1.3 System configuration
The number of feedheaters is determined by the application of the relevant economic factors, as explained in Chapter 1. However, the theoretical bleed point pressures are seldom obtainable because of constraints in blading design.
The actual points used are determined by what is possible with current designs of blades and, in some cases, by standard cylinder modules. On all modern large reheat units, final feed temperature is determined by cold reheat pressure as in all present fossil-fired plant the ultimate HP heaters are fed from the HP cylinder exhaust (i.e., at cold reheat pressure). A typical tubular surface type LP and HP feed system is shown in Fig 3.7.
In this system, the condensate formed in the condenser is pumped by the condenser extraction pump through the gland steam condenser (GSC) and the LP heaters to the high level de-aerator. The elevated position of the de-aerator provides the net positive suction head (NPSH) needs of the BFPs. In this context, the NPSH needs of the BFP are defined as the head equivalent to the pressure necessary, over and above the temperature-dependent saturation pressure at the inlet to the pump, to prevent the feedwater from flashing into steam within the pump. The capacity of the storage tank is available to meet any fluctuations in boiler demand. The feed pumps draw from the de-aerator and discharge through the feed regulating valves and the HP heaters to the boiler.
An alternative system with DC LP heaters and tubular surface HP heaters is illustrated in Fig 3.8. The condensate is extracted from the condenser and pumped through the GSC and the generator coolers up to the first LP DC heater. The condensate cascades from LP1 to LP3 DC heater via LP2. A DC heater extraction pump, pumps the water from LP3 heater up to the high level de-aerator. The BFP draws from the de-aerator tank and discharges to the boiler via the feed regulating valves and the HP heaters.