3.3 De-aerator storage tank
The main function of the de-aerator storage tank is to provide a reservoir of de-aerated feedwater which can be drawn upon by the boiler feed pumps when needed.
The capacity of the tank and the quantities of feed-water stored at the various levels within the tank are determined by the following functional needs:
- (a) To store sufficient feedwater to meet the requirements for flexible operation.
- (b) To provide control levels for the operation of the insurge and outsurge systems without exceeding the high water level.
- (c) Even when working at the top of the normal operating level control band, to accept the contents of the condenser hotwell in the event of the condenser level control valve failing to the open position, without exceeding the high water level.
- (d) From the same initial working level as in (c), to accept half-a-minute's-worth of CMR flow without exceeding the high water level.
- (e) From the level indicated in (c), to accept water accumulated in the condenser hotwell after a unit trip when the extraction pumps are restarted with out exceeding the high water level.
Figure 3.29 shows a cross-section through a typical de-aerator tank for a modern 660 MW unit. Indicated are the various levels and the resultant actions which are initiated.
The highest level to which the condensate can rise is determined by the manufacturer, such that the maximum velocity of the steam flowing across the surface of the water will not cause water pick-up or 'sloshing' of the tank content. In this context, water pick-up is defined as water which is stripped from the tank surface and transported to the de-aerator head by the action of heating steam. Damage to the de-aerator head internals can be caused by the impact of the water which has been carried over. The head can also become flooded due to the drains being unable to cope with the extra water burden.
'Sloshing' of the tank content is caused by the formation of waves in the tank which close the steam pathway and are then propelled the length of the tank by the force created by the heating steam being attracted to the head. For a more complete explanation of the mechanism, reference should be made to the paper on the subject of de-aerator tank instabilities by Cranfield and Wilkinson . On the latest designs of de-aerator, only a fraction of the steam is passed over the tank surface, the majority being introduced local to the head. The small flow across the tank is retained to scavenge any oxygen within the tank and transport it to the head. The head is vented to remove any accumulation of non-condensable gases.
The rest of the working levels in the tank are determined applying rules (a) to (e), above. From Fig 3.29 it will be seen that the insurge and outsurge valves have been provided with separate control bands. A dead band is placed between the two control bands to allow the insurge and outsurge system to operate independently without hunting.