A method of preparing a system for a maintenance operation, said system comprising - a first subsystem containing a super-atmospheric gas, - a second subsystem, and - a third subsystem between the first subsystem and the second subsystem; said first subsystem and said second subsystem being connected via an intermediate subsystem, said intermediate subsystem - capable of allowing gas to flow between the first subsystem and the second subsystem via the third subsystem, and - being located between the first subsystem and the third subsystem; said intermediate subsystem comprising - a first sealing valve seat, - a second valve seat between the first sealing valve seat and the second subsystem, and - a vent opening between the first sealing valve seat and the second valve seat; said third subsystem and said second subsystem being connected via a further intermediate subsystem capable of allowing gas to flow between the third subsystem and the second subsystem, said further intermediate subsystem comprising - a third sealing valve seat, - a fourth valve seat between the third sealing valve seat and the third subsystem, and - a second vent opening between the third sealing valve seat and the fourth valve seat; said method comprising - the discharge of gas present between the first sealing valve seat and the second valve seat via the vent opening before performing a maintenance step of the maintenance operation downstream of said second valve seat with respect to the first sealing valve seat, and - the discharge of gas present between the third sealing valve seat and the fourth valve seat via the second vent opening before performing a maintenance step of the maintenance operation between the second valve seat and the fourth valve seat; characterized in that the method comprises before performing a maintenance step of the maintenance operation with the intermediate subsystem and the further intermediate subsystem in a closed position - applying suction to the vent opening for discharging the gas present between the first sealing valve seat and the second valve seat before performing said maintenance step - the discharge of gas present between the third sealing valve seat and the fourth valve seat via the second vent opening between the second valve seat and the fourth valve seat by applying suction to the second vent opening; wherein the suction is applied to achieve sub-atmospheric pressure in at least one intermediate system (170a, 170b) of the system. - a first subsystem containing a super-atmospheric gas, - a second subsystem, and - a third subsystem between the first subsystem and the second subsystem; - capable of allowing gas to flow between the first subsystem and the second subsystem via the third subsystem, and - being located between the first subsystem and the third subsystem; - a first sealing valve seat, - a second valve seat between the first sealing valve seat and the second subsystem, and - a vent opening between the first sealing valve seat and the second valve seat; - a third sealing valve seat, - a fourth valve seat between the third sealing valve seat and the third subsystem, and - a second vent opening between the third sealing valve seat and the fourth valve seat; - the discharge of gas present between the first sealing valve seat and the second valve seat via the vent opening before performing a maintenance step of the maintenance operation downstream of said second valve seat with respect to the first sealing valve seat, and - the discharge of gas present between the third sealing valve seat and the fourth valve seat via the second vent opening before performing a maintenance step of the maintenance operation between the second valve seat and the fourth valve seat; characterized in that the method comprises before performing a maintenance step of the maintenance operation with the intermediate subsystem and the further intermediate subsystem in a closed position - applying suction to the vent opening for discharging the gas present between the first sealing valve seat and the second valve seat before performing said maintenance step - the discharge of gas present between the third sealing valve seat and the fourth valve seat via the second vent opening between the second valve seat and the fourth valve seat by applying suction to the second vent opening;