Why air vent should be closed maintaining 2kg/cm^2 drum level pressure at starting of boiler operation?

Air vents are provided on the boiler drum to evacuate the air and dissolved gases from the drum. During the boiler start up, the saturated steam will accumulate in the boiler drum and its pressure will start rising.The air and dissolved gases will be released through air vent. As the steam pressure reaches 2 kg/cm2 (above atmospheric pressure 1.03 kg/cm2), we can ensure that the air has been evacuated from the drum. therefore the air vent is closed to prevent the further loss of saturated steam.