IFRs, I was trying to see if it would be OK to use thinner pipes? For example , following the table as a minimum thickness, it is required to use a Sch. 80 pipe on a 4" nozzle. If there is little/no internal pressure at, say, nozzles which are near to the top of the vessel , then it seems excessive. Is there some theory behind the table 5-6a thicknesses?
The nozzle neck thicknesses are not so much for pressure even at the tank bottom but a truly economical way to get metal there for corrosion, stress, reinforcing, erosion, stiffness, etc. It would seem almost nonsensical to pinch pennies on nozzle necks considering the high cost of failure in that part. I'm not aware of any calculations but I was not there when it was written. Perhaps there is someone out there who is?
IFRs, what appears counter-intuitive to me is that if you consider that a manway of lets say 500NB can have a 6mm / 0.236" neck (Table5-4) but a 300NB pipe must have a neck of 0.5" / 12.7mm neck. Which one has a greater impact on vessel - I'd say the manway.
A manway has no fluid motion within it, has no external piping loads and nor is it attached to anything other than the tank that might induce stresses. You may be missing the notes in the table - the minimum manway neck thickness is equal to the thinner of the materials it is joining ( shell and bolting flange ) but can't be less than that listed in the table. I find that most often it is thicker than the tabular numbers.
acknowledged IFRs, and points taken. It is the intention to ensure robust nozzle is installed (small cost compared to vessel) and preserve the tank. Nozzles can be repaired in future. Only my fabricator client is crying at the cost of not being able to use schedule 40 piping.
"A manway has no fluid motion within it, has no external piping loads and nor is it attached to anything other than the tank that might induce stresses. "
Agitators would be the one major exception to this guideline. If you have an agitator installed, the extra metal thickness will offer more rigidity and strength, which an operating mixer needs.
Fixed roof nozzles are supposed to use standard weight line pipe for necks ( figure 5.19 ). They are not give nearly the attention that shell nozzles are, probably because they are not part of the pressure boundary, the tank won't fail in its primary function ( to hold liquid ) if a roof nozzle fails. Again, my opinion only...
