The reliability and availability of a HRSG steam system can be improved by providing a flying switch over into fresh air operation in case of failure of the gas turbine.

The flying switch over can be designed for a planned switch over in case of no availability of the gas turbine as well for a unplanned switch over in case of gas turbine trip.

Especially during the unplanned switch over the pressure of the steam system will decrease due to a temporarily lack of heat input on the HRSG.

The paper will describe different switch over arrangements and their influences on the pressure of the steam system during the switch over.

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