Performing a start-up of a warm boiler is a difficult procedure, since the water inside the tubes are close to saturation. During the start-up, warm air is purged through the boiler, causing rapid boiling in both the evaporator and economiser which results in a high water level in the steam drum.
A mathematical model describing the dynamic performance of a natural circulation boiler is presented. The geometry of the boiler is simplified and discretized in one dimension.
During boiling in the evaporator and economiser steam is produced which displaces the water in the evaporator tubes, resulting in a high water level in the steam drum. A correlation is presented for the relative starting pressure and the relative volume of water occupying the steam drum. Depending on the maximum allowable water level, a minimum starting pressure can be determined and high water levels can thereby be limited.