3.4 De-aerator elevation
The elevation of the de-aerator must be such that the suction head requirements of the feed pumps can bе satisfied under all credible modes of operation. A typical de-aerator tank and suction pipework con-Iguration is shown in Fig 3.30, the tank content, pipe engths and component elevations being indicated.
In addition to the static head indicated, there is tlso the pressure head within the vessel. Under steady tate conditions, the water within the feed pump suction pipework is at the saturation temperature equivalent to the pressure within the de-aerator. To ensure hat boiling within the pipework cannot occur when sudden pressure reduction in the tank takes place during a transient, the water travelling down the suction pipework must gain sufficient static head to prevent boiling. The important factors are the rate at which tank pressure reduces or decays and the rate of flow down the suction pipework. The limiting case found for CEGB plant is a hot restart of a feed pump. In this case the extraction pump is lost, the standby fails to start and the feed pumps draw down the de-aerator tank until the unit trips or is shut down. On unit restart, the water in the condenser which has accumulated after the extraction pump trip, has to be pumped forward to refill the depleted de-aerator. Assuming that the tank is at full-load conditions but only partially full, the 100% cold condensate inflow into a hot depleted tank produces a rapid pressure drop within the tank. The feed pump is started on leak-off, so the feed velocity is at its slowest and hence the time for the column of hot water in the suction pipework to gain static head is at its greatest.
Figure 3.31 shows in graphic form the effect of the hot restart with a typically sized de-aerator tank and suction pipework configuration. To prevent boiling in the suction pipework, the inflow is limited to a suitable flow rate by controlling the opening of the condenser level control valve after a unit trip. For further information on this aspect of de-aerator system design, reference should be made to Dartnell .
Further information on the effect of boiling in feed pipework and the avoidance of 'thermally induced' hydraulic shock is given by Wilkinson and Dartnell .