Can anyone help with what vacuum does the vessel become under a standard for Australia. I believe that the Standards do not include any item under 5 psi before they have too comply but i do not know or can find a standard for vacuum
The vessel / tanks are governed by the pressure vessel code AS1210 which allows the design to full vacuum, ie -101.35 kPa (approx. 15 psi) or by the API 650 / 620 which allows only a small vacuum in the design conditions, up to -1 psi. I'm not sure where is the 5 psi coming from, so I am unable to comment on that. Hope this helps a bit.
apts,
A vacuum is considered an external pressure and if the pressure vs diameter exceeds the curve in Figure 1.3.2 of AS1210 then the vessel should be designed to AS1210. (Curve is P=(31/D + 0.82)kPa (P in kPa & D in m) - and 50 kPa between 0 and ~1 m)