HP (High Pressure) feedwater heater is a heat exchanger that serves to heat boiler feedwater before it is distributed by way of heat exchange between steam extractions of HP (High Pressure) turbine with feedwater from the feedwater pump.
HP feedwater heater is arranged in graduated with direction opposite between feedwater and steam. As a result of heat exchange between steam and feedwater, condensation event will occure that produce condensate. Condensate still has such high temperatures are not discarded but used to help warm up next heaters.
The level of condensate in each of heater is maintained at position set point of 20%. As shown in Figure 1, the control of HP feedwater heater is still relying on feedback control system that regulates the output of heater. The system is only intended to control each heater with no connection to each other. So when one of heaters that has an input steam and condensate (7A and 6A) increased steam load, the heater is still also get the input of large quantities of condensate heater before.
This condition finally will make the level of heater exceeds the limit of 40%, which means turn on Level Switch High (LSH) to open the blowdown valve. In this condition the control system is otherwise not able to keep the level in a safe condition, so that the safety system worked.
Figure 1: Control Feedwater Heater