10.2 De-aerator system
Traditionally the de-aerator storage tank capacity and height have been determined by the constraints de¬scribed in Section 3 of this chapter. For increased unit size, it becomes increasingly costly to accommodate the storage tank size and weight at the traditional high level.
Design studies indicate that a decreased tank capacity may be acceptable. A review of the boiler needs for hot de-aerated water during start-up, storage capacity to meet normal rates of load change and topping up the boiler after a unit trip, has indicated that a tank of about half the previously accepted capacity would be adequate (i.e., about 5 minutes CMR worth of flow).
The rate of pressure decay (and hence the ability of a given de-aerator system to avoid boiling in the feed pump suction pipework during transients) depends on the amount of stored heat (i.e., hot water) in the tank and the rate of introduction of cold feed to the tank. As the amount of hot water is reduced, so is the stored heat.
Provision has to be made to prevent common mode failure of LP heater bled-steam isolating valves causing loss of heating steam to the LP heaters, as this introduces condensate at a reduced temperature to the tank which could cause an unacceptable rate of pressure decay. Diversity of operation or the provision of stored energy local to the valves would be a suitable solution.
All other conditions can be avoided by good operational techniques and suitable sequences to avoid pumping cold condensate into a hot depleted tank.
An alternative solution is to provide a 'pegging' steam supply which would maintain the de-aerator pressure from an alternative steam supply (say cold reheat) in the event of an unacceptable rate of pressure decay being detected.
Modern boiler feed pumps for large capacity turbine-generators being supplied by UK manufacturers to customers overseas, require less suction head than those currently installed in CEGB power stations. The height at which the de-aerator tank must be placed relative to each boiler feed pump depends upon the suction head requirements of the pump, plus an allowance for friction and transients. Any reduction in suction head requirement allows a corresponding reduction in tank elevation. The elevation at which the tank is placed is also influenced by the need to have a suitable floor or steelwork to support the weight of the tank.
When the measures to prevent unacceptable rates of pressure decay during a transient are combined with the new generation of feed pumps which require reduced suction heads, then the combination of a smaller de-aerator tank at a lower level can be achieved. However, the overall system must withstand any transients which could cause loss of suction or boiling within the pipework between the de-aerator tank and the feed pump suction stage. This ability to withstand transient conditions can be demonstrated using the calculation methods outlined in Section 3 of this chapter, allowing for any special features used to minimise pressure decay during normal operation.
The de-aerator storage tank capacity and elevation have to be determined for each case. However, in general, it is expected that tank size can be reduced to about 5 minutes' worth of CMR flow at normal working level, with the tank placed at a significantly reduced elevation.