4.2 Low pressure system configuration - part 1
Figure 3.38 shows the LP feed system for Littlebrook D which uses tubed LP heaters in a horizontal attitude. The heaters are by-passed in two banks, the first group being LP1 and LP2 heaters and the second LP3 heater.
Steam is supplied to each of the three LP heaters from the LP turbines. Each bled-steam line contains a free-acting non-return valve and a bled-steam isolating valve.
The drains formed in LP3 heater are cascaded to LP2 drains flashbox, from where they are drained into LP1 drains flashbox. The combined drains from LP2 and LP1 flashboxes cascade to the condenser flash vessel. The drains from LP1 heater are also cascaded to the condenser flash vessel.
This grouping of LP heaters has been chosen so that if either LP3 or LP2 and LP1 are by-passed, the resultant steam and condensate flows will not overload the individual feed train components. If LP3 heater is by-passed, the condensate temperature to the de-aerator will be at LP2 heater outlet temperature. The steam flow to the de-aerator will increase by about 50% to compensate for the greater enthalpy rise now needed. The de-aerator is designed to allow for this increase in the flow without damage or operational difficulties. When LP3 heater is by-passed, the drains are also isolated from LP2 heater flashbox. If LP1 and LP2 heaters are by-passed, condensate at the temperature of the gland steam condenser outlet is supplied to LP3 heater. LP3 heater then attracts a total of approximately three times the normal steam How. To allow for this large increase in steam flow, the steam inlet and heater internals are designed for this duty. The drain system is also capable of accepting the abnormal drains quantity.
The feed system illustrated has no drain cooling section contained within the heater shells. However, to improve the cycle efficiency, a drain cooler is usually provided to recover as much of the heat from the combined drains out of the lowest pressure LP heater as practicable. The drain cooler can be of the water-to-water type or the more usual flashing type. The flash drain cooler is, in effect, an additional stage of LP feedheating, using the steam flashed from the drains out of LP1 to heat the feedwater. The drains from the flash drain cooler are cascaded to the condenser.
A development to increase cycle efficiency which is to be used for the current PWR feed system (Fig 3.6), is the pumping forward of the drains from a group of LP heaters. Because of the large flows involved in the PWR cycle, it has proved economic to provide the additional pumps and associated equipment needed to inject the LP heater drains into the de-aerator system in order to recover the heat in the drain water.