This is the third and final blog in a three-part series about identifying issues with feedwater heaters. The series discusses why feedwater heaters are important to efficient plant operations, how health of feedwater heaters can be determined, and common level control and feedwater heater problems that occur during operation of the heaters. View the first installment here, and the second installment here.
In a power plant, one of the most common causes of tube failures in a feedwater heater is the improper control of the liquid condensate level in the shell, which also can cause operational and maintenance costs that might lead to premature replacement. The condensate that forms in the shell of each feedwater heater is allowed to flow to the next lowest pressure heater through level control valves, an arrangement called cascading drains. The level control valves control the drain flow to maintain optimum condensate level in each heater. Properly controlling heater levels does two things:
A level control test may be performed to determine the optimum operating level for a feedwater heater by manually altering the heater level and recording the DCA and TTD responses. Lowering the level results in increased drain temperatures and subsequently lower DCA. When the drain cooler is uncovered, there is a sudden step up in drain temperature and reduced DCA resulting from steam entering the drain cooler. TTD is not affected by level as long as it isn’t so high that tubes other than the drain cooler tubes are covered with condensate. Usually there is a level target on the outside of the FWH that shows design level. That is a good starting point.
For example, if a feedwater heater’s drain cooler entrance becomes uncovered when the level drops to 5.5” and its condensing section tube coverage begins around 13”, normal level control can be adjusted to an optimum operating range between 7-9” based on actual level measurement, regardless of the “design” level. Selecting a new optimum operating level control range includes consideration of the existing high-level dump control valve, alarm and bypass switch settings, as well as their range of adjustment and control, both prior to performing the level control test, as well as prior to establishing a new normal control range. Each feedwater heater will have unique behavior relating to level, location of level indicators, and the DCA and TTD performance indices.
It is important for the optimum operating level to also consider the impacts of periodic changes to normal or full-power operating conditions, such as start-up and shutdown, transient operation, heater strings out-of-service, low load operation and similar conditions. Adjustments or tuning of level control valve feedback or gain settings might also be appropriate to reflect the dynamics of the system response to a new normal operating control range. Heater level tests should be considered whenever there are indications of level related problems with the feedwater heater, as well as if there have been significant changes made to the feedwater heater or its operation, such as tube plugging or uprates.
The following is a summary of typical feedwater heater problems in a power plant: