You can use a 6" Weldolet on a 14" header
Only make sure to specify pipe wallthickness (Schedule) for both branch and header. You need to consider both in order to find out whether to use STD, XS, XXS or heavy wall weldolet. The supplier will be the best one to tell you that - MSS SP-97 does not. What I mean is you should specify f. ex. 14" Sch STD x 6" Sch XS in stead of a 14x6" Sch XS weldolet.
The weldolet sizes are based on equal scheduled branch and header and if they are not equal you should let them advise you.

I have this from an old Bonney Forge catalog:

"Where the schedule of the run is greater or less than the schedule of the branch, it is essential that both schedules be specified........."


Make sure that the supplier has a design approval for the fitting (there are many small outfits out there making their own fittings without any documentation)
Try the site http://www.bonneyforge.com/
They have all the dimensions (they invented the weldolet and own the trademark) and you'll be sure the fittings have a design approval. Actually I think dimensions in MSS SP-97 are based on Bonney Forge.

Best regards

Mogens

mgp@kabelnettet.dk

You can also try WFI for information on their version of a Weld-O-Let.  I've used their products at times instead of Bonney Forge with good results and they have provided good technical assistance.  In addition to specifying the schedule of both the run and branch you should take a look at the metalurgy of the pipes, the fitting, and see how it works with your welding code/procedure.

If possible I would avoid this fitting and go with a reducing tee.  Especially if you have any vibration or high stresses.

Geoffrey,

The ASME pipe codes do not address this question directly.  The use of weldolets is approved if the area replacement rules of the relevant code are followed.

I know that 6" weldolet branches are allowed on 14" lines by most major oil companies in 900 and 1500# Class carbon steel systems. Depending on your situation you may need to provide calculations.

Regards,

Gunnar

This is why you need to make sure that the supplier provides the design approval (thus you have to provide both schedules). You should not provide design calculations yourself, as it is normally difficult to obtain the exact geometry from the supplier.
When the weldolets are type- or design approved for the given schedules (and the relevant code) they may also be used for lower ratings than 900#.

Regards
Mogens