If you put the pressure vessel in a sump without liquid in it, there are no external loads. If you submerge the vessel inside the sump in liquid, then you add the static head as external pressure.
If you attach piping to the vessel nozzles, then the nozzles have to be calculated to include external nozzle loads.
As far as what you should do, take my advice. Go and ask a professional pressure vessel design engineer to designed for you. This way you may remain intact, unhurt and you don't risk killing an inocent person when your pressure vessel might explode.
Best regards,
gr2vessels
If you put a vessel in sump, you may need to check flood condition. Rain water (mixed with soil) will have external pressure on the vessel when the area is flooded.
