Couplings Attached by Single Fillet Weld
Code Case 3035

Example of Figure UW-16.2 single fillet attachment:

Table UW-16.1 was introduced to VIII-1 in 2010 as a minimum fitting thickness when attaching fittings with a single fillet weld per Figure UW-16.2, and UW-16(f)(3)(-a), specifically (-a)(-6).

(-6) In lieu of the thickness requirements in UG-45, the minimum wall thickness for fittings shall not be less than that shown in Table UW-16.1 plus the thickness added for corrosion allowance.

However, Table UW-16.1’s minimum thicknesses are greater than some of B16.11’s 3000# coupling wall thicknesses.  Code Case 3035 allows the use of standard 3000# couplings despite not meeting Table UW-16.1’s requirements.

Using a 1.25” 3000#, NPT coupling as an example:

B16.11, Table I-5 gives the OD of a 1.25”, 3000# coupling as 2.25”, and references Table I-2 for the wall thickness, however it does not specify whether you take the bore diameter or the socket wall thickness to determine the threaded couplings minimum wall thickness.

Table I-5
                                                  
Table I-2

There are 3 possible ways to determine the wall thickness, 2 ways from B16.11, and using the OD of 1.25” pipe (1.66”) as the ID of the threaded coupling.

Where OD of the coupling is 2.25” according to B16.11

B16.11 Socket wall thickness:    
TSock    = 0.208” (Tabel I-2)

B16.11 Bore Dia. (1.675”):
This is the method Compress (Codeware Software) uses.
Tbore    = (2.25-1.675)/2
Tbore    = 0.2875”

OD of pipe (1.66”):                          
Tpipe     = (2.25-1.66)/2
Tpipe     = 0.295”

In Table UW-16.1, the minimum wall thickness for a 1.25” fitting that is attached by a single fillet weld is 0.30”, which is greater than all the wall thicknesses that were calculated above.

This brings up a few questions:

1. Q: Does that mean that you can’t attach certain sizes of 3000# couplings to vessels using only a fillet weld?
   • A: No, but you will now need to reference code case 3035 on the data report, calculations, and drawing.  I ran all the coupling sizes through Compress and found this code case only affects sizes: 1/8”,  3/8”, 1-1/4”, 1-1/2”, 3”, 3000# couplings that are attached with a single fillet weld.

2. Q: Have I been doing things wrong this entire time?
   • A: No.  The intent of the Code was to allow the use of standard 3000# fittings, as shown by the new code case and the fact that B16.11 is an acceptable standard according to table U-3 of VIII-1.  It’s likely that table UW-16.1 was meant as a convenient way to reference the B16.11 thickness without vessel manufacturers having to purchase the B16.11 code book.  Vessels built to VIII-1 2010 and later codes were built with the intent of the code.  It’s only recently that commercial software (Compress by Codeware specifically) has added this check.  Before no one questioned the thickness of a standard coupling if it met UG-44 requirements, and I have never come across a case where this has been an issue.  I wouldn’t be surprised if either table UW-16.1 is updated in the future or it’s spelt out in VIII-1 that B16.11 fitting dimensions are fine if they meet the requirements of UG-44.