LWN inside standard diameter in NPS higher than 12 INCH

[Marcsierra]

In LWN (Long Weldig Neck) NPS higher than 12" most of the datasheets propose the inside diameter as the outside diameter of the Pipes acc. ASME 36.1
Which is the reason? Is it to be used as Slip ON Flanges?? If so, why is not allow or recommended directly a SO Flange?

I will appreciate your collaboration to solve this doubt.

 

[LittleInch]

didn't see that figure before so ignore earlier comment - though it might still apply for a "special".

They still call them straight hub welding flanges , but 2013 edition it's now figure 4.

Length is quoted as 229m for NPS4 and lower and 305mm for NPS>4" (though can be amended by agreement). Otherwise dimensions as per Weld Neck Flanges for each size and class rating. Bore is equal to dimension B of welding neck flanges unless by agreement and should not be bigger than the lapped flange bore.

So quite a flexible design....



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