Once-through steam generator and method for starting up a once-through steam generator

A method for starting up a once-through steam generator containing a first gas flue having a containing wall, a combustion chamber, and a plurality of steam generator tubes connected in parallel for conducting a flow medium and each having an outlet side, the plurality of steam generator tubes connected to one another forming a gas-tight evaporator heating surface and forming part of the containing wall of the first gas flue;

• a horizontal gas flue disposed downstream of the first gas flue and having a bottom edge;
• a second gas flue following the first gas flue on a fuel-gas side by way of the horizontal gas flue;
• an outlet header, the plurality of steam generator tubes forming the gas-tight evaporator heating surface opening on the outlet side into the outlet header, the outlet header being common to the plurality of steam generator tubes and disposed at a lower height in comparison with the bottom edge of the horizontal gas flue;
• a bulkhead heating surface directly following the outlet header on a flow-medium side, the bulkhead heating surface disposed in a region of space within the first gas flue above the combustion chamber;
• and a water/steam separating device following the bulkhead heating surface on the flow-medium side, the method which comprises:
• reducing a flow-medium throughput of the plurality of steam generator tubes forming the gas-tight evaporator heating surface after a commencement of an ejection of water from the plurality of steam generator tubes.