PURPOSE: To prevent the counterflow from a stopped vacuum pump equipped with an air ejector to a running vacuum pump and the freezing of the nozzles of the ejectors, by raising the closing speed of system valves.
CONSTITUTION: Each water-sealed-up vacuum pump 10 is appropriate as an air bleeder for the steam condenser 1 of a thermal plant turbine or the like. When the steam condenser is at a low degree of vacuum and the pump is started, a solenoid valve 23 is energized the system valve 5 is opened to suck bled air from the steam condenser 1 into the vacuum pump through pipes 2, 3, the system valve 5 and an ejector bypass valve 6. When the steam condenser has reached a prescribed degree of vacuum higher than the former, a vacuum switch is actuated to energize a solenoid valve 25, close the valve 6 and open an ejector driving valve 16 to suck bled air from the steam condenser 1 into the pump 10 through the ejector 8. At that time, the system valve 5 is controlled through an air-actuated three-port air switching valve 28.