Openings in the shell of a pressure vessel naturally introduce a weakness at that point. The bigger the opening the greater the compensation required to make up for it.

Different pressure vessel design codes use different methods to calculate the amount of compensation required, but in essence they all adopt a formula of calculating how much material has been removed and ‘compensating’ for it by adding up what has been added back in in the form of pipe branch, its weld and possibly some of the flange.
Frequently if the branch is of reasonable schedule (thickness) and hole fairly small then additional compensation may not be required (ignoring nozzle loads or hefty corrosion allowances)
If the compensation is insufficient then additional compensation material may be added by thickening up the branch material, and , or adding a compensating ring.
A compensation ring around the opening adds strength to the hole. As the ring is fully welded it is necessary to give it a ‘tell tale’ hole, designed to allow visible evidence of a leak should there be a leak in the material underneath.
The photograph below shows a compensating ring on a pressure vessel built by Abbott & co. The 6mm telltale hole is located on the right (these can be threaded and have a nipple screwed in). Under certain circumstances compensation may me added to a vessel or air receiver at a later date and the vessel modified to suit.