UG 45 (b)(1) : 12.95 mm
UG 45 (b)(2) : 11.33 mm
UG 45 (b)(3) : 5.88 mm
UG 45 (b)(4) : 11.33 mm
My chosen nozzle thickness is 11.33 mm
How can (3) be smaller than (1) and (2)? In your case, it appears that (3) should be 12.95 mm.
What is your diameter and CA? It seems odd that the external pressure thickness of the head or shell happens to coincide with the standard wall nozzle thickness.
Look at it this way: ...the thickness of a nozzle neck... shall not be less than the smaller of (3) or (4). Perhaps the wording could be simplified to "No thinner than the required shell or head thickness for internal or external pressure (with CA included for both) but need not exceed Standard wall pipe with CA added."
In this case, it should be 11.33 mm as you surmise, but don't forget UG-45(a) and (c).
Take a look at Appendix L, L-7.2.3 (page 562 of the 2007 ed) for a good example.
As an example of (b) only, if a 12" nozzle is inserted into a shell which has a minimum (corroded) thickness for of 0.20" for internal pressure, 0.15" for external pressure, with a CA of 0.10":
(b)(1) is 0.30"
(b)(2) is 0.25"
so (b)(3) is 0.30" (SHELL thickness, not nozzle)
Your minimum nozzle thickness is (b)(3) but need not exceed Standard wall (0.375)(7/8)+0.1 = 0.43" so your minimum nozzle thickness
based on UG-45(b) only is 0.30" Keep in mind that you will check the nozzle thickness for internal pressure and other loadings in UG-45(a) and (c).
Now, if we increase the internal pressure so that the shell requires 0.70" for internal pressure and keep everything else the same:
(b)(1) is 0.80"
(b)(2) is 0.25"
so (b)(3) is 0.80"
(b)(4) is still 0.43", so your so your minimum nozzle thickness
based on UG-45(b) only is 0.43"
jt