When the plant is started up, the trap is fully open. Cold condensate and air are discharged. With rising temperature, the Duo (bimetallic) stainless steel plates bend and pull the stage nozzle into the closing position.

The orifice is then closed immediately below the saturation temperature. Thermodynamic processes taking place in the
stage-nozzle chamber support the closing procedure.

The trap provides automatic air-venting at start-up and during operation. BK 45 and BK 46 can also be used for thermal
air-venting in steam systems.