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The drain cooling section exists to subcool the shell side condensate before it exits the heater and is critical to the health of the heater.
A general layout of the drain cooling section can be found in Figure 1 for a typical horizontal feedwater heater. The drain cooler inlet is at the bottom of the heater, with many older heaters using a “snorkel,” extending into a well or capped nozzle at the bottom of the shell to increase the submergence of the drain cooler entrance. The drain cooling section acts like any common drinking straw. A pressure differential exists between the heater shell and the destination pressure, which is lower, and condensate is drawn up and through the section. While the shell condensate passes through the cooling section, it navigates a set of baffles and is cooled by the tube side feedwater. In this analogy the snorkel is like the bottom of the straw, with the level control system maintaining the condensate level above the drain cooler entrance and below the heater tubes. A level must be controlled to cover this snorkel or the drain cooler
Proper heater level control begins at instrumentation and controls
Good level control starts with an assessment of the heater instrumentation and control mechanisms and benefits from a basic understanding of the drain cooler construction and purpose. This brings us to the first instrument of concern, the level gauge and set points. Under steady state conditions a heater’s level might not be uniform from end to end. Sometimes it takes the shape of a parabola.
Therefore the level gauge should be located near the drain cooling zone section, and the level control set points should be set relevant to the condensate level at the entrance to the drain cooler.
This is what we want to protect.
Additional causes for inaccurate level measurement might be:
- A localized steam velocity profile across the top equalizing leg tap, causing reduced pressure in the equalizing line
- Condensation in a long top equalizing leg
- Loop seals trapping condensate in the top equalizing leg
- Sediment buildup
- Blockage or partial valve closure in the lower leg
- Or high points trapping gas in the lower leg
With accurate level measurement, controls should be able to adjust drain valve position to regulate flow and maintain the proper level for all loads. In some cases, level controls have been found to maintain level at high loads, but have not provided stable level control at lower loads. Level controllers might not provide stable level control for a wide variety of reasons and might result in fluctuating or erratic drain temperatures. Load variations are common in fossil-fired power plants so it is important to evaluate the heater level behavior from higher to lower loads.
Monitoring key performance indicators
To prevent issues with low feedwater heater levels and to reduce alarm frequency, heater levels are sometimes set high. As the heater level increases, the tubes in the condensing zone begin to be covered. This reduces the size of the feedwater heater and reduces plant efficiency, resulting from poorer heat transfer. Optimizing the level does two things – prevent damage to the drain cooler components and maintain good heat transfer efficiency.
There are two performance indicators relative to heater level and heat transfer performance that should be monitored. These are:
- Drain Cooler Approach (DCA)
- Terminal Temperature Difference (TTD)
The DCA is equal to the drain outlet temperature minus the tube-side inlet temperature, Tdrain – TFW,inlet. The DCA can be effectively used as an input for determining the optimum normal operating level. For example, an increase in level will decrease the DCA, resulting from decreased drain outlet temperature. Conversely, a decrease in level, especially to the point where steam enters the drain cooler, will cause the DCA to sharply increase. A typical design value for DCA is 10°F, but some plants designed to operate at this value are seeing +25°F values, indicating a potentially low level operation that could be leading to damage.