This query regards side nozzles (on vertical vessels, I have to presume). It might also be related to horizontal vessels.
When discussing this issue, it is necessary to have a complete understanding of how a pressure vessel is designed and fabricated – and with what type of materials and criteria. Additionally, you can’t lump EVERY conceivable nozzle into the same, generalized, description. Fluid entry, exit, drain, and level nozzles are one thing; Manways and instrument nozzles are another different type of application.
You should be aware that your pressure vessel will most often have either elliptical or torispherical (ASME F&D) heads. Rarely are hemi-heads used. What this leads up to is knowledge of what the “tangent line” on a vessel constitutes. It is also important to know where the SEAM between the cylindrical section (“can”) and the heads is located. Usually, you don’t want to do any cutting or welding within approximately 2 to 3” of the welded seams in order to avoid any added stresses due to heat buildup during the cutting and welding. The specific distance to follow depends on the size of the nozzles / manways involved.
Both the ellipsoidal and torispherical heads have what is called a “straight flange”, an extension of the formed head that follows the diameter of the head and forms the seam with the cylindrical section – on both ends of the vessel. This straight flange may vary with the size and fabricator of the heads, but I generally have seen 2 to 2-1/2 inches of straight length on most straight flanges. From this length, you should be able to come to the realization that it is probably not a good idea to select the straight flange as the location for a nozzle. This is compounded by the fact the “knuckle” radius of each formed head (the point where the head is formed into a cylindrical flange) is a site of very high, concentrated stress because the formed shape has been forced to change direction. I have always used the welded cylindrical seam as my reference point to locate nozzles. That way I keep out of harm’s way in building up more stresses.
As a process engineer, I always decided on the quantity, quality, and location of all pressure vessel nozzles. I would expect the same to be practiced today. It then becomes the responsibility of the vessel designer to either concur with the selections or challenge them. The information and instructions that are transferred from the process engineer to the vessel designer is always started with a detailed Data Sheet of the vessel in question.