"Full Vac" and "Half Vac" have long been used as design conditions. I believe it wasn't until 2004, however, that the Section VIII folks explicitly accepted at least the FV term. From VIII-1 App 3:
full vacuum (FV) — a condition where the internal absolute pressure is 0 psi (0 KPa) and the external absolute pressure on the vessel is 15 psi (100 KPa) (see UG-116)
There's also an interpretation which deals with this, presumably prior to the definition being added to App 3.
I saw a lot of vesselcustomers asking for FV design pressure.
Mostly 0.8 bara was sufficient.
I didn't get that.
When calculated there was a huge wallthickness for that vessels.
Where they wrong with asking for FV?
The full vacuum condition could be the prevailing design condition for vessels with low internal design pressure, with minimal external nozzle loading and small wind / seismic loading. However, the full vacuum design for these vessels should not make a such big impact on the shell design, unless the vacuum rings have not been considered.
As for the higher internal pressure vessels (such as 4.5 MPa), the full vacuum design condition is probably covered by the more onerous internal design pressure, hence it wouldn't make any difference to the shell thickness (obviously, subject also of vessel ID).
The meaning of "4.5 MPa/FV" is that the pressure vessel or similar component is designed for a pressure of 4.5 Mega Pascals (gauge pressure), or Full Vacuum conditions. One Mega Pascal = Millions of Pascals. One MPa (gauge pressure) = 145.03 PSIG. The term FV means Full Vacuum, generally taken as 1 Atmosphere Pressure External to the vessel, with zero absolute pressure inside, equivalent to 15 PSIG External Pressure.
I would suggest you take into consideration some circumstances that might let you find your vessel under vacuum conditions.
Undesired vacuum conditions may occur in vessels for working conditions such as steam generator / steam accumulator or steam heated tank / heat exchanger etc. or in accumulator of highly volatile liquid or for liquid under low vapor pressure conditions.
Condensation will take place during shut down or due to change in temperature. Vacuum is expected if for any reason you vessel is not vented properly (undersized or blocked vent, or even stop valve that by a foolish design or maintenance was installed on vent pipe), or in case your vessel contains dangerous substance that you are not able to vent.
