1.6 Maintenance of system water content
The de-aerator tank provides a reservoir of water on which the feed pump can draw should there be a mismatch between the feed requirements of the boiler and the condensate flowing into the de-aerator tank.
There is typically 7-10 minutes worth of constant maximum rating (CMR) feed waterflow stored in the tank. To allow for normal gains and losses to the de-aerator tank content, insurge and outsurge systems are provided to maintain the tank content as near to a constant value as is operationally necessary. As illustrated in Fig 3.13, the insurge and outsurge connections to and from the reserve feedwater tanks (RFTs) are made to the LP feed system to allow adjustment of the total cycle water content. During normal operation, the large content of the tank smooths out any sudden small changes in demand and so allows the insurge/ outsurge valves to respond in a controlled manner to restore the tank level to normal.
In the event of a low level in the de-aerator tank, the insurge group feeds into the condenser from where it is pumped forward to the de-aerator tank by the condensate extraction pumps. This continues until the water level in the tank is restored to its normal work-ng level. If the de-aerator tank level is higher than the etpoint, the outsurge valve is opened and the excess /ater is discharged from the system to the RFTs until lormal working level is restored. The method by which hese systems are integrated into the LP feed system is escribed in Section 4 of this chapter.