Pressure vessel with a design pressure less than 101 kPa?

Your statement is correct. 102 kPa abs or 1 kPa gauge would be the same.

Yeah, there are some pressure vessels that work with external pressure greater that internal pressure. It's working pressure could be expressed in negative pressure in gauge values.

Whant a classicla example? Submarines... Vacuum pump reservoirs... Suction-side water pump piping. They are all subject to internal pressures smaller than externl ones.

Search CR4 for pressure vessel design under negative pressures. Someone has already showed us how to do that according standards.

• For gauge pressure and absolute pressure: We used to deal with design pressure nominated in any vessel datasheet as a gauge pressure e.g psig, even if it written as "psi" without "g" it will be read as gauge pressure. Unless clearly indicated as absolute pressure psia.

And in no way to guess is it a gauge pressure or absolute pressure? it must be clearly indicated by the owner.

• For vacuum pressure or external pressure: In design of pressure vessels, the term vacuum is used when we need to design this vessel (in addition to its internal pressure) under an external pressure of 14.7 psig or 0 psia. For any other external pressure, it is mandatory for the owner to specify that external pressure for designer to be clearly indicated through vessel datasheet. And, also, the external pressure shall be read as a gauge pressure (even written as psig or psi only) unless clearly indicated as absolute pressure psia.

Difference Between PSI and PSIG?

Equipment Design and Gauge Pressure vs. Absolute Pressure

Thanks for the answers guys!

But can anyone confirm something about vessel working under vacuum conditions. If it is in SI units, there wont be an indication like psia or psig to distinguish whether the pressure designated is absolute / gauage. So in real life, for those vessel in SI units, do they usually use a -ve kPa value to demonstrate that the vessel is working under vacuum conditions? Or do they use a +ve kPa (lower than 101kPa) with something behind kPa (like 'kPa (atm)' ) or something? thanks!