[vishal.khadke]

Dear kannapiran.mech,
Further to what srivallabha has said, let us consider a situation in which a vessel is connected to a vacuum producing equipment. When the vessel is open to atmosphere then the differntial pressure across its wall will be zero. Now when vessel is completely sealed & full vaccum is created in the vessel then differtial pressure accross the walls of the reactor will ~1bar (acting in the inside direction). This is because the atmospheric pressure is 1.013 bar abs. In case of equipments operating under high degree of vacuum,design pressure is generally stated as -1.0 barg OR Full Vacuum (F.V.) in material data sheets. Sometimes it is also possible that the vessel is designed for both positive pressure ( pressure inside greater than atmospheric) as well as full vacuum.
Thanks,
VISHAL