As far as I know no code gives requirements for stiffeners on vessel nozzles.
Normally clients request if stiffeners are required for small bore nozzles having a longer than normal outstand.
This could be for example on the top of a horizontal vessel as nozzles have to be lengthened to physically pass through a platform or nozzles coming out of the side of a horizontal vessel needing a long outstand due to their physical location on the vessel shell or also if the nozzle has a long outstand and it is supporting a heavy valve or level bridle, etc.
Sometimes clients require stiffeners if the nozzle outstand is above a certain length without anything heavy being attached as mentioned above.
As to the size of the stiffeners I normally use 2-off stiffeners separated at approx 90 degrees to each other and positioned at 45 degrees between nozzle and vessel wall – hope you understand. Stiffeners are normally 50mm wide x 6mm or 8mm thick. These are normally welded to the back of the hub on a weld neck flange – take care to avoid bolts.
I have never personally used nozzle stiffeners as an aid in dealing with piping induced nozzle loads – although I have seen it done by others.
Also, nozzles that will support large relief valves, and associated piping commonly are stiffened.
I believe that stiffeners are added for these two cases because of uncertainty in the nozzle loading and because of catastrophic results if there is failure
The Chinese "GB" vessel code uses stiffeners on every nozz.
ASME does not call for stiffeners, because there are mandatory stress calc's. No outrageous nozzle loads, no stiffeners required. Two different design philosophies. And ASME being the more mature one, I'll stick to it.
Stiffeners can be very dangerous on hot (>600°F) vessels if the stiffener will protrude outside the insulation. The thermal stresses due to the temperature difference between nozzle and the stiffener may be sufficient to crack the weld attaching the stiffener to either the shell of the nozzle. Since these cracks would be on the pressure boundary, but are buried below the insulation, detection is challenging. I have observed pressure boundary failures due to this situation, so it's not a theoretical problem.
