Many power plant operators have begun replacing outdated level instrumentation with newer technologies to accurately control feedwater heater levels and decrease their plant’s heat rate. However, assurance of proper performance can only be determined with a feedback reporting system in place.
There are three primary parameters that you can use to monitor individual heater performance. The following definitions and diagram highlight these parameters.
- Feedwater Temperature Rise is the difference between the feedwater outlet temperature and the feedwater inlet temperature. A properly performing heater should meet the manufacturer’s design specifications, provided the level controls are up to the task.
- Terminal Temperature Difference (TTD) provides feedback on the feedwater heater’s performance relative to heat transfer. TTD is defined as the saturation temperature of the extraction steam minus the feedwater outlet temperature. An increase in TTD indicates a reduction in heat transfer, while a decrease indicates an improvement. Typical ranges for TTD on a high-pressure heater with and without a desuperheating zone are -3° F to -5° F and 0° F, respectively. The TTD for low-pressure heaters is typically around 5° F. Steam tables and an accurate pressure reading are required to complete this calculation.
- Drain Cooler Approach (DCA) is a method used to infer feedwater heater levels based on the temperature difference between the drain cooler outlet and the feedwater inlet. A typical value for DCA is 10° F. An increasing DCA temperature difference indicates the level is decreasing and a decreasing DCA temperature indicates a rise in level.