? Please advise me.

 

[Scipio]

Yep, the detail for that configuration is in B31.3, it's on Page 64 of the 2002 edition, Figure 328.5.2b (2), "Face and Back Welds".

 

[PAN]

Scipio,

Thanks again for your help. For "Face and Back Welds", it is inevitable to weld and grind near the flange face (raised face). In the futher, I will strictly control the use of SW flange for small bore piping.

 

[TangoCleveland]

Check thread 97609 for more slipon flange considerations in B31.3

Larry