Feedwater heaters use extraction steam from the turbine to raise the temperature of water destined for the boiler. Water first passes through low-pressure heaters and into the deaerator where excess oxygen is removed. The feedwater then passes into the high-pressure heaters where it is further heated and pressurized. Two separate level control loops should manage each feedwater heater according to ASME standards.
Feedwater heater level is controlled to (1) prevent level from rising into the extraction line; (2) keep the tube surfaces in the condensing zone immersed; and (3) keep the drain cooler flooded. Level instrumentation must withstand moderate to high temperatures and pressures and turbulent conditions.